preface
Recently in view of the Internet company interview asked knowledge points, summed up the Java programmer interview involves most of the interview questions and answers to share with you, I hope to help you review before the interview and find a good job, but also save you on the Internet to search for information time to learn.
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What is the IOC
It’s not an IOC — it’s an Inversion of Control idea. In Java development, IOC means handing your designed objects over to the container for control, rather than the traditional direct control within your objects. How to understand IOC? The key to a good understanding of IOC is to be clear about “who controls whom, what controls, why there is a reversal (there should be a positive reversal), which aspects of the reversal”. Then let’s have an in-depth analysis:
(1) Who controls who, controls what: traditional Java SE programming, we directly create objects in the object through new, is the program to create dependent objects; IoC has a special container to create these objects, that is, the IoC container controls the creation of objects; Who controls whom? The IoC container, of course, controls the objects; Control what? That is, it mainly controls the acquisition of external resources (not just objects including files, etc.).
(2) Why is the inversion, which aspects of the inversion: there is inversion, there is forward, traditional application is by our own active control in the object to directly obtain the dependent object, that is, forward; In inversion, the container helps create and inject dependent objects. Why reverse? Because the container helps us find and inject the dependent object, the object only passively accepts the dependent object, so it is inversion; What has been reversed? The retrieval of dependent objects has been reversed.
As an illustration, traditional programming diagrams actively create related objects and then combine them:
Once the IOC container is in place, these objects are no longer actively created in the client class, as shown in the figure below:
Next, let’s look at four features of IOC
1. Lazy-init lazy loading
Lazy loading of Bean objects (lazy creation)
The default behavior of the ApplicationContext container is to instantiate all Singleton beans ahead of time when the server is started. Pre-instantiation means that all singleton beans are created and configured by the ApplicationContext instance as part of the initialization process.
1.1 Enabling Lazy Loading in XML Mode:
Lazy-init =”” configures lazy loading of bean objects, true or false. False means immediate loading
< bean id = "lazyResult" class = "com.lagou.edu.pojo.Result" lazy - init = "false" > < / bean > duplicate codeCopy the codeLet’s take a look at lazy-init=”false”, which is loaded immediately:
As you can see, the lazyResult bean already exists after the container is started and before the getBean.
And then we set lazy-init=”true” to true
Then let’s F8 take a step down:
LazyResult was found
1.2 Comments enable lazy loading:
@ Lazy:
1.3 Global configuration — default-lazy-init=””
In the bean root tag:
Application scenarios: (1) Enable lazy loading to improve the container startup and operation performance to a certain extent. (2) Enable lazy loading for beans that are not commonly used, so that they can be loaded when they are occasionally used, and the Bean does not need to occupy resources from the beginning
2. FactoryBean and the BeanFactory
2.1 the BeanFactory
The container’s top-level interface, which defines some of the container’s basic behavior, is responsible for producing and managing a factory of beans, using subinterface types such as ApplicationContext
2.2 FactoryBean
There are two kinds of beans in Spring
Common bean
Factory beans can produce bean instances of a particular type (returned to us), which means we can customize the bean creation process.
The static and instantiation methods of Bean creation are similar to factoryBeans. Factorybeans are used a lot, especially in some components of the Spring framework, as well as other frameworks that integrate with the Spring framework
Public interface FactoryBean<T> {// Return the instance created by the FactoryBean. If isSingleton returns true, Map@Nullable T getObject() throws Exception; // Return the bean type created by FactoryBean @nullable Class<? > getObjectType(); Default Boolean isSingleton() {return true; }} Copy the codeCopy the code2.2.1 Create CompanyFactoryBean, implement FactoryBean interface, and rewrite method:
public class CompanyFactoryBean implements FactoryBean<Company> { private String companyInfo; Public void setCompanyInfo(String companyInfo) {this.panyInfo = companyInfo; this.panyInfo = companyInfo; this.panyInfo = companyInfo; } @override public Company getObject() throws Exception {Company Company Company =new Company(); String[] split = companyInfo.split(","); company.setName(split[0]); company.setAddress(split[1]); company.setScale(Integer.parseInt(split[2])); return company; } @Override public Class<? > getObjectType() {return company.class; } @override public Boolean isSingleton() {return true; }} Copy the codeCopy the codepublic class Company { private String name; private String address; private int scale; // omit the getSet and toString} copy codeCopy the code2.2.2 Configuring beans in XML file
<bean id="companyBean" class="com.lagou.edu.factory.CompanyFactoryBean"> <property name="companyInfo" </property> </bean> Copy codeCopy the code2.2.3 test
@org.junit.Test public void test(){ ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml"); Object companyBean = applicationContext.getBean("companyBean"); System.out.println(companyBean); } Company{name=' hook ', address=' zhongguanvillage ', scale=500Copy the codeAlthough in the XML configuration file configured bean class = “com.lagou.edu.factory.CompanyFactoryBean” but returned to the Company type.
How do I return the CompanyFactoryBean type?
Print the results as follows: com.lagou.edu.factory.CompanyFactoryBean@545b995e
3. Rear processor
Spring provides an extended interface for two types of post-processing beans, BeanPostProcessor and BeanFactoryPostProcessor, which are used differently.
Factory initialization (BeanFactory) — > Bean object
After the BeanFactory is initialized, you can do a few things with the BeanFactoryPostProcessor postprocessing
There are some things you can do with the BeanPostProcessor after the Bean object is instantiated (and not complete for the Bean’s entire life cycle)
Note: An object is not necessarily a Springbean, and a Springbean is definitely an object
3.1 SpringBean life cycle diagram
Print as described above. See if it’s consistent:
// Implement BeanNameAware, BeanFactoryAware, ApplicationContextAware, InitializingBean Public class Result implements BeanNameAware, BeanFactoryAware, ApplicationContextAware, InitializingBean, DisposableBean { private String status; private String message; @override public void setBeanFactory(BeanFactory) throws BeansException { System. The out. Println (" 4. BeanFactoryAware: "+ the beanFactory); } @override public void setBeanName(String name) {system.out.println ("3.BeanNameAware: "+name); } @Override public void setApplicationContext(ApplicationContext applicationContext) throws BeansException { System. The out. Println (" 5. ApplicationContextAware: "+ applicationContext); } @Override public void afterPropertiesSet() throws Exception { System.out.println("7.InitializingBean"); } public void initMethodTest(){ System.out.println("8.initMethod"); } @PostConstruct public void postCoustrcut(){ System.out.println("postCoustrcut"); } @predestroy public void PreDestroy (){system.out.println (" execute before destroy "); } @Override public void destroy() throws Exception { System.out.println("DisposableBean"); }} /** Copy the codeCopy the codeIntercepts all */ @Component public class MyBeanPostProcessor implements BeanPostProcessor {@override public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException { if ("lazyResult".equalsIgnoreCase(beanName)){ System.out.println("MyBeanPostProcessor before"); } return bean; } @Override public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException { if ("lazyResult".equalsIgnoreCase(beanName)){ System.out.println("MyBeanPostProcessor After"); } return bean; }} Copy the codeCopy the code/ / XML configuration file: < bean id = "lazyResult" class = "com.lagou.edu.pojo.Result" init - method = "initMethodTest" > < / bean > duplicate codeCopy the code/ / test: @org.junit.Test public void testBeanLazy(){ ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext("classpath:applicationContext.xml"); Object lazyResult = applicationContext.getBean("lazyResult"); System.out.println(lazyResult); applicationContext.close(); } Duplicate codeCopy the codePrint out:
