Spring中 bean的生命周期

• 为什么要了解Spring中 bean的生命周期？

有时候我们需要自定义bean的创建过程，因此了解Spring中 bean的生命周期非常重要。

• 二话不说先上图：

• 在谈具体流程之前先看看Spring官方文档中在介绍原型作用域时的一段话：In contrast to the other scopes, Spring does not manage the complete lifecycle of a prototype bean. The container instantiates, configures, and otherwise assembles a prototype object and hands it to the client, with no further record of that prototype instance. Thus, although initialization lifecycle callback methods are called on all objects regardless of scope, in the case of prototypes, configured destruction lifecycle callbacks are not called. The client code must clean up prototype-scoped objects and release expensive resources that the prototype beans hold. To get the Spring container to release resources held by prototype-scoped beans, try using a custom bean post-processor, which holds a reference to beans that need to be cleaned up.

也就是说Spring并不会管理原型作用域对象的完整生命周期，对象在被Spring处理到上图中的“bean可以使用了”之后就脱离了spring监管，不会调用销毁方法(如果配置了)，除非自定义BeanPostProcessor增加想要的销毁逻辑。

• 接下来介绍上图中的每个步骤：

1. Spring对bean进行实例化。
2. Spring将值及bean的引用注入到当前bean对应的属性中。
3. 如果bean实现了BeanNameAware接口，那么Spring将bean的ID传给setBeanName()方法。
4. 如果bean实现了BeanFactoryAware接口，那么Spring将调用setBeanFactory()方法，同时传入BeanFactory容器实例。
5. 如果bean实现了ApplicationContextAware接口，那么Spring将调用setApplicationContext()方法，同时传入bean所在应用上下文的引用。
6. 如果bean实现了BeanPostProcessor接口，那么Spring将调用他们的postProcessBeforeInitialization()方法。
7. 如果bean实现了InitializingBean接口，那么Spring将调用他们的afterPropertiesSet()方法。类似地，如果<bean>中配置了init-method属性（即指定了自定义的初始化方法，该方法可以抛异常但是不能有参数），则还会接着调用该方法。
8. 如果bean实现了BeanPostProcessor接口，那么Spring将调用他们的postProcessAfterInitialization()方法。
9. bean准备就绪，可以使用了，他们将一直驻留与应用上下文，直到应用上下文被销毁。
10. 如果bean实现了DisposableBean接口，那么Spring将调用他的destroy()方法。类似地，如果<bean>中配置了destroy-method属性（即指定了自定义的销毁方法，和步骤7中的init-method方法一样，该方法可以抛异常但是不能有参数），则还会接着调用该方法。

 package com.spring.beans.cycle;

 import org.springframework.beans.BeansException;
 import org.springframework.beans.factory.BeanFactory;
 import org.springframework.beans.factory.BeanFactoryAware;
 import org.springframework.beans.factory.BeanNameAware;
 import org.springframework.beans.factory.DisposableBean;
 import org.springframework.beans.factory.InitializingBean;
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.ApplicationContextAware;

 public class LolService
         implements InitializingBean, DisposableBean, BeanNameAware, BeanFactoryAware, ApplicationContextAware {

     // 实现DisposableBean接口的方法
     @Override
     public void destroy() throws Exception {
         System.out.println("执行DisposableBean接口的destroy方法");
     }

     // 实现InitializingBean接口的方法
     @Override
     public void afterPropertiesSet() throws Exception {
         System.out.println("执行InitializingBean接口的afterPropertiesSet方法");
     }

     // 通过配置文件中<bean>的init-method属性指定的初始化方法
     public void init() {
         System.out.println("init...");
     }

     // 通过配置文件中<bean>的destroy-method属性指定的销毁方法
     public void destroy0() {
         System.out.println("destroy0...");
     }

     @Override
     public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
         System.out.println("执行ApplicationContextAware接口的setApplicationContext()方法: " + applicationContext);
     }

     @Override
     public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
         System.out.println("执行BeanFactoryAware接口的setBeanFactory()方法: " + beanFactory);
     }

     @Override
     public void setBeanName(String name) {
         System.out.println("执行BeanNameAware接口的setBeanName()方法: " + name);
     }

 }

示例bean：LolService

package com.spring.beans.cycle;

import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.BeanPostProcessor;

public class MyBeanPostProcessor implements BeanPostProcessor {

    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        System.out.println("postProcessBeforeInitialization: " + bean + ", " + beanName);
        return bean;
    }

    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        System.out.println("postProcessAfterInitialization: " + bean + ", " + beanName);
        return bean;
    }

}

BeanPostProcessor实现类

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

    <bean id="lolService" class="com.spring.beans.cycle.LolService" init-method="init" destroy-method="destroy0"></bean>

    <bean class="com.spring.beans.cycle.MyBeanPostProcessor"></bean>
</beans>

配置文件

package com.spring.beans.cycle;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public class Main {

    public static void main(String[] args) {
        ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("beans-cycle.xml");
        LolService service = (LolService) context.getBean("lolService");
        context.close();
    }

}

测试类

Dec 23, 2019 6:38:02 PM org.springframework.context.support.ClassPathXmlApplicationContext prepareRefresh
INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@29ca901e: startup date [Mon Dec 23 18:38:02 CST 2019]; root of context hierarchy
Dec 23, 2019 6:38:02 PM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
INFO: Loading XML bean definitions from class path resource [beans-cycle.xml]
执行BeanNameAware接口的setBeanName()方法: lolService
执行BeanFactoryAware接口的setBeanFactory()方法: org.springframework.beans.factory.support.DefaultListableBeanFactory@7e0b85f9: defining beans [lolService,com.spring.beans.cycle.MyBeanPostProcessor#0]; root of factory hierarchy
执行ApplicationContextAware接口的setApplicationContext()方法: org.springframework.context.support.ClassPathXmlApplicationContext@29ca901e: startup date [Mon Dec 23 18:38:02 CST 2019]; root of context hierarchy
postProcessBeforeInitialization: com.spring.beans.cycle.LolService@77847718, lolService
执行InitializingBean接口的afterPropertiesSet方法
init...
postProcessAfterInitialization: com.spring.beans.cycle.LolService@77847718, lolService
Dec 23, 2019 6:38:03 PM org.springframework.context.support.ClassPathXmlApplicationContext doClose
INFO: Closing org.springframework.context.support.ClassPathXmlApplicationContext@29ca901e: startup date [Mon Dec 23 18:38:02 CST 2019]; root of context hierarchy
执行DisposableBean接口的destroy方法
destroy0...

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