First, reflection application
1. The concept of ortho
This means that we know the definition of the class and the name of the method in the class. We create the object directly and then call the method through the object. For example: Apple Apple = new Apple(); // Direct initialization, "ortho" apple.setprice (15);Copy the code2. The concept of reflection
I don't know what class object I'm initializing is, so I can't use the new keyword to create an object. I need to use the reflection API provided by the JDK to make reflection calls. You need to create objects from the class's path string and invoke methods from the method name string. For example: public class Apple {private int price; public int getPrice() { return price; } public void setPrice(int price) { this.price = price; } public static void main(String[] args) throws Exception{// Normal call // Apple Apple = new Apple(); // create an object // apple.setprice (15); // System.out.println(apple.getprice ()); Class CLZ = class.forname (" com.jbrh.api.apple "); Object appleObj = clz.newinstance (); // Constructor appleConstructor = clz.getconstructor (); // Object appleObj = appleConstructor.newInstance(); Method setPriceMethod = clz.getMethod("setPrice", int.class); // Invoke setPricemethod. invoke(appleObj, 15); GetPriceMethod = clz.getMethod("getPrice"); Println (getPricemethod. invoke(appleObj)); // Invoke the getPrice method system.out.println (getPriceMethod. }}Copy the code3. Reflection mechanism
Java source files go through three phases from creation to execution: 1) source phase 2) Class object phase 3) runtime phaseCopy the code
4. Why learn reflex
If you create class B beforehand, you can just create object B and call its methods, so if I develop my program separately from another guy, who wrote this class B, and we develop it completely independently, I need to create object B in my program, and then call method B to get the student's age, This time maybe the old iron has not finished the development of class B, how to do? This is used to reflect, I only need to agree with him when developing the information of class B, such as class name, field name, method name, I can through the mechanism of reflection when he has not written class B dynamically create B object and call its method. After his class B is created and our code merge deployment is running, I can call B's methods.Copy the code5. Pros and cons of reflection
1, advantages: at run time to obtain the various contents of the class, decompilation, for Java such compile and run language, it is very convenient to create flexible code, these code can be assembled at run time, without the link between the components of the source code, easier to achieve object-oriented. 2. Disadvantages: (1) Reflection consumes system resources, so if you don't need to create an object dynamically, you don't need to use reflection; (2) Reflection can ignore permission checking when calling methods, which may break encapsulation and cause security problems.Copy the codeClass objects and class objects
1) A class object is the product of class loading, which encapsulates all the information of a class (class name, parent class, interface, attribute, method, constructor). 2) An object of a class is an object based on a new class, also known as an instance objectCopy the code
7. Three ways to get class objects
Person = new Person(); Class cla = person.getClass(); 2) Obtain a Class object by its Class name Class cla = Class name.class; Cla = class.forname (" Class name "); cla = class.forname (" Class name "); Scenario: Used to read configuration files and define class names in the configuration files. Read the file and load the class object.Copy the codeClass object
1. Get member variables
package com.mylifes1110.java.bean; import java.lang.reflect.Field; class Person { private String name; public Integer age; protected Double score; @Override public String toString() { return "Person{" + "name='" + name + '\'' + ", age=" + age + ", score=" + score + '}'; } } public class TestReflectFiled { public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException { Person person = new Person(); Class<? extends Person> personClass = person.getClass(); Field[] fields = personClass.getFields(); for (Field field : fields) { System.out.println(field); / / public Java. Lang. Integer com. Mylifes1110. Java beans. Person. Age} / / access to public member variables of modified specified name Field age = personClass.getField("age"); System.out.println(age); / / public Java. Lang. Integer com. Mylifes1110. Java beans. Person. Age / / get member variable age value Object ageValue = age. Get (Person); System.out.println(ageValue); //null // Set the age value age.set(person, 