In-depth analysis and source code analysis of Spring Bean scope and life cycle in Java

Bean scope and life cycle

Bean scope

The scope of a Bean refers to a certain behavior mode of the Bean in the entire Spring framework.

For example, the singleton scope means that there is only one copy of the Bean in the entire Spring, and it is globally shared. Then when someone else modifies the value, what the other person reads is the modified value.

Bean scope classification

• singleton: singleton scope (default scope)

• ##prototype: prototype Scope (multiple instance scope)

• request: request scope

• session: reply scope

• application: Global scope

• websocket: HTTP

# Note# The last four states are values ​​in Spring MVC. In ordinary Spring projects, there are only the first two.

singleton

Bean is in singleton state (singleton) by default, that is, everyone uses For the same object, when we learned the singleton mode before, we all knew that using a singleton can greatly improve performance, so the scope of the Bean in Spring is also the singleton mode by default.

• Description: There is only one instance of the Bean under this scope in the IoC container: obtaining the Bean (that is, obtaining it through applicationContext.getBean and other methods) and assembling the Bean (that is, Injected through @Autowired) are all the same object.

• Scenario: Usually stateless beans use this scope. (Stateless means that the property status of the Bean object does not need to be updated)

prototype

• Description: Each request for a Bean under this scope will create a new instance: Get the Bean (that is, through applicationContext. GetBean and other methods) and assembly beans (that is, injected through @Autowired) are new object instances.

• Scenario: Usually stateful beans use this scope.

request

• Description: Each http request will create a new Bean instance, similar to prototype

• Scenario: An http request and response shared Bean

• Remarks: Limited to use in SpringMVC

#session

• Description: In an http session, define a Bean instance

• Scenario: Shared Bean for user response, for example: recording a user’s login information

• Note: limited to use in SpringMVC

application (understand)

• Description: In an http servlet Context, define a Bean instance

• Scenario: Context information of Web application, for example: recording shared information of an application

• Remarks: Limited to use in SpringMVC

websocket (understand)

• Description: In the life cycle of an HTTP WebSocket, define a Bean instance

• Scenario: In each session of WebSocket, the header information of a Map structure is saved, which will be used to wrap the client message header. After the first initialization, it is the same Bean until the end of WebSocket.

• Note: Limit the use of Spring WebSocket

Singleton scope (singleton) VS global scope (application)

• singleton is the scope of Spring Core, and application is the scope of Spring Web.

• singleton acts on the container of IoC, application Acts on Servlet container

Bean scope settings

@Scope tag can modify either method or class, and @Scope has two settings Method

Use enumeration settings:

@Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)

@Component
public class UserBeans {
  	@Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)
    @Bean
    public User user1() {
        User user = new User();
        user.setId(1);
        user.setName("Gujiu");
        user.setPassword("123456");
        return user;
    }
}

Set the value directly:

@Scope("prototype" )

@Component
public class UserBeans {
    @Scope("prototype")
    @Bean
    public User user1() {
        User user = new User();
        user.setId(1);
        user.setName("Gujiu");
        user.setPassword("123456");
        return user;
    }
}

Spring execution process

• Start container (start project)

• Read configuration file (initialization )

• Use

  xml to register Bean directly

• Configure

  Bean root (scan ) path

• Store the Bean in

  Spring: scan and assemble through class annotations

• will

  Bean Assemble into the required class (get the operation)

Bean life cycle

The so-called life cycle refers to a The entire life process of an object from birth to destruction, we call this process the life cycle of an object.

Life cycle of Bean:

• Instantiate Bean (corresponds to "loading" in JVM, starting from scratch, converting bytecode into memory The objects in the object are just allocated memory) [Buy a rough house]

• Set properties (Bean injection and assembly) [Purchase decoration materials (introducing external resources)]

• Bean Initialization[House Decoration]

• 实现了各种 Aware 通知的方法，如 BeanNameAware、BeanFactoryAware、ApplicationContextAware 的接口方法 [打电话给各个装修的师傅]

• 执行 BeanPostProcessor 初始化前置方法 [师傅勘察环境, 指定装修方案 (前置工作)]

• 执行 @PostConstruct 初始化方法，依赖注入操作之后被执行 [两类装修师傅进行装修]

• 执行自己指定的 init-method 方法 (如果有指定的话) [两类装修师傅进行装修]

• 执行 BeanPostProcessor 初始化后置方法 [装修之后的清理工作]

• 使用 Bean [房子可以入住使用了]

• 销毁 Bean [卖掉房子]

生命周期演示

BeanLifeComponent 类:

//@Component
public class BeanLifeComponent implements BeanNameAware {

    @Override
    public void setBeanName(String s) {
        System.out.println("执行了通知");
    }

    @PostConstruct
    public void postConstruct() {
        System.out.println("执行了 @PostConstruct");
    }

    public void init() {
        System.out.println("执行了 init-method");
    }

    @PreDestroy
    public void preDestroy() {
        System.out.println("执行了销毁方法");
    }
}

xml 配置:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:content="http://www.springframework.org/schema/context"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context https://www.springframework.org/schema/context/spring-context.xsd">

    <bean id="myComponent" class="com.demo.component.BeanLifeComponent"
          init-method="init" ></bean>
</beans>

调用类:

public class App2 {
    public static void main(String[] args) {
        ClassPathXmlApplicationContext applicationContext =
                new ClassPathXmlApplicationContext("spring-config.xml");
        BeanLifeComponent beanLifeComponent =
                applicationContext.getBean("myComponent", BeanLifeComponent.class);
        System.out.println("使用Bean");
        applicationContext.destroy();
    }
}

运行结果展示:

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