Spring(四)之Bean生命周期、BeanPost处理

一、Bean 生命周期

Spring bean的生命周期很容易理解。当bean被实例化时，可能需要执行一些初始化以使其进入可用状态。类似地，当不再需要bean并从容器中移除bean时，可能需要进行一些清理。

虽然，在bean实例化及其销毁之间存在幕后发生的活动列表，但本章仅讨论两个重要的bean生命周期回调方法，这些方法在bean初始化及其销毁时是必需的。

要定义bean的设置和拆卸，我们只需使用initmethod和/或destroy-method参数声明<bean> 。init-method属性指定在实例化时立即在bean上调用的方法。类似地，destroy-method指定在从容器中删除bean之前调用的方法。

示例如下:

(1)编写HelloWord.java

package com.tutorialspoint;

public class HelloWorld {
   private String message;

   public void setMessage(String message){
      this.message = message;
   }
   public void getMessage(){
      System.out.println("Your Message : " + message);
   }
   public void init(){
      System.out.println("Bean is going through init.");
   }
   public void destroy() {
      System.out.println("Bean will destroy now.");
   }
}

(2)编写Beans.xml

<?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-4.0.xsd">

  <bean id = "helloWorld" class = "com.tutorialspoint.HelloWorld" init-method = "init" 
      destroy-method = "destroy">
      <property name = "message" value = "Hello World!"/>
   </bean>

</beans>

(3)编写MainApp.java

package com.tutorialspoint;

import org.springframework.context.support.AbstractApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;


public class MainApp {
   public static void main(String[] args) {
       
             AbstractApplicationContext context = new ClassPathXmlApplicationContext("Beans.xml");

          HelloWorld obj = (HelloWorld) context.getBean("helloWorld");
          obj.getMessage();
          context.registerShutdownHook();
   }
}

二、Bean Post 处理器

该BeanPostProcessor接口定义，你可以实现提供自己的实例化逻辑，依赖解析逻辑等，您还可以实现一些自定义的逻辑Spring容器完成实例化，配置，并通过在一个或多个堵塞初始化Bean后回调方法BeanPostProcessor实现。

您可以配置多个BeanPostProcessor接口，并且您可以通过设置订单属性来控制这些BeanPostProcessor接口的执行顺序，前提是BeanPostProcessor实现了Ordered接口。

BeanPostProcessors在bean（或对象）实例上运行，这意味着Spring IoC容器实例化一个bean实例，然后BeanPostProcessor接口完成它们的工作。

一个ApplicationContext的自动检测与该执行中定义的任何Bean的BeanPostProcessor接口，并注册这些Bean类如后处理器，被通过在容器创建bean的适当调用。

示例如下:

(1)编写HelloWorld.java

package com.tutorialspoint;
public class HelloWorld {
     private String message;

       public void setMessage(String message){
          this.message  = message;
       }
       public void getMessage(){
          System.out.println("Your Message : " + message);
       }
       public void init(){
          System.out.println("Bean is going through init.");
       }
       public void destroy(){
          System.out.println("Bean will destroy now.");
       }
}

(2)编写InitHelloWorld.java

package com.tutorialspoint;

import org.springframework.beans.factory.config.BeanPostProcessor;
import org.springframework.beans.BeansException;

public class InitHelloWorld implements BeanPostProcessor {
   public Object postProcessBeforeInitialization(Object bean, String beanName) 
      throws BeansException {
      
      System.out.println("BeforeInitialization : " + beanName);
      return bean;  // you can return any other object as well
   }
   public Object postProcessAfterInitialization(Object bean, String beanName) 
      throws BeansException {
      
      System.out.println("AfterInitialization : " + beanName);
      return bean;  // you can return any other object as well
   }
}

(3)编写Beans.xml

<?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-4.0.xsd">

 <bean id = "helloWorld" class = "com.tutorialspoint.HelloWorld"
      init-method = "init" destroy-method = "destroy">
      <property name = "message" value = "Hello World!"/>
   </bean>

   <bean class = "com.tutorialspoint.InitHelloWorld" />

</beans>

(4)编写MainApp.java

package com.tutorialspoint;

import org.springframework.context.support.AbstractApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;


public class MainApp {
   public static void main(String[] args) {
       
       AbstractApplicationContext context = new ClassPathXmlApplicationContext("Beans.xml");

          HelloWorld obj = (HelloWorld) context.getBean("helloWorld");
          obj.getMessage();
          context.registerShutdownHook();
   }
}