Super detailed Summary of Java learning materials

Java classic interview questions

• Java based
• • 1. What is the difference between JDK and JRE?
  • 2. What is the difference between == and equals?
  • 3. If two objects have the same hashCode(), equals() must also be true, right?
  • 4. What does final do in Java?
  • 5. How much is math.round (-1.5) in Java?
  • 6. Is String a basic data type?
  • 7. What are the classes for manipulating strings in Java? What’s the difference between them?
  • Is String STR =” I “the same as String STR =new String(” I “)?
  • 9. How to reverse a string?
  • What are the common methods of the String class?
  • 11. Must abstract classes have abstract methods?
  • 12. What are the differences between ordinary classes and abstract classes?
  • 13. Can abstract classes use final?
  • What is the difference between an interface and an abstract class?
  • 15. How many IO streams are there in Java?
  • 16. What is the difference between BIO, NIO and AIO?
  • 17. What are the common methods of Files?
• The container
• • 18. What are Java containers?
  • 19. What’s the difference between Collections and Collections?
  • 20. What is the difference between a List, a Set and a Map?
  • 21. What is the difference between HashMap and Hashtable?
  • 22. How do I decide to use HashMap or TreeMap?
  • 23. How does HashMap work?
  • 24. How does HashSet work?
  • 25. What is the difference between ArrayList and LinkedList?
  • 26. How to convert an array to a List?
  • 27. What is the difference between an ArrayList and a Vector?
  • 28. What’s the difference between Array and ArrayList?
  • 29. What is the difference between poll() and remove() in a Queue?
  • 30. Which collection classes are thread-safe?
  • 31. What is an Iterator?
  • 32. How to use Iterator? What are the characteristics?
  • 33. What is the difference between Iterator and ListIterator?
• multithreading
• • What is the difference between parallelism and concurrency?
  • 36. What is the difference between threads and processes?
  • 37. What is a daemon thread?
  • 38. What are the ways to create a thread?
  • 39. What is the difference between runnable and callable?
  • What are the states of threads?
  • 41. What’s the difference between sleep() and wait()?
  • What is the difference between notify() and notifyAll()?
  • 43. What is the difference between run() and start() for a thread?
  • 44. What are the ways to create a thread pool?
  • What are the states of the thread pool?
  • 46. What is the difference between submit() and execute() methods in the thread pool?
  • 47. How to ensure the safety of multithreading in Java program?
  • 48. What is the upgrade principle of multi-threaded lock?
  • 49. What is a deadlock?
  • 50. How do I prevent deadlocks?
  • 51. What is ThreadLocal? What are the usage scenarios?
  • Talk about the underlying implementation of synchronized.
  • 53. What is the difference between synchronized and volatile?
  • 54. What’s the difference between synchronized and Lock?
  • What is the difference between synchronized and ReentrantLock?
  • 56. How does atomic work?
• reflection
• • 57. What is reflection?
  • 58. What is Java serialization? When is serialization needed?
  • 59. What is a dynamic proxy? What are the applications?
  • 60. How to implement dynamic proxy?
  • 61. Why use cloning?
  • 62. How to implement object cloning?
  • 63. What is the difference between deep copy and shallow copy?
• Java Web
• • 64. What is the difference between a JSP and a servlet?
  • 65. What built-in objects does JSP have? What are the effects?
  • 66. What are the four scopes of JSP?
  • 67. What is the difference between session and cookie?
  • 68. How does session work?
  • 69. Can session be implemented if the client disables cookies?
  • 70. What is the difference between Spring MVC and Struts?
  • 71. How do I avoid SQL injection?
  • 72. What is an XSS attack and how can it be avoided?
  • 73. What is a CSRF attack and how can it be avoided?
• abnormal
• • Throws throws 74.
  • 75. What is the difference between final, finally and Finalize?
  • 76. Which part of the try-catch-finally can be omitted?
  • 77. In try-catch-finally, if a catch returns, finally will be executed?
  • 78. What are the common exception classes?
• network
• • 79. What do HTTP response codes 301 and 302 represent? What’s the difference?
  • 80. What’s the difference between forward and redirect?
  • 81. Describe the differences between TCP and UDP.
  • 82. Why does TCP require three handshakes? Why is that?
  • How does TCP sticky packet come into being?
  • 84. What are the OSI seven-tier models?
  • 85. What are the differences between GET and POST requests?
  • 86. How to achieve cross-domain?
  • 87. How does JSONP work?
• Design patterns
• • 88. Tell me about a design pattern you are familiar with.
  • 89. What is the difference between a simple factory and an abstract factory?
• Spring / Spring MVC
• • 90. Why use Spring?
  • Explain what AOP is?
  • Explain what ioc is.
  • 93. What are the main modules of Spring?
  • What are the common injection methods in Spring?
  • 95. Are beans in Spring thread-safe?
  • 96. How many scopes does Spring support for beans?
  • 97. What are the methods for Spring to automate bean assembly?
  • What are the ways in which Spring transactions are implemented?
  • 99. What about Transaction isolation in Spring?
  • 100. What about the Spring MVC workflow?
  • 101. What components does Spring MVC have?
  • 102. What does @requestMapping do?
  • 103. What does @autowired do?
• Spring Boot / Spring Cloud
• • 104. What is Spring Boot?
  • 105. Why Spring Boot?
  • 106. What is the Spring Boot core configuration file?
  • 107. What are the types of Spring Boot configuration files? What’s the difference?
  • 108. How can Spring Boot implement hot deployment?
  • 109. What is the difference between JPA and Hibernate?
  • 110. What is Spring Cloud?
  • 111. What does the Spring Cloud circuit breaker do?
  • 112. What are the core components of Spring Cloud?
• Hibernate
• • 113. Why use Hibernate?
  • 114. What is ORM Framework?
  • 115. How to view printed SQL statements in The console in Hibernate?
  • How many query methods does Hibernate have?
  • 117. Can Hibernate entity classes be defined as final?
  • 118. What is the difference between using Integer and int for mapping in Hibernate?
  • 119. How does Hibernate work?
  • 120. The difference between get() and load()?
  • 121. What about Hibernate’s caching mechanism?
  • 122. What are the states of Hibernate objects?
  • What is the difference between getCurrentSession and openSession in Hibernate?
  • 124. Do Hibernate entity classes have to have parameterless constructors? Why is that?
• Mybatis
• • 125. What is the difference between #{} and ${} in mybatis?
  • 126. How many pagination methods are there in Mybatis?
  • 127. Advantages and disadvantages of Mybatis
  • 128. What is the difference between logical paging and physical paging in Mybatis?
  • 129. Does Mybatis support lazy loading? What is the principle of lazy loading?
  • What about level 1 cache and level 2 cache in Mybatis?
  • 131. What are the differences between Mybatis and Hibernate?
  • 132. What executors do Mybatis have?
  • 133. What is the implementation principle of mybatis paging plug-in?
  • 134. How to write a custom plug-in for Mybatis?
• RabbitMQ
• • 135. What are the usage scenarios for RabbitMQ?
  • 136. What are the important roles of RabbitMQ?
  • 137. What are the important components of RabbitMQ?
  • 138. What is vhost for rabbitMQ?
  • 139. How are rabbitMQ messages sent?
  • 140. How does RabbitMQ ensure message stability?
  • 141. How can RabbitMQ avoid message loss?
  • 142. What are the conditions for successful message persistence?
  • 143. What are the disadvantages of RabbitMQ persistence?
  • 144. How many broadcast types are available for RabbitMQ?
  • 145. How does RabbitMQ implement delayed message queuing?
  • 146. What are rabbitMQ clusters for?
  • 147. What are the types of RabbitMQ nodes?
  • 148. What should I pay attention to when setting up rabbitMQ clusters?
  • 149. Is each rabbitMQ node a full copy of the other nodes? Why is that?
  • 150. What happens when the only disk node in the RabbitMQ cluster crashes?
  • 151. Does RabbitMQ have a stop order for cluster nodes?
• Kafka
• • Can Kafka be used independently of ZooKeeper? Why is that?
  • 153. How many data retention strategies does Kafka have?
  • 154. Kafka sets 7 days and 10 gigabytes of clean data at the same time, and on the fifth day it reaches 10 gigabytes of clean data.
  • 155. What can cause Kafka to run slowly?
  • 156. What should I pay attention to when using kafka clusters?
• Zookeeper
• • 157. What is Zookeeper?
  • 158. What functions does ZooKeeper have?
  • 159. How many deployment modes are available for ZooKeeper?
  • 160. How does ZooKeeper synchronize the status of the primary and secondary nodes?
  • 161. Why have a primary node in a cluster?
  • 162. There are three servers in the cluster and one node is down. Is ZooKeeper still available?
  • 163. What is the notification mechanism of ZooKeeper?
• MySql
• • 164. What are the three paradigms of databases?
  • Mysql > alter table alter table alter table alter table alter table alter table alter table alter table alter table alter table alter table alter table alter table
  • 166. How do I obtain the current database version?
  • 167. What is ACID?
  • 168. What is the difference between char and varchar?
  • 169. What is the difference between float and double?
  • 170. What is the difference between inner join, left join, right join in mysql?
  • 171. How is the mysql index implemented?
  • 172. How to verify that the mysql index meets the requirement?
  • 173. What about transaction isolation of databases?
  • 174. What is the common mysql engine?
  • 175. What about row and table locks in mysql?
  • 176. What about optimistic and pessimistic locks?
  • 177. What are the methods used to troubleshoot mysql problems?
  • 178. How to optimize mysql performance?
• Redis
• • 179. What is redis? What are the usage scenarios?
  • 180. What functions does Redis have?
  • 181. What’s the difference between Redis and memecache?
  • 182. Why is Redis single threaded?
  • 183. What is cache penetration? How to solve it?
  • 184. What data types are supported by Redis?
  • 185. What Java clients does Redis support?
  • 186. What are the differences between Jedis and Redisson?
  • 187. How to ensure the consistency of cache and database data?
  • 188. How many ways can REDis persist?
  • 189. How does Redis implement distributed lock?
  • 190. What are the defects of redis distributed locks?
  • 191. How does Redis optimize memory?
  • 192. What are the Redis elimination strategies?
  • 193. What are the common performance issues with Redis? How to solve it?
• JVM
• • 194. What are the main components of the JVM? And its role?
  • 195. What about the JVM runtime data area?
  • 196. What is the difference between stacks?
  • 198. What is the parental delegation model?
  • 199. What about the execution of class loading?
  • 200. How to determine whether an object can be reclaimed?
  • 201. What reference types are available in Java?
  • 202. What garbage collection algorithms does the JVM have?
  • 203. What garbage collectors does the JVM have?
  • 204. What about CMS garbage collector?
  • 205. What are the new generation garbage collectors and old generation garbage collectors? What’s the difference?
  • 206. Describe briefly how generational garbage collector works.
  • 207. What about JVM tuning tools?
  • 208. What are the common JVM tuning parameters?

