java - 为什么在spring中创建了两个对象？

Question

我越来越熟悉 Spring。奇怪的是，下面的代码调用构造函数两次，而我希望它被调用一次。有人可以帮忙吗？

package com.tutorialspoint;

import java.util.List;
import org.springframework.context.event.ContextStartedEvent;
import org.springframework.context.ApplicationListener;

public class DemoClass implements ApplicationListener<ContextStartedEvent> {

    private String message;
    private int nrOfMessages;



    public DemoClass(String mes, int nr) {
        message = mes;
        nrOfMessages = nr;
        System.out.println("Demo class constructor. Parameters: " + mes + " " + nr);
    }

    // a setter method to set List
    public void setNrOfMessages(int nr) {
        this.nrOfMessages = nr;
    }


    // Message setter
    public void setMessage(String message) {
        this.message = message;
    }

    // prints and returns all the elements of the list.
    public void dumpContents() {
            System.out.println("Message: " + message + " Nr of messages: " + nrOfMessages);
        }

    public void onApplicationEvent(ContextStartedEvent event) {
          System.out.println("ContextStartedEvent Received");
       }


}

bean 和 main:

package com.tutorialspoint;

import java.io.FileNotFoundException;
import java.util.List;
import org.springframework.context.support.AbstractApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import org.springframework.context.ConfigurableApplicationContext;

public class MainApp {


    public static void main(String[] args) {

        ConfigurableApplicationContext  context = new ClassPathXmlApplicationContext("Beans.xml");

        context.start();

        // Create an object
        DemoClass obj = (DemoClass) context.getBean("democlassBean");

        // Dump contents
        obj.dumpContents();

    }
}

bean

<?xml version="1.0" encoding="UTF-8"?>

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
    http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
    http://www.springframework.org/schema/context
    http://www.springframework.org/schema/context/spring-context-3.0.xsd">


   <bean id="democlassBean" class="com.tutorialspoint.DemoClass"
    scope="prototype">
      <constructor-arg value="Hello world beans."/>
      <constructor-arg value="300"/>        
   </bean>

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


</beans>

这是输出:

Demo class constructor. Parameters: Hello world beans. 300
BeforeInitialization : democlassBean
AfterInitialization : democlassBean
ContextStartedEvent Received 
Demo class constructor. Parameters: Hello world beans. 300    
BeforeInitialization : democlassBean
AfterInitialization : democlassBean
Message: Hello world beans. Nr of messages: 300

您可以看到构造函数被调用两次 - 为什么？？？

---

这里还有 initHelloWorld 的代码:

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
   }

}

Answer 1

看来，用原型(prototype)bean来充当ApplicationListener并不是个好主意。检查http://forum.spring.io/forum/spring-projects/container/35965-applicationlistener-interface-makes-beans-eagerly-instantiated了解详情。

你可以让它工作，但需要一些额外的步骤(在带内衬的帖子中描述)。请注意，链接的帖子有些旧(2007 年)，其中涵盖的一些实现细节可能不再有效。

如果您真的关心创建的实例数量 - 创建两个类 - 一个作为原型(prototype)，另一个(单例)充当 ApplicationListener 怎么样？