Spring原理之Bean生命周期

 
Spring Bean的生命周期是指Bean从创建到销毁的整个过程，大致分为4个阶段：

• Bean的实例化阶段：Spring容器根据BeanDefinition信息，通过反射创建对象；
  
• Bean的初始化阶段：对Spring创建的Bean进行属性填充、Aware接口方法回调、执行Bean的后置处理器、执行初始化方法、AOP等
  
• Bean的完成阶段：将生成的Bean放到单例池（singletonObjects）中
  
• Bean的销毁阶段：Spring 容器关闭前，执行Bean的销毁方法。
  

 
Bean的实例化流程【加载Bean信息】

• 将Bean的信息封装成一个BeanDefinition对象
  

1. --org.springframework.beans.factory.support.BeanDefinitionReaderUtils#createBeanDefinition
3. public static AbstractBeanDefinition createBeanDefinition(
4.       String parentName, String className, ClassLoader classLoader) throws ClassNotFoundException {
5.    GenericBeanDefinition bd = new GenericBeanDefinition();
6.    bd.setParentName(parentName);
7.    if (className != null) {
8.       if (classLoader != null) {
9.          bd.setBeanClass(ClassUtils.forName(className, classLoader));
10.       }
11.       else {
12.          bd.setBeanClassName(className);
13.       }
14.    }
15.    return bd;
16. }

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• 将所有的BeanDefinition存储到BeanDefinitionMap中
  

1. --org.springframework.beans.factory.support.BeanDefinitionReaderUtils#registerBeanDefinition
3. public static void registerBeanDefinition(
4.       BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
5.       throws BeanDefinitionStoreException {
7.    // Register bean definition under primary name.
8.    String beanName = definitionHolder.getBeanName();
9.    registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());
11.    // Register aliases for bean name, if any.
12.    String[] aliases = definitionHolder.getAliases();
13.    if (aliases != null) {
14.       for (String alias : aliases) {
15.          registry.registerAlias(beanName, alias);
16.       }
17.    }
18. }

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• 通过BeanFactory的后置处理器对BeanDefinitionMap进行操作【详见 Spring之后置处理器】
  
• Spring框架(ApplicaitonContext)对BeanDefinitionMap进行遍历，利用反射创建Bean对象
  

1. public void preInstantiateSingletons() throws BeansException {
2.    
3.    List<String> beanNames = new ArrayList<String>(this.beanDefinitionNames);
4.    for (String beanName : beanNames) {
5.             //省略代码
6.       getBean(beanName);
7.    }
8. }

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Bean的初始化阶段【生成Bean】

• 创建Bean的实例
  

1. --org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#doCreateBean
3. protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
4.       throws BeanCreationException {
6.    //实例化Bean
7.    BeanWrapper instanceWrapper = null;
8.    if (mbd.isSingleton()) {
9.       instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
10.    }
11.    if (instanceWrapper == null) {
12.       instanceWrapper = createBeanInstance(beanName, mbd, args);
13.    }
14.    final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
15.    Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
17.     //代码省略
18.         ...
20.    Object exposedObject = bean;
21.    try {
22.       //属性赋值（依赖注入）
23.       populateBean(beanName, mbd, instanceWrapper);
24.       if (exposedObject != null) {
25.          //初始化：包含Aware接口方法回调、BeanPostProcessor接口的回调、初始化方法的调用
26.          exposedObject = initializeBean(beanName, exposedObject, mbd);
27.       }
28.    }
30.    return exposedObject;
31. }

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• 给Bean的属性赋值【依赖注入的方式详见 2. Spring IoC&DI】
  

1. --org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#populateBean
3. protected void populateBean(String beanName, RootBeanDefinition mbd, BeanWrapper bw) {
4.     PropertyValues pvs = mbd.getPropertyValues();
5.     //代码省略
6.     ...
7.     this.applyPropertyValues(beanName, mbd, bw, (PropertyValues)pvs);
8. }

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• Aware接口方法回调
  

1. --org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#invokeAwareMethods
3. private void invokeAwareMethods(String beanName, Object bean) {
4.     if (bean instanceof Aware) {
5.         if (bean instanceof BeanNameAware) {
6.             ((BeanNameAware)bean).setBeanName(beanName);
7.         }
9.         if (bean instanceof BeanClassLoaderAware) {
10.             ((BeanClassLoaderAware)bean).setBeanClassLoader(this.getBeanClassLoader());
11.         }
13.         if (bean instanceof BeanFactoryAware) {
14.             ((BeanFactoryAware)bean).setBeanFactory(this);
15.         }
16.     }
18. }