Java based

1. What is the difference between JDK and JRE?

2. What is the difference between == and equals?

= = interpretation

The effect is different for the base type and the reference type ==, as shown below:

Basic type: compares whether the values are the same; Reference type: Compares whether references are the same;

Code examples:

String x = "string";
String y = "string";
String z = new String("string");
System.out.println(x==y); // true
System.out.println(x==z); // false
System.out.println(x.equals(y)); // true
System.out.println(x.equals(z)); // true
Copy the code

Since x and y refer to the same reference, == is also true, while the new String() method overwrites the memory space, so == is false, and equals always compares values, so it is true.

Equals reading

Equals is essentially equal to ==, except that String and Integer and others have overridden the equals method to make it a value comparison. Take a look at the code below.

First, equals compares an object with the same value by default, as follows:

class Cat { public Cat(String name) { this.name = name; } private String name; public String getName() { return name; } public void setName(String name) { this.name = name; }} Cat c1 = new Cat(" Cat "); Cat c2 = new Cat(" Cat "); System.out.println(c1.equals(c2)); // falseCopy the code

The output is false to our surprise. Equals equals equals equals equals equals equals equals equals equals equals equals equals equals

public boolean equals(Object obj) {
    return (this == obj);
}
Copy the code

Equals is essentially equal to ==. Why do two strings with the same value return true? The code is as follows:

String s1 = new String(" s1 "); String s2 = new String(" "); System.out.println(s1.equals(s2)); // trueCopy the code

Similarly, when we go to String equals, we find the answer as follows:

public boolean equals(Object anObject) { if (this == anObject) { return true; } if (anObject instanceof String) { String anotherString = (String)anObject; int n = value.length; if (n == anotherString.value.length) { char v1[] = value; char v2[] = anotherString.value; int i = 0; while (n-- ! = 0) { if (v1[i] ! = v2[i]) return false; i++; } return true; } } return false; }Copy the code

String overrides Object’s equals method, changing reference comparison to value comparison.

Summary: == is a value comparison for primitive types and a reference comparison for reference types; Equals by default is a reference comparison, but many classes have redone equals (String, Integer, etc.) as a value comparison, so in general equals compares whether or not values are equal.

3. If two objects have the same hashCode(), equals() must also be true, right?

String str1 = "Call "; String str2 = ""; System. Out. Println (the String. Format (" | str2 str1: % d: % d ", str1. HashCode (), str2. HashCode ())); System.out.println(str1.equals(str2)); The result of the execution: str1:1179395 | str2:1179395 falseCopy the code

4. What does final do in Java?

Classes that are final are called final classes and cannot be inherited. Final methods cannot be overridden. A final variable is called a constant, which must be initialized, after which its value cannot be changed.

5. How much is math.round (-1.5) in Java?

6. Is String a basic data type?

Strings are not part of the base type. There are eight basic types: byte, Boolean, char, short, int, float, long, and double. Strings are objects.

7. What are the classes for manipulating strings in Java? What’s the difference between them?

Is String STR =” I “the same as String STR =new String(” I”)?

No, because memory is allocated differently. String STR =” I “, which the Java virtual machine allocates to the constant pool; String STR =new String(” I “) is allocated to the heap.

9. How to reverse a string?

// StringBuffer reverse
StringBuffer stringBuffer = new StringBuffer();
stringBuffer.append("abcdefg");
System.out.println(stringBuffer.reverse()); // gfedcba
// StringBuilder reverse
StringBuilder stringBuilder = new StringBuilder();
stringBuilder.append("abcdefg");
System.out.println(stringBuilder.reverse()); // gfedcba
Copy the code

What are the common methods of the String class?

IndexOf () : returns the indexOf the specified character. CharAt () : Returns the character at the specified index. Replace () : string replacement. Trim () : Removes whitespace at both ends of a string. Split () : Split string, returns a split array of strings. GetBytes () : Returns a byte array of strings. Length () : Returns the length of the string. ToLowerCase () : converts the string toLowerCase. ToUpperCase () : converts a string toUpperCase characters. Substring () : intercepts a string. Equals () : String comparison.

11. Must abstract classes have abstract methods?

abstract class Cat { public static void sayHi() { System.out.println("hi~"); }}Copy the code

12. What are the differences between ordinary classes and abstract classes?

Ordinary classes cannot contain abstract methods. Abstract classes can contain abstract methods. Abstract classes cannot be instantiated directly; ordinary classes can be instantiated directly.

13. Can abstract classes use final?

What is the difference between an interface and an abstract class?

Implementation: Subclasses of abstract classes inherit using extends; Interfaces must be implemented using IMPLEMENTS. Constructors: Abstract classes can have constructors; No interface exists. Main method: Abstract classes can have main methods and we can run them; An interface cannot have a main method. Number of implementations: A class can implement many interfaces; But only one abstract class can be inherited. Access modifiers: Methods in an interface use the public modifier by default; Methods in abstract classes can be arbitrary access modifiers.

15. How many IO streams are there in Java?

16. What is the difference between BIO, NIO and AIO?

BIO: Block IO Synchronous blocking IO is the traditional IO that we usually use. It is characterized by simple mode and convenient use, and low concurrent processing capability. NIO: New IO synchronous non-blocking IO is an upgrade of traditional IO. The client and server communicate through channels, realizing multiplexing. AIO: Asynchronous IO is an upgrade of NIO, also known as NIO2, which implements Asynchronous non-blocking I/O operations based on events and callbacks.

17. What are the common methods of Files?

The container

18. What are Java containers?

19. What’s the difference between Collections and Collections?

Java.util. Collection is a Collection interface (a top-level interface of the Collection class). It provides generic interface methods for performing basic operations on collection objects. The Collection interface has many concrete implementations in the Java class library. The significance of Collection interface is to provide maximum unified operation mode for various specific collections, and its direct inheritance interfaces are List and Set. Collections is a utility/helper class of the collection class that provides a set of static methods for sorting, searching, and thread-safe operations on the elements in the collection.

20. What is the difference between a List, a Set and a Map?

21. What is the difference between HashMap and Hashtable?

HashMap removes the contains method from HashTable, but adds containsValue () and containsKey () methods. HashTable is synchronous, whereas HashMap is non-synchronous, making hashTable more efficient. A hashMap allows empty key values, whereas a hashTable does not.

22. How do I decide to use HashMap or TreeMap?

23. How does HashMap work?

Overview of HashMap: HashMap is an asynchronous implementation of the Map interface based on hash tables. This implementation provides all optional mapping operations and allows null values and NULL keys. This class does not guarantee the order of the mapping, and in particular it does not guarantee that the order is constant. The data structure of HashMap: In the Java programming language, the most basic structure is two kinds, one is an array, the other is a mock pointer (reference), all data structures can be constructed with these two basic structures, HashMap is no exception. A HashMap is actually a “linked list hash” data structure, which is a combination of arrays and lists. When we put an element into a Hashmap, we first recalculate the hash value based on the hashcode of the key. The hash value is calculated to the element’s location in the array (subscript). If the array already contains other elements at that location, the elements at that location are stored as a linked list, and the new elements are added in the header. The first one goes to the end of the chain. If there is no element at that location in the array, the element is placed directly at that location in the array. In Jdk 1.8, the implementation of HashMap is optimized. When there are more than eight nodes in the list, the list will be converted to a red-black tree to improve query efficiency, from O(n) to O(logn).

24. How does HashSet work?

25. What is the difference between ArrayList and LinkedList?

The most obvious difference is that the underlying data structure of ArrrayList is an array, which supports random access, while the underlying data structure of LinkedList is a two-way circular LinkedList, which does not support random access. Using subscripts to access an element, the time complexity of ArrayList is O(1) and LinkedList is O(n).

26. How to convert an array to a List?

27. What is the difference between an ArrayList and a Vector?

Vector is synchronous, ArrayList is not. However, if you are looking to make changes to the list during iteration, you should use CopyOnWriteArrayList. ArrayList is faster than Vector because it is synchronized and does not overload. ArrayList is more general because we can easily get synchronous and read-only lists using the Collections utility class.

28. What’s the difference between Array and ArrayList?

29. What is the difference between poll() and remove() in a Queue?

Poll () and remove() both fetch an element from the queue, but poll() returns null on failure to fetch an element, and remove() throws an exception on failure.

30. Which collection classes are thread-safe?

31. What is an Iterator?

An iterator is a design pattern that is an object that can iterate over and select objects in a sequence without requiring the developer to understand the underlying structure of that sequence. Iterators are often referred to as “lightweight” objects because they are inexpensive to create.

32. How to use Iterator? What are the characteristics?

33. What is the difference between Iterator and ListIterator?

Iterator can be used to iterate over sets and lists, but ListIterator can only be used to iterate over lists. Iterator can traverse only forward over sets; ListIterator can traverse both forward and backward. ListIterator implements the Iterator interface and includes other functions such as adding elements, replacing elements, retrieving the previous and next element indexes, and so on.

multithreading

What is the difference between parallelism and concurrency?

Parallelism is when two or more events occur at the same time; Concurrency is when two or more events occur at the same time interval. Parallelism is multiple events on different entities, and concurrency is multiple events on the same entity. “Simultaneous” processing of multiple tasks on one processor, multiple tasks on multiple processors. Such as Hadoop distributed cluster.

So the goal of concurrent programming is to make full use of every core of the processor to achieve maximum processing performance.

36. What is the difference between threads and processes?

37. What is a daemon thread?

A daemon thread, or daemon thread, is a service thread, specifically a service thread to another thread.

38. What are the ways to create a thread?

39. What is the difference between runnable and callable?

A bit of a deep question, but also see a Java programmer to learn the breadth of knowledge.

The return value of the run() method in the Runnable interface is void, and all it does is execute the code in the run() method; The Call () method in the Callable interface, which returns a value, is a generic type that can be used in conjunction with Future and FutureTask to retrieve the result of asynchronous execution.

What are the states of threads?

41. What’s the difference between sleep() and wait()?

Sleep () : The method is a static method of Thread class, which makes the calling Thread go to sleep and give the execution opportunity to other threads. After the sleep time ends, the Thread enters the ready state and compets with other threads for CPU execution time. Because sleep() is static, it cannot change the lock of an object. When sleep() is called in a synchronized block, the thread goes to sleep, but the lock is not released, and other threads cannot access the object.

Wait () : Wait () is a method of the Object class. When a thread executes a wait method, it enters the wait pool associated with the Object and releases the lock of the Object so that other threads can access the wait. NotifyAll and notify methods are used to wake up the waiting thread

What is the difference between notify() and notifyAll()?

43. What is the difference between run() and start() for a thread?

Each Thread completes its operations through a method run() corresponding to a particular Thread object, called the Thread body. Start a Thread by calling the start() method of the Thread class.

The start() method is used to start a thread, truly implementing multithreading. Instead of waiting for the body of the run method to finish executing, you can continue with the following code. The thread is in a ready state and is not running. The Thread class then calls the method run() to complete its running state. The method run() is called the Thread body, which contains the contents of the Thread to be executed. The run method ends and the Thread terminates. The CPU then schedules other threads.

