Spring IOC - Bean的初始化

        在bean的初始化阶段，bean已经被实例化及属性填充了，此时的bean已相对成熟，接下来的初始化阶段还会做一些额外的工作对bean做进一步处理，主要包括以下四个方面：

1. 调用aware接口方法

2. 初始化前：调用初始化前的后置处理器方法

3. 初始化：调用InitializingBean接口的afterPropertiesSet方法

4. 初始化后：调用初始化后的后置处理器方法

1. Bean初始化主流程

        AbstractAutowireCapableBeanFactory#initializeBean

        运用aware回调接口、afterPropertiesSet方法、后置处理器完成bean实例的初始化

// 运用aware回调接口、afterPropertiesSet方法、后置处理器完成bean实例的初始化
protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) {if (System.getSecurityManager() != null) {AccessController.doPrivileged((PrivilegedAction<Object>) () -> {invokeAwareMethods(beanName, bean);return null;}, getAccessControlContext());}else {// 1.调用aware接口的方法invokeAwareMethods(beanName, bean);}Object wrappedBean = bean;if (mbd == null || !mbd.isSynthetic()) {// 2.调用初始化前后置处理器wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);}try {// 3.调用初始化方法，其实就是afterPropertiesSet方法invokeInitMethods(beanName, wrappedBean, mbd);}catch (Throwable ex) {throw new BeanCreationException((mbd != null ? mbd.getResourceDescription() : null),beanName, "Invocation of init method failed", ex);}if (mbd == null || !mbd.isSynthetic()) {// 4.调用初始化后后置处理器wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);}return wrappedBean;
}

2. Aware接口调用

        如果Bean实现了BeanNameAware、BeanClassLoaderAware、BeanFactoryAware接口，则会调用相应接口的方法。

• BeanNameAware：在Bean实例化后进行扩展操作，可以获取Bean的名称
• BeanClassLoaderAware：在Bean实例化后进行扩展操作，可以获取Bean的类加载器
• BeanFactoryAware：在Bean实例化后进行扩展操作，可以获取Bean所属的BeanFactory

private void invokeAwareMethods(final String beanName, final Object bean) {if (bean instanceof Aware) {if (bean instanceof BeanNameAware) {((BeanNameAware) bean).setBeanName(beanName);}if (bean instanceof BeanClassLoaderAware) {ClassLoader bcl = getBeanClassLoader();if (bcl != null) {((BeanClassLoaderAware) bean).setBeanClassLoader(bcl);}}if (bean instanceof BeanFactoryAware) {((BeanFactoryAware) bean).setBeanFactory(AbstractAutowireCapableBeanFactory.this);}}
}

3. 初始化前：调用初始化前的后置处理器方法

        这里涉及到了Bean后置处理器：BeanPostProcessor，其作用如下：

1. spring会自动从它的所有的bean定义中检测BeanPostProcessor类型的bean定义，然后实例化它们，再将它们应用于随后创建的每一个bean实例

2. 在bean实例的初始化方法回调之前调用BeanPostProcessor的postProcessBeforeInitialization的方法

3. 在bean实例的初始化方法回调之后调用BeanPostProcessor的postProcessAfterInitialization的方法

4. 通常，通过标记接口等方式填充bean的后置处理器会实现postProcessBeforeInitialization方法，而将bean包装为代理的后置处理器会实现postProcessAfterInitialization方法

public interface BeanPostProcessor {@Nullabledefault Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {return bean;}@Nullabledefault Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {return bean;}
}

@Override
public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName)throws BeansException {Object result = existingBean;for (BeanPostProcessor processor : getBeanPostProcessors()) {Object current = processor.postProcessBeforeInitialization(result, beanName);if (current == null) {return result;}result = current;}return result;
}

        例如ApplicationContextAwareProcessor后置处理器，其重写了postProcessBeforeInitialization方法，如果bean实现了EnvironmentAware、EmbeddedValueResolverAware、ResourceLoaderAware、ApplicationEventPublisherAware、MessageSourceAware、ApplicationContextAware接口，则会调用响应接口的方法

• EnvironmentAware：在Bean实例化后进行扩展操作，可以获取Environment实例
• EmbeddedValueResolverAware：在Bean实例化后进行扩展操作，可以获取StringValueResolver实例，可用于解析占位符
• ResourceLoaderAware：在Bean实例化后进行扩展操作，可以获取ResourceLoader实例
• ApplicationEventPublisherAware：在Bean实例化后进行扩展操作，可以获取ApplicationEventPublisher实例
• MessageSourceAware：在Bean实例化后进行扩展操作，可以获取MessageSource实例
• ApplicationContextAware：在Bean实例化后进行扩展操作，可以获取ApplicationContext实例

4. 初始化：调用InitializingBean接口的afterPropertiesSet方法或自定义的init方法

protected void invokeInitMethods(String beanName, final Object bean, @Nullable RootBeanDefinition mbd)throws Throwable {boolean isInitializingBean = (bean instanceof InitializingBean);if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) {if (logger.isTraceEnabled()) {logger.trace("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");}if (System.getSecurityManager() != null) {try {AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () -> {((InitializingBean) bean).afterPropertiesSet();return null;}, getAccessControlContext());}catch (PrivilegedActionException pae) {throw pae.getException();}}else {// 调用afterPropertiesSet方法((InitializingBean) bean).afterPropertiesSet();}}if (mbd != null && bean.getClass() != NullBean.class) {String initMethodName = mbd.getInitMethodName();if (StringUtils.hasLength(initMethodName) &&!(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&!mbd.isExternallyManagedInitMethod(initMethodName)) {// 调用自定义的init方法invokeCustomInitMethod(beanName, bean, mbd);}}
}

5. 初始化后：调用初始化后的后置处理器方法

@Override
public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)throws BeansException {Object result = existingBean;for (BeanPostProcessor processor : getBeanPostProcessors()) {Object current = processor.postProcessAfterInitialization(result, beanName);if (current == null) {return result;}result = current;}return result;
}