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;

}

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Spring中的后置处理器BeanPostProcessor讲解

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