BeanPostProcessor的使用和源码解析

BeanPostProcessor接口作用：

如果我们想在Spring容器中完成bean实例化、配置以及其他初始化方法前后要添加一些自己逻辑处理。我们需要定义一个或多个BeanPostProcessor接口实现类，然后注册到Spring IoC容器中。

/**
 * bean后置处理器
 * @author zss
 *
 */
public class PostProcessor implements BeanPostProcessor {

    @Override
    public Object postProcessBeforeInitialization(Object bean,
            String beanName) throws BeansException {
        if ("narCodeService".equals(beanName)) {//过滤掉bean实例ID为narCodeService
            return bean;
        }
        System.out.println("后置处理器处理bean=【"+beanName+"】开始");
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return bean;
    }

    @Override
    public Object postProcessAfterInitialization(Object bean,
            String beanName) throws BeansException {
        if ("narCodeService".equals(beanName)) {
            return bean;
        }
        System.out.println("后置处理器处理bean=【"+beanName+"】完毕!");
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return bean;
    }

}
注意:接口中两个方法不能返回null，如果返回null那么在后续初始化方法将报空指针异常或者通过getBean()方法获取不到bena实例对象
     因为后置处理器从Spring IoC容器中取出bean实例对象没有再次放回IoC容器中

将Spring的后置处理器PostProcessor配置到Spring配置文件中

<?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">

    <!-- 定义一个bean -->
     <bean id="narCodeService" class="com.test.service.impl.NarCodeServiceImpl">
     </bean>
    <bean id="beanLifecycle" class="com.test.spring.BeanLifecycle" init-method="init" destroy-method="close">
        <property name="name" value="张三"></property>
        <property name="sex" value="男"></property>
    </bean>

    <!-- Spring后置处理器 -->
    <bean id="postProcessor" class="com.test.spring.PostProcessor"/>
</beans>

源码分析：

@Override
    public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            // Prepare this context for refreshing.
            prepareRefresh();

            // Tell the subclass to refresh the internal bean factory.
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

            // Prepare the bean factory for use in this context.
            prepareBeanFactory(beanFactory);

            try {
                // Allows post-processing of the bean factory in context subclasses.
                postProcessBeanFactory(beanFactory);

                // Invoke factory processors registered as beans in the context.
                invokeBeanFactoryPostProcessors(beanFactory);

                // 在这里把所有实现了BeanPostProcessor接口注册到容器的beanPostProcessors里面
                registerBeanPostProcessors(beanFactory);

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()) {
            wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
        } try {
            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()) {
            wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
        } return wrappedBean;
    }

applyBeanPostProcessorsBeforeInitialization和applyBeanPostProcessorsAfterInitialization的具体代码分别为:

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;
    }

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

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

根据以上代码，我们得知，在invokeInitMethods的执行前后，spring会分别调用所有的BeanPostProcessor，执行其中的方法，那么invokeInitMethods的具体内容我们仍需要看下，发现此方法主要作用有两个：1、判断bean是否继承了InitializingBean，如果继承接口，执行afterPropertiesSet()方法，2、获得是否设置了init-method属性，如果设置了，就执行设置的方法

protected void invokeInitMethods(String beanName, final Object bean, RootBeanDefinition mbd) throws Throwable { boolean isInitializingBean = (bean instanceof InitializingBean); if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) { if (logger.isDebugEnabled()) {
                logger.debug("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");
            } if (System.getSecurityManager() != null) { try {
                    AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
                        @Override public Object run() throws Exception {
                            ((InitializingBean) bean).afterPropertiesSet(); return null;
                        }
                    }, getAccessControlContext());
                } catch (PrivilegedActionException pae) { throw pae.getException();
                }
            } else {
                ((InitializingBean) bean).afterPropertiesSet();
            }
        } if (mbd != null) {
            String initMethodName = mbd.getInitMethodName(); if (initMethodName != null && !(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&
                    !mbd.isExternallyManagedInitMethod(initMethodName)) {
                invokeCustomInitMethod(beanName, bean, mbd);
            }
        }
    }

根据以上描述，我们可以看到原有推断有一些问题，两个方法的执行主要是在bean完成初始化之后，准备执行默认方法时候对bean进行包装。

三、应用场景

几个典型的应用如：

1、解析bean的注解，将注解中的字段转化为属性

2、统一将属性在执行前，注入bean中，如数据库访问的sqlMap，如严重服务，这样不需要每个bean都配置属性

3、打印日志，记录时间等。