The run() method is local to the thread, just a function in the thread, not multithreaded. If you call run() directly, it is just like calling a normal function. If you call run() directly, you have to wait for the run() method to complete before you can execute the following code. Therefore, there is only one execution path, and there is no thread characteristic at all. So use the start() method instead of the run() method for multithreaded execution.

44. What are the ways to create a thread pool?

What are the states of the thread pool?

Thread pools are in five states: Running, ShutDown, Stop, Tidying, and Terminated. Thread pool each state switch block diagram:

46. What is the difference between submit() and execute() methods in the thread pool?

47. How to ensure the safety of multithreading in Java program?

Thread safety is embodied in three aspects:

Atomicity: provides mutually exclusive access and only one thread can operate on data at a time (atomic,synchronized); Visibility: Changes made by one thread to main memory can be seen by other threads in a timely manner (synchronized,volatile); Orderliness: One thread observes the order in which instructions are executed in another thread, and the observation is often disorderly due to instruction reordering (happens-before principle).

48. What is the upgrade principle of multi-threaded lock?

49. What is a deadlock?

A deadlock is a phenomenon in which two or more processes are blocked during execution, either by competing for resources or by communicating with each other, and cannot proceed without external action. The system is said to be in a deadlock state or a deadlock occurs in the system. These processes that are always waiting for each other are called deadlocked processes. An error at the operating system level, it is short for process deadlock. It was first proposed by Dijkstra in 1965 while working on banker algorithms. It is one of the most difficult problems to deal with in computer operating systems and in the whole field of concurrent programming.

50. How do I prevent deadlocks?

51. What is ThreadLocal? What are the usage scenarios?

Thread-local variables are variables that are limited within a thread and are owned by the thread itself and are not shared between multiple threads. Java provides a ThreadLocal class to support thread-local variables as a way to achieve thread-safety. However, be careful when using thread-local variables in a managed environment, such as a Web server, where the life of a worker thread is longer than the life of any application variable. Java applications run the risk of memory leaks if any thread-local variables are not released after work is done.

Talk about the underlying implementation of synchronized.

53. What is the difference between synchronized and volatile?

Volatile essentially tells the JVM that the value of the current variable in the register (working memory) is indeterminate and needs to be read from main memory; Synchronized locks the current variable so that only the current thread can access it and other threads are blocked. Volatile can only be used at the variable level; Synchronized can be used at the variable, method, and class levels. Volatile only enables change visibility of variables, not atomicity. Synchronized can guarantee the change visibility and atomicity of variables. Volatile does not block threads; Synchronized can cause threads to block. Volatile variables are not optimized by the compiler; Variables of the synchronized tag can be optimized by the compiler.

54. What’s the difference between synchronized and Lock?

What is the difference between synchronized and ReentrantLock?

Synchronized is a keyword like if, else, for, and while. ReentrantLock is a class. This is the essential difference between synchronized and while. Since ReentrantLock is a class, it provides more flexible features than synchronized. It can be inherited, can have methods, and can have a variety of class variables. ReentrantLock has more extensibility than synchronized in several aspects:

ReentrantLock allows you to set the wait time for acquiring locks, so deadlocks are avoided. ReentrantLock allows you to obtain information about various locks. ReentrantLock allows you to flexibly implement multiple notifications

Unsafe Lock (ReentrantLock) refers to the Unsafe park method, whereas synchronized refers to the Mark Word object header.

56. How does atomic work?

reflection

57. What is reflection?

58. What is Java serialization? When is serialization needed?

Simply put, it is to store the state of various objects in memory (i.e. instance variables, not methods), and can be saved to read out the state of the object. Although you can save object States in various ways of your own, Java gives you a mechanism for saving object states that should be better than your own, and that is serialization.

When serialization is required:

A) When you want to save the state of an object in memory to a file or database; B) when you want to use a socket to send objects over the network; C) when you want to transfer objects over RMI;

59. What is a dynamic proxy? What are the applications?

60. How to implement dynamic proxy?

You must first define an interface and have an InvocationHandler(to which you pass objects from the class that implements the interface) process the class. There is also a utility class called Proxy(commonly called Proxy because calling newInstance() produces Proxy objects, but it is just a utility class that produces Proxy objects). Using the InvocationHandler, splice the source code of the proxy class, compile it to generate the binary code of the proxy class, use the loader to load, and instantiate the proxy object, and finally return.

Copy the object

61. Why use cloning?

62. How to implement object cloning?

There are two ways:

1). Implement Cloneable interface and override clone() method in Object class; 2). Implement Serializable interface, clone through object serialization and deserialization, can realize the real deep clone, the code is as follows:

import java.io.ByteArrayInputStream; import java.io.ByteArrayOutputStream; import java.io.ObjectInputStream; import java.io.ObjectOutputStream; import java.io.Serializable; public class MyUtil { private MyUtil() { throw new AssertionError(); } @SuppressWarnings("unchecked") public static <T extends Serializable> T clone(T obj) throws Exception { ByteArrayOutputStream bout = new ByteArrayOutputStream(); ObjectOutputStream oos = new ObjectOutputStream(bout); oos.writeObject(obj); ByteArrayInputStream bin = new ByteArrayInputStream(bout.toByteArray()); ObjectInputStream ois = new ObjectInputStream(bin); return (T) ois.readObject(); / / description: Calling ByteArrayInputStream // or the close method of the ByteArrayOutputStream object makes no sense // These two memory-based streams can free resources as soon as the garbage collector cleans up the object, // This is different from releasing external resources (such as file streams)}}Copy the code

Here is the test code:

import java.io.Serializable; /** ** @author nnngu ** / class Person implements Serializable {private static final Long serialVersionUID = -9102017020286042305L; private String name; // private int age; Private Car Car; Public Person(String name, int age, Car Car) {this.name = name; this.age = age; this.car = car; } public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } public Car getCar() { return car; } public void setCar(Car car) { this.car = car; } @Override public String toString() { return "Person [name=" + name + ", age=" + age + ", car=" + car + "]"; @author nnngu ** / class Car implements Serializable {private static final Long serialVersionUID = -5713945027627603702L; private String brand; Private int maxSpeed; Public Car(String brand, int maxSpeed) {this.brand = brand; this.maxSpeed = maxSpeed; } public String getBrand() { return brand; } public void setBrand(String brand) { this.brand = brand; } public int getMaxSpeed() { return maxSpeed; } public void setMaxSpeed(int maxSpeed) { this.maxSpeed = maxSpeed; } @Override public String toString() { return "Car [brand=" + brand + ", maxSpeed=" + maxSpeed + "]"; }} class CloneTest {public static void main(String[] args) {try {Person p1 = new Person(" ", 33, new Car(" "), 300)); Person p2 = MyUtil.clone(p1); P2.getcar ().setbrand ("BYD"); Println (p1); Person (p1); Person (p1); } catch (Exception e) { e.printStackTrace(); }}}Copy the code

Note: based on the realization of serialization and deserialization cloning is not only a deep, more important is through the generic limit, you can check out whether the Object supports serialization to clone, the inspection is done by the compiler, not throw an exception at run time, this is the solution using the Object class is obviously better than the method of clone cloned objects. It’s always better to expose problems at compile time than to leave them to run time.

63. What is the difference between deep copy and shallow copy?

Java Web

64. What is the difference between a JSP and a servlet?

65. What built-in objects does JSP have? What are the effects?

JSP has nine built-in objects:

Request: Encapsulates a client request that contains parameters from a GET or POST request; Response: Encapsulates the server’s response to the client. PageContext: This object is used to get other objects; Session: The object that encapsulates the user’s session; Application: encapsulates the object of the server running environment; Out: Outputs the output stream object that the server responds to. Config: indicates the configuration object of the Web application. Page: The JSP page itself (equivalent to this in Java programs); Exception: An object that encapsulates the exception thrown by the page.

66. What are the four scopes of JSP?

67. What is the difference between session and cookie?

HTTP is a stateless protocol, so when the server needs to record the user status, it needs to use some mechanism to identify the specific user, which is Session. In a typical scenario like a shopping cart, when you click the order button, HTTP is stateless, so you don’t know which user is doing it, so the server creates a special Session for a particular user, identifies that user, and keeps track of that user, so it knows how many books are in the shopping cart. This Session is stored on the server and has a unique identifier. There are many ways to save sessions on the server, including memory, databases, and files. Session transfer should also be considered when clustering. In large websites, there is usually a special cluster of Session servers to store user sessions. In this case, Session information is stored in memory, and some caching services such as Memcached are used to store sessions.

How does the server identify a particular client? This is where Cookie comes in. Each TIME an HTTP request is made, the client sends a Cookie to the server. In fact, most applications use cookies to realize Session tracking. When a Session is created for the first time, the server tells the client in the HTTP protocol that it needs to record a Session ID in the Cookie, and sends this Session ID to the server for each subsequent request. I knew who you were.

Someone asked, what if cookies are disabled on the client’s browser? In this case, session tracking is typically done using a technique called URL rewriting, where each HTTP interaction is followed by a parameter such as SID = XXXXX that the server identifies the user.

In fact, cookies can also be used in some user-friendly scenarios. Imagine you log in to a website once, but the next time you log in, you don’t want to enter your account again. What do you do? This information can be written into the Cookie. When you visit the website, the script of the website page can read this information and automatically fill in the user name for you, which can be convenient for users. That’s how Cookie got its name, to give users a little sweetener.

So, to sum up: Session is a data structure stored on the server to track user status. This data can be stored in clusters, databases, files. Cookie is a mechanism for the client to save user information, which is used to record some user information and also a way to realize the Session.

68. How does session work?

69. Can session be implemented if the client disables cookies?

Cookie and Session are generally considered to be two independent things. Session uses the scheme of maintaining state on the server side, while Cookie uses the scheme of maintaining state on the client side. But why can’t you get sessions if you disable cookies? The Session ID is used to determine the server Session corresponding to the current Session, and the Session ID is transmitted through the Cookie. If the Cookie is disabled, the Session ID is lost and the Session cannot be obtained.

Assume that the user closes the Cookie to use the Session, it can be implemented in the following ways:

Set “session.use_trans_sid = 1” in the php.ini configuration file, or enable the “-enable-trans-sid” option at compile time to enable PHP to automatically pass the session ID across pages. Manually pass the Session ID through the URL and hide the form. Save Session IDS in files, databases, etc., and call them manually during page crossing.

70. What is the difference between Spring MVC and Struts?

71. How do I avoid SQL injection?

This function is called in the JSP to check for package invalid characters in the JSP page judgment code, using regular expressions to filter incoming parameter strings

72. What is an XSS attack and how can it be avoided?

73. What is a CSRF attack and how can it be avoided?

CSRF (Cross-site Request Forgery) is also known as one-click attack or session riding. Generally speaking, the attacker counterfeits the user’s browser request and sends it to the website that the user has authenticated himself to visit, so that the target website receives it and mistakenly thinks it is the user’s real operation and executes the command. Often used to steal accounts, transfer money, send false messages and so on. The attacker takes advantage of the vulnerability of the website to verify the request, the website can confirm that the request is from the user’s browser, but cannot verify whether the request is from the user’s real intention of the operation behavior.

