BeanPostProcessor详解

转自： https://www.jianshu.com/p/d26e8ec9c077?

BeanPostProcessor也称为Bean后置处理器，它是Spring中定义的接口，在Spring容器的创建过程中（具体为Bean初始化前后）会回调BeanPostProcessor中定义的两个方法。BeanPostProcessor的源码如下

public interface BeanPostProcessor {

    Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException;

    Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException;

}

其中postProcessBeforeInitialization方法会在每一个bean对象的初始化方法调用之前回调；postProcessAfterInitialization方法会在每个bean对象的初始化方法调用之后被回调。具体执行时期可以参考Spring中Bean的生命周期源码探究。

自定义BeanPostProcessor

查看BeanPostProcessor源码，可以看到它两个方法的参数都相同，其中第一个参数Object bean表示当前正在初始化的bean对象。此外两个方法都返回Object类型的实例，返回值既可以是将入参Object bean原封不动的返回出去，也可以对当前bean进行包装再返回。来看看下面的自定义BeanPostProcessor

/**
 * 后置处理器：初始化前后进行处理工作
 * 需要将后置处理器加入到容器中
 */
@Component
public class MyBeanPostProcessor implements BeanPostProcessor {

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

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

}

Spring容器中加入MyBeanPostProcessor之后，针对容器中每个创建的Bean对象（Spring自身创建的Bean和应用程序创建的Bean），都会回调postProcessBeforeInitialization和postProcessAfterInitialization方法。

执行原理

BeanPostProcessor的执行是定义在容器的刷新过程中，容器刷新对象具体的方法为：AbstractApplicationContext.refresh()。
在refresh方法执行的调用栈中会去调用AbstractAutowireCapableBeanFactory.doCreateBean()方法，该方法节选源码如下

protected Object initializeBean(final String beanName, final Object bean, RootBeanDefinition mbd) {

    if (mbd == null || !mbd.isSynthetic()) {
        wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
    }

    invokeInitMethods(beanName, wrappedBean, mbd);

    if (mbd == null || !mbd.isSynthetic()) {
        wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
    }
    return wrappedBean;
}

看到在调用初始化方法前后会分别调用applyBeanPostProcessorsBeforeInitialization()和applyBeanPostProcessorsAfterInitialization()。applyBeanPostProcessorsBeforeInitialization()方法的源码如下

public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName)
        throws BeansException {

    Object result = existingBean;
    //获取所有的BeanPostProcessor进行遍历
    for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
        result = beanProcessor.postProcessBeforeInitialization(result, beanName);
        if (result == null) {
            return result;
        }
    }
    return result;
}

可以看到其逻辑为遍历得到容器中所有的BeanPostProcessor，然后一次执行postProcessBeforeInitialization，一但返回null，就跳出for循环不执行后面的BeanPostProcessor.postProcessorsBeforeInitialization()。也就是说如果返回的是null那么我们通过getBean方法将得不到目标Bean。
applyBeanPostProcessorsBeforeInitialization()方法的逻辑和上面一致，就是将循环执行的beanProcessor.postProcessBeforeInitialization()替换成beanProcessor.postProcessAfterInitialization()
Spring底层的很多功能特性都是借助BeanPostProcessor的子类来实现。

常见BeanPostProcessor分析

下图是debug过程中，ApplicationContext对象中的包含的BeanPostProcessor。具体包含哪些BeanPostProcessor和具体应用程序相关，除了下标3中的MyBeanPostProcessor为自定义的BeanPostProcessor，其余均为Spring自带的BeanPostProcessor。

 

下面来分析一下ApplicationContextAwareProcessor和AutowiredAnnotationProcessor的执行原理。

ApplicationContextAwareProcessor

ApplicationContextAwareProcessor后置处理器的作用是，当应用程序定义的Bean实现ApplicationContextAware接口时注入ApplicationContext对象。

@Component
public class Car implements ApplicationContextAware {

    private ApplicationContext applicationContext;

    public Car(){
        System.out.println("car instance...");
    }

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        System.out.println("ApplicationContextAware...setApplicationContext()");
        this.applicationContext = applicationContext;
    }
}

那Car是如何通过实现ApplicationContextAware接口就能获得ApplicationContext对象呢？答案是通过ApplicationContextAwareProcessor后置处理器来实现，我们来看看ApplicationContextAwareProcessor的源码

class ApplicationContextAwareProcessor implements BeanPostProcessor {

    private final ConfigurableApplicationContext applicationContext;

    private final StringValueResolver embeddedValueResolver;

    /**
     * Create a new ApplicationContextAwareProcessor for the given context.
     */
    public ApplicationContextAwareProcessor(ConfigurableApplicationContext applicationContext) {
        this.applicationContext = applicationContext;
        this.embeddedValueResolver = new EmbeddedValueResolver(applicationContext.getBeanFactory());
    }