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• 调用BeanPostProcessor.postProcessBeforeInitialization方法【BeanPostProcessor接口的具体实现详见  Spring之后置处理器】
  
  
  
  • 如果用户没有编写自定义的BeanPostProcessor，则该步骤执行空方法
    
  
  

1. --org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#applyBeanPostProcessorsBeforeInitialization
3. public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName)
4.       throws BeansException {
6.    Object result = existingBean;
7.    for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
8.       result = beanProcessor.postProcessBeforeInitialization(result, beanName);
9.       if (result == null) {
10.          return result;
11.       }
12.    }
13.    return result;
14. }

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• 调用Bean的初始化方法【详见 5. Bean的初始化方式】
  

1. --org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#invokeInitMethods
3. protected void invokeInitMethods(String beanName, final Object bean, RootBeanDefinition mbd)
4.       throws Throwable {
6.     //调用InitializingBean的实现类
7.    boolean isInitializingBean = (bean instanceof InitializingBean);
8.    if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) {
9.       if (System.getSecurityManager() != null) {
10.          try {
11.             AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
12.                @Override
13.                public Object run() throws Exception {
14.                   ((InitializingBean) bean).afterPropertiesSet();
15.                   return null;
16.                }
17.             }, getAccessControlContext());
18.          }
19.          catch (PrivilegedActionException pae) {
20.             throw pae.getException();
21.          }
22.       }
23.       else {
24.          ((InitializingBean) bean).afterPropertiesSet();
25.       }
26.    }
28.     //调用init-method
29.    if (mbd != null) {
30.       String initMethodName = mbd.getInitMethodName();
31.       if (initMethodName != null && !(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&
32.             !mbd.isExternallyManagedInitMethod(initMethodName)) {
33.          invokeCustomInitMethod(beanName, bean, mbd);
34.       }
35.    }
36. }

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• 调用BeanPostProcessor.postProcessAfterInitialization方法【详见  Spring之后置处理器】
  
  
  
  • 如果用户没有编写自定义的BeanPostProcessor，则该步骤执行空方法
    
  
  

1. --org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#applyBeanPostProcessorsAfterInitialization
3. public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
4.       throws BeansException {
6.    Object result = existingBean;
7.    for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
8.       result = beanProcessor.postProcessAfterInitialization(result, beanName);
9.       if (result == null) {
10.          return result;
11.       }
12.    }
13.    return result;
14. }

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Bean的完成阶段【将Bean注入单例池】

• 创建好的Bean对象会存储到名为singletonObjects的Map中
  

1. --org.springframework.beans.factory.support.DefaultSingletonBeanRegistry#addSingleton
3. protected void addSingleton(String beanName, Object singletonObject) {
4.    synchronized (this.singletonObjects) {
5.       this.singletonObjects.put(beanName, (singletonObject != null ? singletonObject : NULL_OBJECT));
6.       this.singletonFactories.remove(beanName);
7.       this.earlySingletonObjects.remove(beanName);
8.       this.registeredSingletons.add(beanName);
9.    }
10. }

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• 获取Bean
  

1. --org.springframework.beans.factory.support.DefaultSingletonBeanRegistry#getSingleton(java.lang.String, boolean)
3. protected Object getSingleton(String beanName, boolean allowEarlyReference) {
4.    Object singletonObject = this.singletonObjects.get(beanName);
5.    if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
6.       synchronized (this.singletonObjects) {
7.          singletonObject = this.earlySingletonObjects.get(beanName);
8.          if (singletonObject == null && allowEarlyReference) {
9.             ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
10.             if (singletonFactory != null) {
11.                singletonObject = singletonFactory.getObject();
12.                this.earlySingletonObjects.put(beanName, singletonObject);
13.                this.singletonFactories.remove(beanName);
14.             }
15.          }
16.       }
17.    }
18.    return (singletonObject != NULL_OBJECT ? singletonObject : null);
19. }

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Bean的销毁阶段

• 调用Bean的销毁方法（在容器关闭的时候调用）
  

1. --org.springframework.context.support.AbstractApplicationContext#destroyBeans
3. protected void destroyBeans() {
4.     this.getBeanFactory().destroySingletons();
5. }

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