How to avoid:

1. Validate the HTTP Referer field

The Referer field in the HTTP header records the source address of the HTTP request. Typically, a request to access a security-restricted page comes from the same site, and if a hacker wants to carry out a CSRF attack on it, he can only construct the request on his own site. Therefore, you can defend against CSRF attacks by validating the Referer value.

2. Use captcha

Verification codes are added to key operation pages. After receiving a request, the background checks the verification codes to defend against CSRF. But this approach is not very user friendly.

3. Add token to request address and validate

The reason why CSRF attack can be successful is that the hacker can completely forge the user’s request, and all the user authentication information in the request is in the cookie, so the hacker can directly use the user’s own cookie to pass the security authentication without knowing the authentication information. The key to defending against CSRF is to put information in the request that a hacker cannot forge and that does not exist in a cookie. A randomly generated token can be added to the HTTP request as a parameter, and an interceptor can be established on the server side to verify the token. If there is no token in the request or the token content is incorrect, the request may be rejected as a CSRF attack. This is more secure than checking the Referer. Tokens can be generated after the user logs in and placed in the session, and then taken out of the session on each request and compared with the tokens in the request. The difficulty with this approach, however, is how to add tokens to the request as parameters. For GET requests, the token is appended to the request address so that the URL becomes http://url? Csrftoken = tokenvalue. For POST requests, the token is added at the end of the form, adding the token as a parameter to the request.

4. Customize and validate properties in HTTP headers

This method also uses the token and validates it. Unlike the previous method, instead of placing the token in the HTTP request as a parameter, it places it in a custom attribute in the HTTP header. With the XMLHttpRequest class, you can add the CSRFToken HTTP header attribute and put the token value into all of the class requests at once. This eliminates the inconvenience of adding the token to the request in the previous method, and the XMLHttpRequest address is not logged in the browser’s address bar, and the token is not leaked to other sites through the Referer.

abnormal

Throws throws 74.

Throws throws. Throws declares all exception information that a method may throw. Throws declares exceptions but does not process them. A throw is a specific type of exception thrown.

75. What is the difference between final, finally and Finalize?

76. Which part of the try-catch-finally can be omitted?

A: Catch can be omitted

The reason:

More strictly, try is only good for runtime exceptions, and try+catch is good for run-time exceptions + normal exceptions. That is, if you just try a normal exception and don’t catch it, the compilation will fail because the compiler dictates that if you catch a normal exception, you must display the declaration with a catch for further processing. Run-time exceptions are not specified this way at compile time, so a catch can be omitted, and the compiler feels comfortable adding it.

In theory, any code that the compiler looks at is likely to have a problem, so even if you try everything, it’s just a layer on top of what it normally does at runtime. But once you add a try to a piece of code, you explicitly promise the compiler to catch exceptions that the code might throw instead of throwing them up. If it is a normal exception, the compiler requires that it must be caught with a catch for further processing. If there is a runtime exception, catch and discard and +finally sweep, or add a catch catch for further processing.

The addition of finally is a “sweep” processing that takes place whether or not an exception is caught.

77. In try-catch-finally, if a catch returns, finally will be executed?

/* * Java interview question -- Will the code in finally execute if there is a return statement ina catch? */ public class FinallyDemo2 { public static void main(String[] args) { System.out.println(getInt()); } public static int getInt() { int a = 10; try { System.out.println(a / 0); a = 20; } catch (ArithmeticException e) { a = 30; return a; /* * return a return 30 instead of a; The return path forms a *. But it finds finally after it, so it continues with finally, a=40 * goes back to the previous path again, and returns 30. Once the return path is formed, a here is no longer an A variable. The constant 30 */} finally {a = 40; } // return a; }}Copy the code

package com.java_02; /* * Java interview question -- Will the code in finally execute if there is a return statement ina catch? */ public class FinallyDemo2 { public static void main(String[] args) { System.out.println(getInt()); } public static int getInt() { int a = 10; try { System.out.println(a / 0); a = 20; } catch (ArithmeticException e) { a = 30; return a; /* * return a return 30 instead of a; The return path forms a *. But it finds finally after it, so it continues with finally, a=40 * goes back to the previous path again, and returns 30. Once the return path is formed, a here is no longer an A variable. The constant 30 */} finally {a = 40; return a; } // return a; // return a; }}Copy the code

78. What are the common exception classes?

NullPointerException: Thrown when an application attempts to access an empty object. SQLException: An exception that provides information about a database access error or other error. IndexOutOfBoundsException: indicates a sort index (such as an array, a string, or vector) thrown when beyond range. NumberFormatException: Thrown when an application tries to convert a string to a numeric type, but the string cannot be converted to a proper format. FileNotFoundException: Thrown when an attempt to open a file represented by the specified pathname fails. IOException: Throws an I/O exception when it occurs. This class is a generic class for exceptions generated by failed or interrupted I/O operations. ClassCastException: Thrown when an attempt is made to cast an object into a subclass that is not an instance. ArrayStoreException: An exception thrown when attempting to store an object of the wrong type into an array of objects. IllegalArgumentException: An exception thrown to indicate that an invalid or incorrect argument has been passed to a method. ArithmeticException: Throws the exception when the arithmetic condition of the exception is present. For example, an instance of this class is thrown when an integer is “divided by zero”. NegativeArraySizeException: if the application is trying to create an array with negative size, the exception is thrown. NoSuchMethodException: Thrown when a particular method cannot be found. SecurityException: An exception thrown by the security manager to indicate a security violation. UnsupportedOperationException: when does not support the requested operation, throw the exception. RuntimeExceptionRuntimeException: are those that may be in the normal operation of the Java virtual machine exceptions thrown during the period of the superclass.

network

79. What do HTTP response codes 301 and 302 represent? What’s the difference?

A: 301,302 are HTTP state codes that represent a URL transfer.

The difference between:

301 redirect: 301 stands for Permanently Moved. Redirect: 302 stands for Temporarily Moved (Temporarily Moved).

80. What’s the difference between forward and redirect?

81. Describe the differences between TCP and UDP.

TCP connection-oriented (for example, dial up to establish a connection before making a phone call); UDP is connectionless, that is, no connection is required before sending data. TCP provides reliable services. That is to say, data transmitted through the TCP connection is error-free, not lost, not repeated, and in order to arrive; UDP does its best to deliver, i.e. reliable delivery is not guaranteed. Tcp realizes reliable transmission through checksum, retransmission control, serial number identification, sliding window and acknowledgement. For example, when the packet is lost, the sequence control can also be carried out for the subcontracting out of order. UDP has better real-time, higher work efficiency than TCP, suitable for high-speed transmission and real-time communication or broadcast communication. Each TCP connection can be point-to-point only. UDP supports one-to-one, one-to-many, many-to-one and many-to-many interactive communication. TCP requires more system resources, while UDP requires less.

82. Why does TCP require three handshakes? Why is that?

How does TCP sticky packet come into being?

①. Sticky packets are generated by the sender

The client and server that use THE TCP protocol to transmit data often maintain a long connection state (there is no sticky packet for sending data once a connection). The two sides can transmit data continuously when the connection is constantly open. However, when the number of packets sent is too small, TCP by default enables the Nagle algorithm to merge these smaller packets and send them (buffer data sending is a heap pressure process). This merge takes place in the send buffer, which means the data is sent out in a sticky packet state.

②. Sticky packets are generated on the receiver

When the receiver uses TCP protocol to receive data, the process is as follows: data to the receiver, from the bottom of the network model to the transmission layer, the transmission layer TCP protocol processing is to put it into the receiving buffer, and then the application layer to actively obtain (C language recv, read and other functions); The problem is that the data reading function we call in the program can’t take the data out of the buffer in time, and the next data comes and part of it is put into the end of the buffer, and when we read the data, it is a sticky packet. (Data loading speed > data loading speed of the application layer)

84. What are the OSI seven-tier models?

85. What are the differences between GET and POST requests?

GET is harmless when the browser falls back, while POST resubmits the request. The URL generated by GET can be bookmarked, but not by POST. GET requests are actively cached by browsers, whereas POST requests are not, unless set manually. GET requests can only be url encoded, while POST supports multiple encoding methods. GET request parameters are retained in browser history, while parameters in POST are not. GET requests pass parameters in the URL with length limits, whereas POST does not. GET accepts only ASCII characters for the data type of the argument, while POST has no restrictions. GET is less secure than POST because parameters are exposed directly to the URL and therefore cannot be used to pass sensitive information. The GET argument is passed through the URL, and the POST is placed in the Request body.

86. How to achieve cross-domain?

<! -- The following ports are represented by port 10000: domainA 10001: domainB --> <! -- localhost:10000 --> <script> var iframe = document.createElement('iframe'); iframe.style.display = 'none'; Var state = 0; / / prevent page refresh infinite iframe. Onload = function () {if (state = = = 1) {the console. The log (JSON. Parse (iframe. ContentWindow. Name)); / / remove create iframe iframe. ContentWindow. Document. Write (' '); iframe.contentWindow.close(); document.body.removeChild(iframe); } else if(state === 0) { state = 1; // Load complete, point to current domain, // Blocked a frame with origin "http://localhost:10000" from accessing a cross-origin frame. iframe.contentWindow.location = 'http://localhost:10000/proxy.html'; }}; iframe.src = 'http://localhost:10001'; document.body.appendChild(iframe); </script> <! -- localhost:10001 --> <! DOCTYPE html> ... <script> window.name = JSON.stringify({a: 1, b: 2}); </script> </html>Copy the code

<! -- The following ports are represented by port 10000: domainA 10001: domainB --> <! -- localhost:10000 --> <iframe src="http://localhost:10001/msg.html" name="myPostMessage" style="display:none;" > </iframe> <script> function main() { LSsetItem('test', 'Test: ' + new Date()); LSgetItem('test', function(value) { console.log('value: ' + value); }); LSremoveItem('test'); } var callbacks = {}; window.addEventListener('message', function(event) { if (event.source === frames['myPostMessage']) { console.log(event) var data = /^#localStorage#(\d+)(null)? #([\S\s]*)/.exec(event.data); if (data) { if (callbacks[data[1]]) { callbacks[data[1]](data[2] === 'null' ? null : data[3]); } delete callbacks[data[1]]; } } }, false); var domain = '*'; Function LSsetItem(key, value) {var obj = {setItem: key, value: value}; frames['myPostMessage'].postMessage(JSON.stringify(obj), domain); Function LSgetItem(key, callback) {var identifier = new Date().getTime(); var obj = { identifier: identifier, getItem: key }; callbacks[identifier] = callback; frames['myPostMessage'].postMessage(JSON.stringify(obj), domain); } function LSremoveItem(key) {var obj = {removeItem: key}; frames['myPostMessage'].postMessage(JSON.stringify(obj), domain); } </script> <! -- localhost:10001 --> <script> window.addEventListener('message', function(event) { console.log('Receiver debugging', event); if (event.origin == 'http://localhost:10000') { var data = JSON.parse(event.data); if ('setItem' in data) { localStorage.setItem(data.setItem, data.value); } else if ('getItem' in data) { var gotItem = localStorage.getItem(data.getItem); event.source.postMessage( '#localStorage#' + data.identifier + (gotItem === null ? 'null#' : '#' + gotItem), event.origin ); } else if ('removeItem' in data) { localStorage.removeItem(data.removeItem); } } }, false); </script>Copy the code

87. How does JSONP work?

Jsonp (json+padding) is used to dynamically create a script tag, using the SRC attribute of the script tag to retrieve javascript scripts in any domain. Through this feature (or vulnerability), the server does not return the JSON format, but returns a javascript code that calls a function. The call is made in SRC, which is cross-domain.

