Spring扩展点之BeanPostProcessor

前言

BeanPostProcessor接口是Spring中一个非常重要的接口，它的接口定义如下

public interface BeanPostProcessor {

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

当你实现了这个接口的时候，Spring会保证在每一个bean对象初始化方法调用之前调用postProcessBeforeInitialization方法，在初始化方法调用之后调用postProcessAfterInitialization

BeanPostProcessor的注册

看过我之前写的IOC源码分析系列文章的同学应该对这个都比较有印象

Spring在执行到这的时候会把所有实现BeanPostProcessor接口的实现类都注册到BeanFactory中，一起来看一下实现的细节

    protected void registerBeanPostProcessors(ConfigurableListableBeanFactory beanFactory) {
        PostProcessorRegistrationDelegate.registerBeanPostProcessors(beanFactory, this);
    }
public static void registerBeanPostProcessors(
            ConfigurableListableBeanFactory beanFactory, AbstractApplicationContext applicationContext) {
        //获取所有BeanPostProcessor的实现类
        String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);


        int beanProcessorTargetCount = beanFactory.getBeanPostProcessorCount() + 1 + postProcessorNames.length;
        beanFactory.addBeanPostProcessor(new BeanPostProcessorChecker(beanFactory, beanProcessorTargetCount));

          // 这里把实现PriorityOrdered接口,Ordered 接口的BeanPostProcessors 和其他类型的BeanPostProcessors 区分开
        List<BeanPostProcessor> priorityOrderedPostProcessors = new ArrayList<>();
        List<BeanPostProcessor> internalPostProcessors = new ArrayList<>();
        List<String> orderedPostProcessorNames = new ArrayList<>();
        List<String> nonOrderedPostProcessorNames = new ArrayList<>();
        for (String ppName : postProcessorNames) {
            if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
                BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
                priorityOrderedPostProcessors.add(pp);
                if (pp instanceof MergedBeanDefinitionPostProcessor) {
                    internalPostProcessors.add(pp);
                }
            }
            else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
                orderedPostProcessorNames.add(ppName);
            }
            else {
                nonOrderedPostProcessorNames.add(ppName);
            }
        }

        //对实现了PriorityOrdered接口的按优先级排序
        sortPostProcessors(beanFactory, priorityOrderedPostProcessors);
       //这里就是注册了，下面会说
        registerBeanPostProcessors(beanFactory, priorityOrderedPostProcessors);

        
        List<BeanPostProcessor> orderedPostProcessors = new ArrayList<>();
        for (String ppName : orderedPostProcessorNames) {
            BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
            orderedPostProcessors.add(pp);
            if (pp instanceof MergedBeanDefinitionPostProcessor) {
                internalPostProcessors.add(pp);
            }
        }
        sortPostProcessors(beanFactory, orderedPostProcessors);
       //注册
        registerBeanPostProcessors(beanFactory, orderedPostProcessors);

        List<BeanPostProcessor> nonOrderedPostProcessors = new ArrayList<>();
        for (String ppName : nonOrderedPostProcessorNames) {
            BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
            nonOrderedPostProcessors.add(pp);
            if (pp instanceof MergedBeanDefinitionPostProcessor) {
                internalPostProcessors.add(pp);
            }
        }
        //注册
        registerBeanPostProcessors(beanFactory, nonOrderedPostProcessors);

        // 最后注册常规的
        sortPostProcessors(beanFactory, internalPostProcessors);
        registerBeanPostProcessors(beanFactory, internalPostProcessors);

        beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(applicationContext));
    }

可以看到上方的代码就是把这些BeanPostProcessor分为了几类，然后分别根据规则排序后注册进BeanFactory中，而BeanFactory中其实就只是维护了一个BeanPostProcessor的列表而已

  private final List<BeanPostProcessor> beanPostProcessors = new ArrayList();

private static void registerBeanPostProcessors(
            ConfigurableListableBeanFactory beanFactory, List<BeanPostProcessor> postProcessors) {

        for (BeanPostProcessor postProcessor : postProcessors) {
            beanFactory.addBeanPostProcessor(postProcessor);
        }
    }

public void addBeanPostProcessor(BeanPostProcessor beanPostProcessor) {
        Assert.notNull(beanPostProcessor, "BeanPostProcessor must not be null");
        this.beanPostProcessors.remove(beanPostProcessor);
        this.beanPostProcessors.add(beanPostProcessor);
        if (beanPostProcessor instanceof InstantiationAwareBeanPostProcessor) {
            this.hasInstantiationAwareBeanPostProcessors = true;
        }

        if (beanPostProcessor instanceof DestructionAwareBeanPostProcessor) {
            this.hasDestructionAwareBeanPostProcessors = true;
        }

    }

执行原理

我们知道Bean的初始化是在定义在容器的刷新过程中，而具体的实现则是由AbstractAutowireCapableBeanFactory.initializeBean()方法完成的。在这个方法中就包含了BeanPostProcessor的调用逻辑

protected Object initializeBean(final String beanName, final Object bean, RootBeanDefinition mbd) {
        if (System.getSecurityManager() != null) {
            AccessController.doPrivileged(new PrivilegedAction<Object>() {
                @Override
                public Object run() {
                    invokeAwareMethods(beanName, bean);
                    return null;
                }
            }, getAccessControlContext());
        }
        else {
            invokeAwareMethods(beanName, bean);
        }
 
        Object wrappedBean = bean;
        if (mbd == null || !mbd.isSynthetic()) {
            // BeanPostProcessors 的Before 方法
            wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
        }
 
        try {
            // 调用初始化方法
            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()) {
            // BeanPostProcessors 的After方法
            wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
        }
        return wrappedBean;
    }

而这里面的执行逻辑我们也可以猜到，无非就是循环遍历所有的BeanPostProcessor，然后一一执行

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

    Object result = existingBean;
    for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
        result = beanProcessor.postProcessBeforeInitialization(result, beanName);
        if (result == null) {
            return result;
        }
    }
    return result;}

其中applyBeanPostProcessorsAfterInitialization的实现内容跟这个是一样的

但是这里面有一个主意的点，那就是如果具体的实现一但返回null，那么就会跳出for循环，后面的就得不到机会执行了

常见用例

查看这个接口的继承体系，可以看到这个接口的实现类是非常多的，各个实现类的功能如果感兴趣大家可以去慢慢挖掘一下