    @Override
    public Object postProcessBeforeInitialization(final Object bean, String beanName) throws BeansException {
        AccessControlContext acc = null;
        // 这里bean是Car,它实现了ApplicationContextAware接口
        if (System.getSecurityManager() != null &&
                (bean instanceof EnvironmentAware || bean instanceof EmbeddedValueResolverAware ||
                        bean instanceof ResourceLoaderAware || bean instanceof ApplicationEventPublisherAware ||
                        bean instanceof MessageSourceAware || bean instanceof ApplicationContextAware)) {
            invokeAwareInterfaces(bean);
        }

        return bean;
    }

    private void invokeAwareInterfaces(Object bean) {
        if (bean instanceof Aware) {
            if (bean instanceof EnvironmentAware) {
                ((EnvironmentAware) bean).setEnvironment(this.applicationContext.getEnvironment());
            }
            if (bean instanceof EmbeddedValueResolverAware) {
                ((EmbeddedValueResolverAware) bean).setEmbeddedValueResolver(this.embeddedValueResolver);
            }
            if (bean instanceof ResourceLoaderAware) {
                ((ResourceLoaderAware) bean).setResourceLoader(this.applicationContext);
            }
            if (bean instanceof ApplicationEventPublisherAware) {
                ((ApplicationEventPublisherAware) bean).setApplicationEventPublisher(this.applicationContext);
            }
            if (bean instanceof MessageSourceAware) {
                ((MessageSourceAware) bean).setMessageSource(this.applicationContext);
            }
            if (bean instanceof ApplicationContextAware) {
                // 会执行这里回调car重写的setApplicationContext方法，然后将this.applicationContext注入给Car
                ((ApplicationContextAware) bean).setApplicationContext(this.applicationContext);
            }
        }
    }

    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) {
        return bean;
    }

}

InitDestroyAnnotationBeanPostProcessor

InitDestroyAnnotationBeanPostProcessor后置处理器是用来处理自定义的初始化方法和销毁方法。Spring中提供了3种自定义初始化和销毁方法：

1. 通过@Bean指定init-method和destroy-method属性
2. Bean实现InitializingBean（定义初始化逻辑），DisposableBean（定义销毁逻辑）;
3. @PostConstruct：在bean创建完成并且属性赋值完成；来执行初始化方法@PreDestroy：在容器销毁bean之前通知我们进行清理工作

InitDestroyAnnotationBeanPostProcessor的作用就是让第3种方式生效。先看看如何使用@PostConstruct和@PreDestroy注解。

@Component
public class Car {

    public Car(){
        System.out.println("car instance...");
    }

    /**
     * 自定义的初始化方法
     */
    @PostConstruct
    public void init(){
        System.out.println("car ... init...");
    }

    /**
     * 自定义的销毁方法
     */
    @PreDestroy
    public void detory(){
        System.out.println("car ... detory...");
    }
}

InitDestroyAnnotationBeanPostProcessor会在Bean创建的时候通过反射的方式查找包含@PostConstruct和@PreDestroy注解的方法，然后再通过反射执行方法。我们来看看InitDestroyAnnotationBeanPostProcessor.postProcessBeforeInitialization()的源码

public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
    // 获取bean的metadata
    LifecycleMetadata metadata = findLifecycleMetadata(bean.getClass());
    try {
        // 执行@PostConstruct指定的init方法
        metadata.invokeInitMethods(bean, beanName);
    }
    catch (InvocationTargetException ex) {
        throw new BeanCreationException(beanName, "Invocation of init method failed", ex.getTargetException());
    }
    catch (Throwable ex) {
        throw new BeanCreationException(beanName, "Failed to invoke init method", ex);
    }
    return bean;
}

metadata中已经解析出initMethods和destroyMethods，其具体内容如下。

 

metadata.invokeInitMethods(bean, beanName);就是根据反射执行init方法。

InstantiationAwareBeanPostProcessor

InstantiationAwareBeanPostProcessor是BeanPostProcessor的子接口。

public interface InstantiationAwareBeanPostProcessor extends BeanPostProcessor {

    //实例化Bean之前调用
    Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException;

    //实例化Bean之后调用
    boolean postProcessAfterInstantiation(Object bean, String beanName) throws BeansException;

    //设置Bean对象中的某个属性时调用
    PropertyValues postProcessPropertyValues(
            PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeansException;

}

这两个接口的功能基本类似，不过要注意InstantiationAwareBeanPostProcessor和BeanPostProcessor的方法名区别。InstantiationAwareBeanPostProcessor中是Instantiation， 而BeanPostProcessor是Initialization。