Design patterns

88. Tell me about a design pattern you are familiar with.

The singleton pattern

To put it simply, there is only one instance of a class in an application, and you can’t go to new because constructors are private and usually get their instances through the getInstance() method.

The return value of getInstance() is a reference to an object, not a new instance, so don’t mistake it for multiple objects. The singleton pattern is also easy to implement, so just look at the demo

public class Singleton {

private static Singleton singleton;

private Singleton() {

}

public static Singleton getInstance() { if (singleton == null) { singleton = new Singleton(); } return singleton; }}

As is my custom, I’m going to comment it all out in case you don’t understand it, but this code is so simple that I didn’t comment it, so if you don’t understand a few lines of code, you can go to sleep and read my blog when you wake up and maybe you’ll understand it.

The above is the most basic method, also known as lazy writing (thread unsafe). Here are a few more singletons:

Lazy writing (thread-safe)

public class Singleton { private static Singleton instance; private Singleton (){} public static synchronized Singleton getInstance() { if (instance == null) { instance = new Singleton(); } return instance; }}Copy the code

Hanhan style

public class Singleton { private static Singleton instance = new Singleton(); private Singleton (){} public static Singleton getInstance() { return instance; }}Copy the code

Static inner class

public class Singleton { private static class SingletonHolder { private static final Singleton INSTANCE = new Singleton(); } private Singleton (){} public static final Singleton getInstance() { return SingletonHolder.INSTANCE; }}Copy the code

The enumeration

public enum Singleton {  
   INSTANCE;  
   public void whateverMethod() {  
   }  
}
Copy the code

This approach, championed by Effective Java author Josh Bloch, not only avoids multithreaded synchronization problems, but also prevents deserialization from recreating new objects, which is a strong barrier. However, since the enum feature was added in 1.5, It feels strange to write in this way. Double check lock

public class Singleton { private volatile static Singleton singleton; private Singleton (){} public static Singleton getSingleton() { if (singleton == null) { synchronized (Singleton.class) { if (singleton == null) { singleton = new Singleton(); } } } return singleton; }}Copy the code

Conclusion: I personally prefer static inner class writing and hungry Chinese writing. In fact, these two methods can handle most cases. There are other ways to write it, but it depends on your business needs.

Observer model

A one-to-many dependency between objects in which all dependent objects are notified and automatically updated when an object’s state changes.

Observer pattern UML diagrams

People who can’t understand the picture come here with a small bench to give you an example: Suppose there are three people, Mei (female, 22), Xiao Wang and Xiao Li. Xiao Mei is very beautiful. Xiao Wang and Xiao Li are two programmers who keep an eye on xiao Mei’s every move. One day, Mei said, “Who will accompany me to play games?” This sentence was heard by Xiao Wang and Xiao Li, the result was very happy, Dally, in a short time, Xiao Wang rushed to the door of Xiao Mei, here, Xiao Mei is the observer, Xiao Wang and Xiao Li are the observers, the observer sent a message, and then the observers process accordingly, see the code:

Public interface Person {void getMessage(String s); public interface Person {void getMessage(String s); }Copy the code

This interface is equivalent to the phone numbers of Xiao Wang and Xiao Li. When Xiao Mei sends a notice, she will call getMessage. Dialing the phone is the call interface

Public class implements Person {private String name = "implements "; Public LaoWang() {} @override public void getMessage(String s) {system.out.println (name + "); }} public implements Person {private String name = "implements "; Public LaoLi() {} @override public void getMessage(String s) {system.out.println (String s); }}Copy the code

The code is very simple, let’s look at mei’s code:

public class XiaoMei { List<Person> list = new ArrayList<Person>(); public XiaoMei(){ } public void addPerson(Person person){ list.add(person); } public void notifyPerson() {for(Person Person :list){notifyPerson() {Person. Whoever comes first can play games with me!" ); }}}Copy the code

Let’s write a test class to see if this is true

public class Test { public static void main(String[] args) { XiaoMei xiao_mei = new XiaoMei(); LaoWang lao_wang = new LaoWang(); LaoLi lao_li = new LaoLi(); // Xiao Wang and Xiao Li registered xiao_mei.addperson (lao_wang); xiao_mei.addPerson(lao_li); // Xiao Mei sends notifications to Xiao Wang and Xiao Li. Xiao_mei.notifyperson (); }}Copy the code

Perfect ~

Decorator pattern

The existing business logic is further encapsulated to add additional functions. For example, THE IO flow in Java uses decorator mode. When users use it, they can assemble it arbitrarily to achieve their desired effect. For example, I want to eat a sandwich. First, I need a big sausage. I like to eat cream. So how do we write the code? First, we need to write a Food class that all other foods inherit from. Look at the code:

public class Food {

   private String food_name;

   public Food() {
   }

   public Food(String food_name) {
       this.food_name = food_name;
   }

   public String make() {
       return food_name;
   };
}
Copy the code

The code is so simple that I won’t explain it, and then we’ll write some subclasses to inherit it:

Public class Bread extends Food {private Food basic_food; public Bread(Food basic_food) { this.basic_food = basic_food; } public String make() {return basic_food. Make ()+"+ bread "; }} public extends Food {private Food basic_food; public Cream(Food basic_food) { this.basic_food = basic_food; } public String make() {return basic_food. Make ()+"+ butter "; }} public class Vegetable extends Food {private Food basic_food; public Vegetable(Food basic_food) { this.basic_food = basic_food; } public String make() {return basic_food. Make ()+"+ vegetable "; }}Copy the code

The constructor passes in a parameter of type Food, and then adds some logic to the make method. If you still don’t understand why this is the case, just look at my Test class

public class Test { public static void main(String[] args) { Food food = new Bread(new Vegetable(new Cream(new The Food (" sausage ")))); System.out.println(food.make()); }}Copy the code

See no one layer encapsulation, away from you, we see: the most I new a sausage inside, outside the sausage I wrapped with a layer of cream, the cream of the outside and I add a layer of vegetables, bread, is the most I put outside is very image, ha ha ~ this design pattern is just like a real life touch, understand? So let’s look at the results

The results

One sandwich is ready

Adapter mode

Connecting two very different things together, like a transformer in real life. Suppose a mobile phone charger needs a voltage of 20V, but the normal voltage is 220V, this time need a transformer, 220V voltage into 20V voltage, so that the transformer will be 20V voltage and mobile phone.

public class Test { public static void main(String[] args) { Phone phone = new Phone(); VoltageAdapter adapter = new VoltageAdapter(); phone.setAdapter(adapter); phone.charge(); Class Phone {public static final int V = 220; // Normal voltage 220v, is a constant private VoltageAdapter adapter; Public void charge() {adapter.changevoltage (); } public void setAdapter(VoltageAdapter adapter) { this.adapter = adapter; Public void changeVoltage() {system.out.println (" charging..."); ); Println (" original voltage: "+ Phone.V + "V"); Println (" voltage after transformer conversion :" + (phle.v-200) + "V"); }}Copy the code

Factory Pattern Simple factory pattern: One abstract interface, implementation classes for multiple abstract interfaces, and one factory class for instantiating the abstract interface

// abstract class Car {public void run(); public void stop(); } // Class Benz implements Car {public void run() {system.out.println ("Benz implements Car... ); } public void stop() {system.out.println ("Benz stopped... ); }} class Ford implements Car {public void run() {system.out.println ("Ford implements Car... ); } public void stop() {system.out.println ("Ford stopped... ); Public static Car getCarInstance(String type) {Car c = null; if ("Benz".equals(type)) { c = new Benz(); } if ("Ford".equals(type)) { c = new Ford(); } return c; } } public class Test { public static void main(String[] args) { Car c = Factory.getCarInstance("Benz"); if (c ! = null) { c.run(); c.stop(); } else {system.out.println (" can't make this car... ") ); }}}Copy the code

Factory method Pattern: There are four roles: Abstract Factory pattern, Concrete factory pattern, Abstract product pattern, and Concrete product pattern. Instead of instantiating a concrete product by a factory class, the product is instantiated by a subclass of the abstract factory

Public interface Moveable {void run(); Public class Plane implements Moveable {@override public void run() {system.out.println (" Plane....") ); } } public class Broom implements Moveable { @Override public void run() { System.out.println("broom....." ); }} Public Abstract Class VehicleFactory {abstract Moveable create(); } public class PlaneFactory extends VehicleFactory {public Moveable create() {return new Plane(); } } public class BroomFactory extends VehicleFactory { public Moveable create() { return new Broom(); Public class Test {public static void main(String[] args) {VehicleFactory factory = new BroomFactory(); Moveable m = factory.create(); m.run(); }}Copy the code

Abstract Factory pattern: Unlike the factory method pattern, where the factory produces a single product, the abstract factory pattern produces multiple products

Public Abstract class AbstractFactory {public Abstract Vehicle createVehicle(); public abstract Weapon createWeapon(); public abstract Food createFood(); } // Concrete factory class, where Food,Vehicle, Weapon are abstract classes,  public class DefaultFactory extends AbstractFactory{ @Override public Food createFood() { return new Apple(); } @Override public Vehicle createVehicle() { return new Car(); } @Override public Weapon createWeapon() { return new AK47(); Public class Test {public static void main(String[] args) {AbstractFactory f = new DefaultFactory(); Vehicle v = f.createVehicle(); v.run(); Weapon w = f.createWeapon(); w.shoot(); Food a = f.createFood(); a.printName(); }}Copy the code

Proxy mode

There are two types, static proxy and dynamic proxy. Let’s talk about static agents. I’m not going to talk about a lot of theoretical stuff, and even if I did, you wouldn’t understand it. What real role, abstract role, proxy role, delegate role… It’s a mess. I can’t read it. When I was learning the agent mode, I went to the Internet and looked up a lot of information. When I opened the links, they were basically for you to analyze what roles there were. There were a lot of theories, which seemed very laborious. Let’s not talk about anything. Let’s talk about real life. (Note: I’m not denying theoretical knowledge here, I just think that sometimes theoretical knowledge is hard to understand, you are here to learn, not to nitpick.) After a certain age, we have to get married, and it is a very troublesome thing to get married (including those who are pushed by their parents). Rich family may look for master of ceremonies to preside over the wedding, appear lively, foreign atmosphere ~ well, now the business of wedding celebration company came, we only need to give money, wedding celebration company can help us arrange a whole set of wedding process. The process looks something like this: the family urge marriage – > the families of men and women both parties agreed to marry the day of the zodiac – > looking for a company of wedding – > at the appointed time for the wedding ceremony – > after marriage Wedding how the company intends to make the wedding program, after finished the wedding wedding company will do, we know nothing… Don’t worry, it’s not a black agent, we just give people the money, they will do the job for us. So, the wedding agency here is the proxy role, now you see what the proxy role is.

