Spring 中的Aware接口

Spring中提供一些Aware相关接口，像是BeanFactoryAware、 ApplicationContextAware、ResourceLoaderAware、ServletContextAware等等，实作这些 Aware接口的Bean在被初始之后，可以取得一些相对应的资源，例如实作BeanFactoryAware的Bean在初始后，Spring容器将会注入BeanFactory的实例，而实作ApplicationContextAware的Bean，在Bean被初始后，将会被注入 ApplicationContext的实例等等。
Bean取得BeanFactory、ApplicationContextAware的实例目的是什么，一般的目的就是要取得一些档案资源的存取、相 关讯息资源或是那些被注入的实例所提供的机制，例如ApplicationContextAware提供了publishEvent()方法，可以支持基于Observer模式的事件传播机制。
ApplicationContextAware接口的定义如下：

public interface ApplicationContextAware {
    void setApplicationContext(ApplicationContext context);
}

public interface ApplicationContextAware {
    void setApplicationContext(ApplicationContext context);
}

我们这边示范如何透过实作ApplicationContextAware注入ApplicationContext来实现事件传播，首先我们的HelloBean如下：

import org.springframework.context.*;
public class HelloBean implements ApplicationContextAware {

    private ApplicationContext applicationContext;
    private String helloWord = "Hello!World!";

    public void setApplicationContext(ApplicationContext context) {
        this.applicationContext = context;
    }

    public void setHelloWord(String helloWord) {
        this.helloWord = helloWord;
    }

    public String getHelloWord() {
        applicationContext.publishEvent(
               new PropertyGettedEvent("[" + helloWord + "] is getted"));
        return helloWord;
    }
}

import org.springframework.context.*;
public class HelloBean implements ApplicationContextAware {

    private ApplicationContext applicationContext;
    private String helloWord = "Hello!World!";

    public void setApplicationContext(ApplicationContext context) {
        this.applicationContext = context;
    }

    public void setHelloWord(String helloWord) {
        this.helloWord = helloWord;
    }

    public String getHelloWord() {
        applicationContext.publishEvent(
               new PropertyGettedEvent("[" + helloWord + "] is getted"));
        return helloWord;
    }
}

ApplicationContext会由Spring容器注入，publishEvent()方法需要一个继承ApplicationEvent的对象，我们的PropertyGettedEvent继承了ApplicationEvent，如下：

import org.springframework.context.*;
public class PropertyGettedEvent extends ApplicationEvent {
    public PropertyGettedEvent(Object source) {
        super(source);
    }
}

import org.springframework.context.*;
public class PropertyGettedEvent extends ApplicationEvent {
    public PropertyGettedEvent(Object source) {
        super(source);
    }
}

当ApplicationContext执行publishEvent()后，会自动寻找实作ApplicationListener接口的对象并通知其发生对应事件，我们实作了PropertyGettedListener如下：

import org.springframework.context.*;
public class PropertyGettedListener implements ApplicationListener {
    public void onApplicationEvent(ApplicationEvent event) {
        System.out.println(event.getSource().toString());
    }
}

import org.springframework.context.*;
public class PropertyGettedListener implements ApplicationListener {
    public void onApplicationEvent(ApplicationEvent event) {
        System.out.println(event.getSource().toString());   
    }
}

Listener必须被实例化，这我们可以在Bean定义档中加以定义：

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

<!DOCTYPE beans PUBLIC "-//SPRING/DTD BEAN/EN" "http://www.springframework.org/dtd/spring-beans.dtd">

<beans>
    <bean id="propertyGetterListener" class="onlyfun.caterpillar.PropertyGettedListener"/>
    <bean id="helloBean" class="cn.edu.ccnu.inc.HelloBean">
        <property name="helloWord"><value>Hello!Justin!</value></property>
    </bean>
</beans>