Spring中的后置处理器BeanPostProcessor讲解

Spring中提供了很多PostProcessor供开发者进行拓展，例如：BeanPostProcessor、BeanFactoryPostProcessor、BeanValidationPostProcessor等一系列后处理器。他们的使用方式大多类似，了解其中一个并掌握他的使用方式，其他的可以触类旁通。

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。

BeanPostProcessor API：

public interface BeanPostProcessor {  

    /**
     * Apply this BeanPostProcessor to the given new bean instance <i>before</i> any bean
     * initialization callbacks (like InitializingBean's {@code afterPropertiesSet}
     * or a custom init-method). The bean will already be populated with property values.
     */
    //实例化、依赖注入完毕，在调用显示的初始化之前完成一些定制的初始化任务
    Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException;  

    /**
     * Apply this BeanPostProcessor to the given new bean instance <i>after</i> any bean
     * initialization callbacks (like InitializingBean's {@code afterPropertiesSet}
     * or a custom init-method). The bean will already be populated with property values.
     */
    //实例化、依赖注入、初始化完毕时执行
    Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException;  

}

BeanPostProcessor接口提供了两个供开发者自定义的方法：postProcessBeforeInitialization、postProcessAfterInitialization。

postProcessBeforeInitialization：该方法主要针对spring在bean初始化时调用初始化方法前进行自定义处理。

postProcessAfterInitialization：该方法主要针对spring在bean初始化时调用初始化方法后进行自定义处理。

测试代码：

com.test.model.Cat:

package com.test.model;

/**
 * 测试bean
 */
public class Cat {
  private String name;
  private int age;

  public void say() {
    System.out.println("name:" + name);
    System.out.println("age:" + age);
  }

  public String getName() {
    return name;
  }

  public void setName(String name) {
    this.name = name;
  }

  public int getAge() {
    return age;
  }

  public void setAge(int age) {
    this.age = age;
  }
}

spring容器中配置cat，和cat的beanPostProcessor：

<!--配置bean并初始化-->
    <bean id="cat" class="com.test.model.Cat" >
      <property name="name" value="HelloKitty" />
      <property name="age" value="1" />
    </bean>
    <bean id="catBeanPostProcessor" class="com.test.postprocessor.CatBeanPostProcessor" />

com.test.postprocessor.CatBeanPostProcessor:

package com.test.postprocessor;

import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.BeanPostProcessor;
import com.test.model.Cat;

/**
 * 自定义后处理器
 */
public class CatBeanPostProcessor implements BeanPostProcessor{

    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        if (bean instanceof Cat) {
              //输出原始属性
              Cat cat = (Cat) bean;
              cat.say();
              return bean;
            }
         return bean;
    }

    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        if (bean instanceof Cat) {
              //修改属性值，并返回
              Cat cat = (Cat) bean;
              cat.setName("hello maomi");
              cat.setAge(3);
              return cat;
            }
         return bean;
    }

}

IndexController：

package com.cy.controller;

import javax.servlet.ServletContext;
import javax.servlet.http.HttpServletRequest;

import org.springframework.context.ApplicationContext;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.context.support.WebApplicationContextUtils;

import com.test.model.Cat;

@Controller
public class IndexController {

    //到index首页
    @RequestMapping(value="index")
    public String index(HttpServletRequest request){
        /**
         * 访问index同时，从容器中获取已经被初始化之后处理过的cat，打印信息
         */
        ServletContext servletContext = request.getSession().getServletContext();
        ApplicationContext ac = WebApplicationContextUtils.getWebApplicationContext(servletContext);
        Cat c = (Cat) ac.getBean("cat");
        c.say();

        return "index";
    }
}

观察结果：

容器启动时，输出：

name:HelloKitty

age:1

访问项目http://localhost:8080/demo/index，index链接时，输出：

name:hello maomi

age:3

--------------------------------------------------------------------------------------------------------------------------

可以看到通过后处理器处理过后的bean信息已经改变。最后，看看源码中如何调用自定义实现的。

在初始化bean方法中：AbstractAutowireCapableBeanFactory.java

/**
 * 初始化bean
 */
protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) {
  //省略部分无关代码
  Object wrappedBean = bean;
  //初始化前
  if (mbd == null || !mbd.isSynthetic()) {
    wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
  }

  try {
    //调用初始化方法初始化bean
    invokeInitMethods(beanName, wrappedBean, mbd);
  }
  catch (Throwable ex) {
    throw new BeanCreationException(
        (mbd != null ? mbd.getResourceDescription() : null),
        beanName, "Invocation of init method failed", ex);
  }
  //初始化后
  if (mbd == null || !mbd.isSynthetic()) {
    wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
  }
  return wrappedBean;
}
//postProcessBeforeInitialization方法调用
@Override
public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName)
    throws BeansException {

  Object result = existingBean;
  for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
    //调用自定义postProcessBeforeInitialization方法
    Object current = beanProcessor.postProcessBeforeInitialization(result, beanName);
    if (current == null) {
      return result;
    }
    result = current;
  }
  return result;
}
//postProcessAfterInitialization方法调用
@Override
public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
    throws BeansException {

  Object result = existingBean;
  for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
    //自定义postProcessAfterInitialization方法调用
    Object current = beanProcessor.postProcessAfterInitialization(result, beanName);
    if (current == null) {
      return result;
    }
    result = current;
  }
  return result;
}