See the code implementation:

Public interface ProxyInterface {// Marry (); // Public interface ProxyInterface {// Marry (); Void eat(); void eat(); //void shit(); }Copy the code

Civilized society, acting to have a meal, acting shit what I will not write, social weathering ~ ~ ~ can understand good, we look at the code of the wedding company:

public class WeddingCompany implements ProxyInterface { private ProxyInterface proxyInterface; public WeddingCompany(ProxyInterface proxyInterface) { this.proxyInterface = proxyInterface; } @override public void marry() {system.out.println (" we are married "); System.out.println(" We are making preparations for marriage "); System.out.println("...") ); System.out.println(" Gift purchase...") ); System.out.println("...") ); System.out.println(" I can get married "); proxyInterface.marry(); System.out.println(" After the marriage, we need to do follow-up processing, you can go home, the rest of our company to do "); }}Copy the code

See, the wedding company needs to do a lot of things, let’s look at the marriage family code:

Public class NormalHome implements ProxyInterface{@override public void marry() {system.out.println (" we are married ~ "); }}Copy the code

This has been very obvious, the marriage family only need to get married, and the wedding company to take care of everything, before and after the things are the wedding company to do, I heard that now the wedding company is very profitable, this is the reason, the work is much, can not make money? Let’s look at the test class code:

public class Test { public static void main(String[] args) { ProxyInterface proxyInterface = new WeddingCompany(new NormalHome()); proxyInterface.marry(); }}Copy the code

The running results are as follows:

89. What is the difference between a simple factory and an abstract factory?

Spring / Spring MVC

90. Why use Spring?

Explain what AOP is?

AOP (aspect-oriented Programming, aspect-oriented Programming), can be said to be OOP (Object-oriented Programming, object-oriented Programming) complement and perfect. OOP introduces concepts such as encapsulation, inheritance, and polymorphism to build an object hierarchy that simulates a collection of common behaviors. OOP is helpless when we need to introduce common behavior to discrete objects. That is, OOP allows you to define top-to-bottom relationships, but not left-to-right relationships. For example, the logging function. Logging code tends to be spread horizontally across all object hierarchies, regardless of the core functionality of the object to which it is spread. The same is true for other types of code, such as security, exception handling, and transparency persistence. This scattering of extraneous code is called cross-cutting code, and in OOP design it leads to a lot of code duplication, which is not conducive to reuse of modules.

AOP, on the other hand, uses a technique called “crosscutting” to peel apart the insides of wrapped objects and encapsulate common behavior that affects multiple classes into a reusable module called “Aspect,” or Aspect. The so-called “aspect”, simply speaking, is to encapsulate the logic or responsibility that has nothing to do with business, but is called by business modules together, so as to reduce the repetitive code of the system, reduce the degree of coupling between modules, and facilitate the future operability and maintainability. AOP represents a horizontal relationship, if the “object” is a hollow cylinder, which encapsulates the properties and behavior of the object; Aspect-oriented programming, then, is like a sharp knife, cutting open these hollow cylinders to get information inside them. And the cut surface is the so-called “aspect”. Then, with a masterful hand, it restored the cut surfaces without leaving a trace.

Using “crosscutting” techniques, AOP divides a software system into two parts: core concerns and crosscutting concerns. The main flow of business processing is the core concern, and the less relevant part is the crosscutting concern. One of the things about crosscutting concerns is that they often occur at multiple points of the core concern, and they are basically similar everywhere. Such as permission authentication, logging, and transaction processing. Aop’s role is to separate the various concerns in a system, separating core concerns from crosscutting concerns. As Adam Magee, senior solution architect at Avanade, says, the core idea of AOP is to “separate the business logic in your application from the generic services that support it.”

Explain what ioc is.

93. What are the main modules of Spring?

The Spring framework has so far integrated more than 20 modules. These modules are divided into core containers, data access/integration, Web, AOP (faceted programming), tools, messaging, and test modules as shown below.

What are the common injection methods in Spring?

95. Are beans in Spring thread-safe?

Whether the beans in the Spring container are thread-safe or not, the container itself does not provide a thread-safe policy for beans, so it can be said that the beans in the Spring container do not have thread-safe features, but the specific scope of beans should be combined to study.

96. How many scopes does Spring support for beans?

97. What are the methods for Spring to automate bean assembly?

The Spring container is responsible for creating the beans in the application and coordinating the relationships between these objects by ID. As developers, we need to tell Spring which beans to create and how to assemble them together.

There are two ways to assemble beans in Spring:

Implicit bean discovery mechanisms and auto-assembly are configured for display in Java code or XML

Of course, these methods can also be used together.

What are the ways in which Spring transactions are implemented?

99. What about Transaction isolation in Spring?

The transaction isolation level refers to the degree to which data changes made by one transaction are isolated from those made by another parallel transaction. When multiple transactions access the same data simultaneously, the following problems may occur if the necessary isolation mechanisms are not in place:

Dirty read: a transaction reads uncommitted update data from another transaction. Phantom read: For example, the first transaction modifies the data in a table, such as the modification involves “all rows” in the table. At the same time, the second transaction also modifies the data in the table by inserting a “new row” into the table. Then, as if in an illusion, a user operating on the first transaction will discover that there are still unmodified rows in the table. Unrepeatable reads: For example, if two identical SELECT statements are executed in the same transaction, but no DDL statements are executed in the same transaction, the results are inconsistent. This is called unrepeatable reads.

100. What about the Spring MVC workflow?

101. What components does Spring MVC have?

The core components of Spring MVC:

DispatcherServlet: central Controller that forwards requests to specific control class Controller: Controller that handles requests HandlerMapping: ModelAndView: the data returned by the service layer and the encapsulation class of the view layer ViewResolver: the ViewResolver resolves the specific view Interceptors: Interceptor, which intercepts the request we define and then does the processing

102. What does @requestMapping do?

103. What does @autowired do?

blog.csdn.net/u013257679/article/details/52295106

Spring Boot / Spring Cloud

104. What is Spring Boot?

In the big family of Spring framework, there are many derivative frameworks, such as Spring, SpringMvc framework, etc. The core content of Spring is inversion of control (IOC) and dependency injection (DI). The so-called inversion of control is not a technology, but an idea. Operationally created in a Spring configuration file, dependency injection means that the Spring container provides resources for an object in the application, such as reference objects, constant data, and so on.

SpringBoot is a framework, a new programming specification, his generation simplifies the use of the framework, the so-called simplification refers to the simplification of the Spring many frameworks required a large number of and tedious configuration files, so SpringBoot is a framework to serve the framework, the scope of service is simplified configuration files.

105. Why Spring Boot?

106. What is the Spring Boot core configuration file?

Spring Boot provides two common configuration files:

Properties file YML file

107. What are the types of Spring Boot configuration files? What’s the difference?

108. How can Spring Boot implement hot deployment?

SpringBoot hot deployment can be implemented in two ways:

① Use Spring Loaded

Add the following code to the project:

<build> <plugins> <plugin> <! GroupId </groupId> <artifactId> Spring-boot-maven-plugin </artifactId> <dependencies> <! Spring hot deployment --> <! -- The dependency can't be downloaded here. --> <dependency> <groupId>org. springFramework </groupId> <artifactId>springloaded</artifactId> <version>1.2.6.RELEASE</version> </dependency> </dependencies> </dependencies> </plugins> </build>Copy the code

After adding, you need to use MVN command to run:



Find Edit configurations in your IDEA first and do the following :(click the “+” in the top left corner and then select maven to bring up the right pane. Type in the directive shown in the red underlined area. You can name the directive (here named MvnSpringBootRun).

Click save and the IDEA project will appear in the running part. Click the green arrow to run

(2). Use the spring – the boot – devtools

Add dependencies to project POM files:

<! <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-devtools</artifactId> </dependency>Copy the code

Then: using shift + CTRL + Alt + “/” the shortcut key (IDEA) choose “Registry” and then check the compiler. The automake. Allow. The when. App. Running

109. What is the difference between JPA and Hibernate?

110. What is Spring Cloud?

Literally, Spring Cloud is a framework dedicated to distributed systems and Cloud services.

Spring Cloud is a new member of the entire Spring family and a logical outgrowth of the recent boom in Cloud services.

Spring Cloud provides developers with tools to quickly build common patterns in distributed systems, such as:

Configuration Management service Registration and Discovery Circuit breaker Intelligent Routing Service Invocation Load balancing Micro proxy control bus one-time token Global lock Leadership election Distributed session Cluster status Distributed message……

Using Spring Cloud developers can implement services and applications that implement these patterns right out of the box. These services can run in any environment, including distributed environments, developers’ own laptops, and a variety of hosting platforms.

111. What does the Spring Cloud circuit breaker do?

112. What are the core components of Spring Cloud?

Service discovery — Netflix Eureka

A RESTful service that locates mid-tier services running in an AWS Region. It consists of two components: Eureka server and Eureka client. The Eureka server serves as the service registry server. The Eureka client is a Java client designed to simplify interactions with servers, act as a polling load balancer, and provide failover support for services. Netflix uses a separate client in its production environment that provides weighted load balancing based on traffic, resource utilization, and error status.

2. Customer side load balancing — Netflix Ribbon

The Ribbon provides client-side software load balancing algorithms. The Ribbon client component provides a comprehensive set of configuration options, such as connection timeout, retry, retry algorithm, and more. The Ribbon comes with pluggable, customizable load balancing components.

③ Circuit breaker — Netflix Hystrix

A circuit breaker prevents an application from trying multiple times to perform an operation that is likely to fail, allowing it to continue without waiting for recovery or wasting CPU cycles when it determines that the failure is persistent. Breaker mode also enables the application to detect whether the fault has been resolved. If the problem appears to have been corrected, the application can try to invoke the action.

4. Service gateway — Netflix Zuul

Nginx-like, reverse proxy functionality, but Netflix has added some of its own features to work with other components.

⑤. Distributed configuration — Spring Cloud Config

This is still static and needs to work with The Spring Cloud Bus for dynamic configuration updates.

Hibernate

113. Why use Hibernate?

The code of JDBC access database is encapsulated, which greatly simplifies the tedious repetitive code of data access layer. Hibernate is a mainstream persistence framework based on JDBC and an excellent ORM implementation. Hibernate uses Java reflection rather than bytecode enhancers to achieve transparency. Hibernate performs very well because it is a lightweight framework. The flexibility of mapping is excellent. It supports a variety of relational databases, from one-to-one to many-to-many complex relationships.