18); System.out.println(person); / / Person {name = 'null', the age = 18, score = null} / / get all the member variables, not limited by the modifier Field [] declaredFields = personClass. GetDeclaredFields (); for (Field declaredField : declaredFields) { System.out.println(declaredField); } /* private java.lang.String com.mylifes1110.java.bean.Person.name public java.lang.Integer Com. Mylifes1110. Java beans. Person. Age protected Java. Lang. Double com. Mylifes1110. Java beans. Person. Score * / / / get a member of the specified name variable, Not limited by the modifier Field name = personClass. GetDeclaredField (" name "); // Ignore the security check for access modifiers name.setaccessible (true); Object nameValue = name.get(person); System.out.println(nameValue); //null } }Copy the code2. Get the constructor
package com.mylifes1110.java.bean; import java.lang.reflect.Constructor; import java.lang.reflect.InvocationTargetException; class Person { private String name; public Integer age; protected Double score; public Person() {} public Person(String name, Integer age, Double score) { this.name = name; this.age = age; this.score = score; } @Override public String toString() { return "Person{" + "name='" + name + '\'' + ", age=" + age + ", score=" + score + '}'; } } public class TestReflectConstructor { public static void main(String[] args) throws IllegalAccessException, NoSuchMethodException, InvocationTargetException, InstantiationException { Person person = new Person(); Class<? extends Person> personClass = person.getClass(); Public Constructor<? extends Person> constructor = personClass.getConstructor(); System.out.println(constructor); . / / public com. Mylifes1110. Java bean. The Person (s)/Person/create an instance using no arguments construct object p1 = constructor. NewInstance (); System.out.println(p1); //Person{name='null', age=null, score=null} Constructor<? extends Person> constructor2 = personClass.getConstructor(String.class, Integer.class, Double.class); System.out.println(constructor2); //public com.mylifes1110.java.bean.Person(java.lang.String,java.lang.Integer,java.lang.Double) Person p2 = Constructor2. NewInstance (" Ziph, "18, 100.00); System.out.println(p2); Person{name='Ziph', age=18, score=100.0} Constructor<? >[] constructors = personClass.getConstructors(); for (Constructor<? > constructor1 : constructors) { System.out.println(constructor1); } /* public com.mylifes1110.java.bean.Person() public com.mylifes1110.java.bean.Person(java.lang.String,java.lang.Integer,java.lang.Double) */ } }Copy the code3. Acquisition method
package com.mylifes1110.java.bean; import java.lang.reflect.InvocationTargetException; import java.lang.reflect.Method; class Person { private String name; public Integer age; protected Double score; Public void jump() {system.out.println (" Jump." ); } public void run(Integer meters) {system.out.println (" I ran today "+ meters +" meters ". ); } @Override public String toString() { return "Person{" + "name='" + name + '\'' + ", age=" + age + ", score=" + score + '}'; } } public class TestReflectMethod { public static void main(String[] args) throws IllegalAccessException, NoSuchMethodException, InvocationTargetException { Person person = new Person(); Class<? extends Person> personClass = person.getClass(); Jump = personclass.getMethod ("jump"); Jump. Invoke (person); // Jump up. Method run = personClass.getMethod("run", integer.class); // Invoke run.invoke(person, 2000); // I ran 2000 meters today. [] methods = personClass.getmethods (); for (Method method : methods) { System.out.println(method); System.out.println(method.getName()); } /* public void com.mylifes1110.java.bean.Person.run(java.lang.Integer) run public java.lang.String com.mylifes1110.java.bean.Person.toString() toString public void com.mylifes1110.java.bean.Person.jump() jump public final void java.lang.Object.wait() throws java.lang.InterruptedException wait public final void java.lang.Object.wait(long,int) throws java.lang.InterruptedException wait public final native void java.lang.Object.wait(long) throws java.lang.InterruptedException wait public boolean java.lang.Object.equals(java.lang.Object) equals public native int java.lang.Object.hashCode() hashCode public final native java.lang.Class java.lang.Object.getClass() getClass public final native void java.lang.Object.notify() notify public final native void java.lang.Object.notifyAll() notifyAll */ } }Copy the code4. Get the class name
public class TestReflectClassName { public static void main(String[] args) { Person person = new Person(); Class<? extends Person> personClass = person.getClass(); String className = personClass.getName(); System.out.println(className); //com.mylifes1110.java.bean.Person } }Copy the code