114. What is ORM Framework?

115. How to view printed SQL statements in The console in Hibernate?

Reference: blog.csdn.net/Randy_Wang_/article/details/79460306

How many query methods does Hibernate have?

HQL: Hibernate Query Language. Query = session.createQuery(" From Customer where name =?") ); Query. SetParameter (0, "old master "); Query.list(); QBC: Query By Criteria.(Conditional Query) Criteria = session.createcriteria (customer.class); Criteria. Add (Restrictions. Eq ("name", "sister ")); List<Customer> list = criteria.list(); SQL: SQLQuery query = session.createSQLQuery("select * from customer"); List<Object[]> list = query.list(); SQLQuery query = session.createSQLQuery("select * from customer"); query.addEntity(Customer.class); List<Customer> list = query.list();Copy the code

117. Can Hibernate entity classes be defined as final?

It is possible to define Hibernate’s entity classes as final, but this is not a good idea. Because Hibernate uses proxy patterns to improve performance in the case of deferred association, if you define your entity class as final, Hibernate can no longer use proxies because Java doesn’t allow extensions to final classes, thus limiting the means to improve performance. However, if your persistent class implements an interface in which all public methods defined in the entity class are declared, you can avoid the aforementioned adverse consequences.

118. What is the difference between using Integer and int for mapping in Hibernate?

119. How does Hibernate work?

Hibernate works:

Configuration config = new Configuration().configure(); / / read and parse the hibernate. CFG. XML configuration files By hibernate. CFG. The read and parse the XML mapping information through SessionFactory sf = config. BuildSessionFactory (); SessionFactory Session = sf.openSession(); // Open Sesssion Transaction tx = session.beginTransaction(); Tx.mit (); Transation persistent operate (); Session closes SesstionFactory

120. The difference between get() and load()?

121. What about Hibernate’s caching mechanism?

The cache in Hibernate is divided into level 1 cache and level 2 cache.

Level 1 caches are session-level caches that are transaction-wide and built-in and cannot be unloaded. The level 2 cache is the SesionFactory cache and is valid from the start to the end of the application. This parameter is optional. Level-2 cache is disabled by default and must be manually enabled. After the database is saved, a copy is cached in memory. If the database is updated, it needs to be updated synchronously.

What kind of data is appropriate to store in the second-level cache?

Rarely modified data Post last reply time frequently queried data e-commerce site not very important data, allowing occasional concurrent data to occur data constant data that will not be accessed concurrently

Extension: Hibernate’s second-level cache does not support distributed caching by default. Use memcahe, Redis, and other central caches instead of secondary caches.

122. What are the states of Hibernate objects?

What is the difference between getCurrentSession and openSession in Hibernate?

OpenSession is to open a new session object, and a new session is opened each time it is used. If it is used for several times in a row, the obtained session is not the same object, and the close method should be called to close the session after using it.

GetCurrentSession getCurrentSession getCurrentSession getCurrentSession getCurrentSession getCurrentSession getCurrentSession getCurrentSession getCurrentSession getCurrentSession getCurrentSession getCurrentSession getCurrentSession getCurrentSession getCurrentSession So that’s one of the differences from openSession. In a nutshell, getCurrentSession is: if it’s already in use, use the old one, if it’s not, build a new one.

Note: In real development, getCurrentSession is used more often because it is usually the same transaction (i.e. using a database), so openSession is not used in general or openSession is an older set of interfaces.

124. Do Hibernate entity classes have to have parameterless constructors? Why is that?

Mybatis

125. What is the difference between #{} and ${} in mybatis?

126. How many pagination methods are there in Mybatis?

Array pages SQL page interceptor pages RowBounds pages

127. Advantages and disadvantages of Mybatis

128. What is the difference between logical paging and physical paging in Mybatis?

Physical pages are not necessarily faster than logical pages, and logical pages are not necessarily faster than physical pages. Physical paging is always better than logical paging: there is no need to impose pressure on the application side that belongs to the database side, even if there is an advantage in speed, but other performance advantages more than make up for it.

129. Does Mybatis support lazy loading? What is the principle of lazy loading?

What about level 1 cache and level 2 cache in Mybatis?

Level 1 Cache: A HashMap local Cache based on PerpetualCache, with storage scope of Session. When a Session is flushed or closed, all caches in that Session are cleared, enabling Level 1 Cache by default.

Level 2 caches have the same mechanism as Level 1 caches, with PerpetualCache and HashMap storage by default, but with Mapper(Namespace) storage scope and customizable storage source, such as Ehcache. Level 2 caching is not enabled by default. To enable level 2 caching, use the Level 2 cache attribute class to implement the Serializable interface (which can be used to store the state of objects), which can be configured in its mapping file.

If a C/U/D operation is performed on a Session (level 1) or Namespaces (level 2), all caches in the select area will be cleared by default.

131. What are the differences between Mybatis and Hibernate?

132. What executors do Mybatis have?

Mybatis has three basic executors:

SimpleExecutor: Every time an Update or select is performed, a Statement object is opened and the Statement object is closed immediately. ReuseExecutor: Performs update or select, searches for a Statement object using the SQL as its key, uses it if it exists, and creates it if it does not exist. After the Statement object is used, it is stored in the Map for future use. In short, the Statement object is reused. BatchExecutor: Update (no SELECT, JDBC batch does not support SELECT), add all SQL to the batch (addBatch()), and wait for execution (executeBatch()), which caches multiple Statement objects. Each Statement object is addBatch() and waits for the executeBatch() batch to be executed one by one. Same as JDBC batch processing.

133. What is the implementation principle of mybatis paging plug-in?

134. How to write a custom plug-in for Mybatis?

From: blog.csdn.net/qq_30051265/article/details/80266434

Mybatis intercepts four Mybatis objects (Executor, StatementHandler, ParameterHandler, ResultSetHandler) in the following ways: StatementHandler: intercepts the processing of Sql syntax builds ParameterHandler: intercepts the processing of parameters ResultSetHandler: intercepts the processing of result sets

Mybatis custom plug-in must implement the Interceptor interface:

public interface Interceptor {
    Object intercept(Invocation invocation) throws Throwable;
    Object plugin(Object target);
    void setProperties(Properties properties);
}
Copy the code

Plugin method: generate dynamic proxy object according to signatureMap setProperties method: setProperties property custom plug-in demo:

// ExamplePlugin.java @Intercepts({@Signature( type= Executor.class, method = "update", args = {MappedStatement.class,Object.class})}) public class ExamplePlugin implements Interceptor { public Object intercept(Invocation invocation) throws Throwable { Object target = invocation.getTarget(); Method Method = Invocation. GetMethod (); Object[] args = Invocation. GetArgs (); // Method parameters // do something...... Method executes the code block Object result = Invocation.proceed() before intercepting; // do something ....... Method executes a code block after intercepting return result; } public Object plugin(Object target) { return Plugin.wrap(target, this); } public void setProperties(Properties properties) { } }Copy the code

An @intercepts can be configured with multiple @signatures. The parameters in @signature are defined as follows: Type: intercepting class. Method: intercepts the Executor update method. Args: indicates method parameters.

RabbitMQ

135. What are the usage scenarios for RabbitMQ?

①. For asynchronous communication across systems, message queues can be used wherever asynchronous interaction is required. For example, in addition to making phone calls (synchronous), we also need to send text messages and E-mail (asynchronous) communication methods.

②. Coupling between multiple applications, because messages are platform – and language-independent and semantically no longer function calls, is better suited as a loosely coupled interface between multiple applications. Message queue-based coupling does not require both sender and receiver to be online at the same time. In enterprise application integration (EAI), file transfer, shared databases, message queues, and remote procedure calls can all be used as integration methods.

③. For the synchronous variation steps within the application, such as order processing, the front-end application can put the order information into the queue, and the back-end application can obtain the message processing from the queue in turn. A large number of orders at peak time can be backlogged in the queue and processed slowly. Because synchronization usually means blocking, and blocking a large number of threads can degrade computer performance.

(4). The message-driven architecture (EDA), the system is decomposed into the message queue, and news producers and news consumers, a process can according to need to split into several phases (stages), phase connection with queue up, the result of the previous Stage processing into the queue, after a phase get messages from the queue to continue processing.

⑤. Applications need more flexible coupling, such as publish and subscribe, for example, routing rules can be specified.

⑥. Cross-lan, even cross-city communication (CDN industry), such as Beijing machine room and Guangzhou machine room application communication.

136. What are the important roles of RabbitMQ?

137. What are the important components of RabbitMQ?

ConnectionFactory: A manager used in application code to establish a connection between the application and Rabbit. Channel: The Channel used by the message push. Exchange: Used to receive and distribute messages. Queue: Used to store messages from producers. RoutingKey: Used to assign producer data to the switch. BindingKey: Used to bind exchange messages to queues.

138. What is vhost for rabbitMQ?

139. How are rabbitMQ messages sent?

A TCP connection is created between the client and rabbit Server. Once TCP is opened and authenticated (authentication is the username and password you send to the Rabbit server), Your client and RabbitMQ create an AMQP channel. A channel is a virtual connection created over “real” TCP. Amqp commands are sent over the channel and each channel has a unique ID. Subscription queues are all done through this channel.

140. How does RabbitMQ ensure message stability?

141. How can RabbitMQ avoid message loss?

Message Persistence ACK acknowledgement mechanism Sets the cluster mirroring mode message compensation mechanism

142. What are the conditions for successful message persistence?

143. What are the disadvantages of RabbitMQ persistence?

The disadvantage of persistence is that it reduces the throughput of the server by using disk rather than memory storage. SSDS can be used to mitigate throughput issues.

144. How many broadcast types are available for RabbitMQ?

145. How does RabbitMQ implement delayed message queuing?

After the message expires, it enters the dead letter exchange and is forwarded to the delay consumption queue by the exchange to realize the delay function. Use the Rabbitmq-delayed -message-exchange plug-in for delay.

146. What are rabbitMQ clusters for?

147. What are the types of RabbitMQ nodes?

Disk node: Messages are stored to disk. Memory node: Messages are stored in memory. Messages are lost when the server is restarted. The performance is higher than that of disks.

148. What should I pay attention to when setting up rabbitMQ clusters?

149. Is each rabbitMQ node a full copy of the other nodes? Why is that?

No, for two reasons:

Storage space considerations: If each node has a full copy of all queues, the new node does not add storage space, but adds more redundant data. Performance considerations: If every message needs to be fully copied to every cluster node, the new node does not improve the ability to process messages, at best maintaining the same or worse performance as a single node.

150. What happens when the only disk node in the RabbitMQ cluster crashes?

151. Does RabbitMQ have a stop order for cluster nodes?

RabbitMQ requires that the cluster be stopped in the order in which the memory nodes should be shut down first and the disk nodes last. If the order is reversed, messages may be lost.

Kafka

Can Kafka be used independently of ZooKeeper? Why is that?

Kafka cannot be used independently of ZooKeeper because Kafka uses ZooKeeper to manage and coordinate kafka node servers.

153. How many data retention strategies does Kafka have?

154. Kafka sets 7 days and 10 gigabytes of clean data at the same time, and on the fifth day it reaches 10 gigabytes of clean data.

At this point Kafka will perform a data purge, which will be performed regardless of the time and size of the purge.

155. What can cause Kafka to run slowly?

156. What should I pay attention to when using kafka clusters?

It is better not to have more clusters than 7, because the more nodes, the longer the message replication takes and the lower the throughput of the whole group. Set the number of clusters to an odd number because more than half of the failed clusters will become unusable. Set it to an odd number for higher fault tolerance.

Zookeeper

157. What is Zookeeper?

Zookeeper is a distributed, open source distributed application coordination service. It is an open source implementation of Google Chubby and an important component of Hadoop and hbase. It provides consistency services for distributed applications, including configuration and maintenance, domain name service, distributed synchronization, and group service.

158. What functions does ZooKeeper have?

159. How many deployment modes are available for ZooKeeper?

Zookeeper can be deployed in three modes:

Single-node deployment: Runs on one cluster. Cluster deployment: Multiple clusters run. Pseudo-cluster deployment: One cluster starts multiple ZooKeeper instances.

160. How does ZooKeeper synchronize the status of the primary and secondary nodes?

161. Why have a primary node in a cluster?

In a distributed environment, some business logic needs to be executed by only one machine in the cluster, and the results can be shared by other machines. This can greatly reduce double computation and improve performance, so the primary node is needed.

162. There are three servers in the cluster and one node is down. Is ZooKeeper still available?

163. What is the notification mechanism of ZooKeeper?

The client will set up a Watcher event for a ZNode. When the ZNode changes, the client will receive a Notification from ZooKeeper. Then the client can make business changes according to the zNode changes.

MySql

164. What are the three paradigms of databases?

First normal Form: Emphasizes atomicity of columns, that is, each column of a database table is an indivisible atomic data item. Second normal Form: Requires that the attributes of the entity depend entirely on the primary key. Complete dependency means that there cannot be attributes that depend on only part of the primary key. Third normal form: any non-primary property does not depend on any other non-primary property.

Mysql > alter table alter table alter table alter table alter table alter table alter table alter table alter table alter table alter table alter table alter table

166. How do I obtain the current database version?

Use select version() to get the current MySQL database version.

167. What is ACID?

168. What is the difference between char and varchar?

Char (n) : Fixed-length types, such as subscribe char(10). When you type “ABC”, they still take up 10 bytes. The other seven are empty bytes. Chat advantages: high efficiency; Disadvantages: Takes up space; Application scenario: Char is suitable for storing the MD5 value of a password with a fixed length. Varchar (n) : Variable length. The value stored is the number of bytes taken up by each value plus the number of bytes used to record its length. Therefore, it is appropriate to consider Varcahr in terms of space; Char is appropriate in terms of efficiency, but there is a tradeoff between the two.

169. What is the difference between float and double?

170. What is the difference between inner join, left join, right join in mysql?

Inner join keywords: inner join; Left join: left join; Right join: right join.

The inner link is to display the matched associated data; The left link shows all the tables on the left, and the table on the right shows the data that meets the conditions; The right join is just the opposite.

171. How is the mysql index implemented?

172. How to verify that the mysql index meets the requirement?

Use Explain to see how SQL executes queries to see if your index meets your requirements.

Explain syntax: explain select * from table where type=1

173. What about transaction isolation of databases?

174. What is the common mysql engine?

InnoDB engine: The InnoDB engine provides acid transaction support, row-level locking and foreign key constraints, and is designed for database systems with large data volumes. When MySQL is running, InnoDB creates a buffer pool in memory to buffer data and indexes. However, this engine does not support full-text search and is slow to start. It does not save the number of rows in a table. Therefore, when performing select count(*) from table, you need to scan the entire table. Because of the small granularity of locks, write operations do not lock the entire table. Therefore, using locks in scenarios with high concurrency can improve efficiency.

MyIASM engine: the default engine for MySQL, but does not support transactions, row-level locks and foreign keys. Therefore, when inserting and updating statements, that is, when writing, the table needs to be locked, resulting in a loss of efficiency. Unlike InnoDB, MyIASM saves the number of rows in a table, so when you select count(*) from a table, you can read the saved values directly without scanning the entire table. Therefore, MyIASM can be used as the database engine of choice if the tables are being read far more than written, and transaction support is not required.

175. What about row and table locks in mysql?

176. What about optimistic and pessimistic locks?

Optimistic lock: Every time I go to get data, I will not lock it because I think others will not change it, but I will judge whether others have updated the data in the meantime before submitting the update. Pessimistic lock: Every time you try to get data, you assume that someone else will change it, so every time you try to get data, you lock it, so that someone else will block it until the lock is released.

Add a version field to the table, and the value of each successful change increases by 1. In this way, each time you change the version of the database, check whether the version you have is consistent with the current version of the database. If the version is inconsistent, do not change it.

177. What are the methods used to troubleshoot mysql problems?

178. How to optimize mysql performance?

Create an index for the search field. Avoid select * and list the fields you want to query. Vertical split table. Select the correct storage engine

Redis

179. What is redis? What are the usage scenarios?

Redis is an open source, network-enabled, memory-based and persistent logging, key-value database written in ANSI C, and provides multiple language apis.

Redis application scenarios: High concurrent data Read/write Massive data read/write data with high scalability requirements

180. What functions does Redis have?

181. What’s the difference between Redis and memecache?

All of memcached’s values are simple strings, and redis, as an alternative, supports richer data types redis is much faster than Memcached and redis can persist its data

182. Why is Redis single threaded?

183. What is cache penetration? How to solve it?

Cache penetration: The system queries data that does not exist. If the cache does not match, the system queries the data from the database. If no data is found, the system does not write the data into the cache.

Solution: The simplest and most crude way If a query returns empty data (either nonexistent or a system failure), we cache the empty result, but its expiration time is short, no more than five minutes.

184. What data types are supported by Redis?

185. What Java clients does Redis support?

Redisson, Jedis, lettuce, etc. Redisson is officially recommended.

186. What are the differences between Jedis and Redisson?

187. How to ensure the consistency of cache and database data?

Set the cache expiration time appropriately. Update Redis synchronously when adding, changing, and deleting databases. Transaction mechanism can be used to ensure data consistency.

188. How many ways can REDis persist?

189. How does Redis implement distributed lock?

Redis distributed lock is actually in the system to occupy a “pit”, other programs also want to occupy the “pit”, occupy a successful can continue to execute, failed can only give up or later retry.

A trap is usually occupied by a program using the setnx(Set if not exists) instruction.

190. What are the defects of redis distributed locks?

191. How does Redis optimize memory?

Use hashes whenever possible. Hashes use very little memory, so you should abstract your data model into a hash as much as possible.

For example, if you have a user object in your Web system, do not set a separate key for the user’s name, last name, email address, and password. Instead, store all of the user’s information in a hash table.

192. What are the Redis elimination strategies?

193. What are the common performance issues with Redis? How to solve it?

When the primary server writes a memory snapshot, the main process is blocked. When a large number of snapshots are taken, the performance is greatly affected and services are interrupted intermittently. Therefore, do not write a memory snapshot on the primary server. Redis master/slave replication performance problem, in order to master/slave replication speed and connection stability, the master/slave library had better be in the same LAN.

JVM

194. What are the main components of the JVM? And its role?

The Runtime Data Area of the ClassLoader is the Native Interface of the Execution Engine.

What components do: The Java code is converted into bytecode by the ClassLoader, and the bytecode is loaded into memory by the Runtime Data Area. Bytecode files are only a set of instructions for the JVM, and cannot be directly delivered to the underlying operating system for execution. Therefore, a specific command parser Execution Engine is required to translate the bytecode into the underlying system instructions and then send them to the CPU for Execution. In this process, the Native Interface of other languages is called to realize the function of the entire program.

195. What about the JVM runtime data area?

196. What is the difference between stacks?

1. Stack memory stores local variables and heap memory stores entities;
2. Stack memory is updated faster than heap memory because local variables have a short lifetime;
3. Variables stored in stack memory are released as soon as their life cycle ends, while entities stored in heap memory are collected irregularly by the garbage collection mechanism.
4. What are queues and stacks? What’s the difference? Queues and stacks are used to prestore data. Queues allow first-in, first-out retrieval of elements, but there are exceptions where the Deque interface allows retrieval of elements from both ends. A stack is similar to a queue, but it runs last-in, first-out retrieval of elements.

198. What is the parental delegation model?

199. What about the execution of class loading?

Class loading consists of the following five steps:

Load: according to the search path to find the corresponding class file and import; Check: Check the correctness of the loaded class file; Preparation: allocate memory space for static variables in a class; Resolution: The process by which the virtual machine replaces symbolic references in a constant pool with direct references. A symbolic reference is understood as an identifier, whereas a direct reference points directly to an address in memory. Initialization: Performs initialization on static variables and static code blocks.

200. How to determine whether an object can be reclaimed?

201. What reference types are available in Java?

Strong reference Soft reference Weak reference Virtual reference (ghost reference/phantom reference)

202. What garbage collection algorithms does the JVM have?

203. What garbage collectors does the JVM have?

Serial: The original single-threaded Serial garbage collector. Serial Old: An older version of the Serial garbage collector, which is also single-threaded, can be an alternative to the CMS garbage collector. ParNew: is the multithreaded version of Serial. Parallel and ParNew collectors are similarly multithreaded, but Parallel is a throughput first collector that can sacrifice wait time for system throughput. Parallel Old is the older version of Parallel, Parallel uses the copy memory reclamation algorithm, Parallel Old uses the mark-collation memory reclamation algorithm. CMS: A collector whose goal is to achieve minimum pause times, ideal for B/S systems. G1: A GC implementation that takes into account both throughput and pause times and is the default GC option in JDK 9 onwards.

204. What about CMS garbage collector?

205. What are the new generation garbage collectors and old generation garbage collectors? What’s the difference?

Serial, ParNew, Parallel Avenge, Serial Old, Parallel Old, CMS Collector: G1

The new generation garbage collector generally adopts replication algorithm. The advantage of replication algorithm is high efficiency, but the disadvantage is low memory utilization. Old-time recyclers generally use a mark-collation algorithm for garbage collection.

206. Describe briefly how generational garbage collector works.

207. What about JVM tuning tools?

The JDK comes with many monitoring tools, which are located in the BIN directory of the JDK. The two most commonly used view monitoring tools are JConsole and JVisualVM.

Jconsole: For monitoring memory, threads, classes, etc in the JVM Jvisualvm: The JDK comes with a versatile analysis tool, can analyze: memory snapshot, thread snapshot, program deadlock, monitoring memory changes, GC changes, etc.

208. What are the common JVM tuning parameters?