spring 事件機制

spring的事件機制使用的是觀察者模式
觀察者模式學習http://www.runoob.com/design-pattern/observer-pattern.html
ApplicationEvent ： 具體的事件對象，充當介質
這是一個抽象類，spring提供了幾種實現 ApplicationContextEvent，ContextClosedEvent，ContextRefreshedEvent，ContextStartedEvent，ContextStoppedEvent和RequestHandledEvent

ApplicationEvent 源碼如下：

public abstract class ApplicationEvent extends EventObject {

    /** use serialVersionUID from Spring 1.2 for interoperability */
    private static final long serialVersionUID = 7099057708183571937L;

    /** System time when the event happened */
    private final long timestamp;


    /**
     * Create a new ApplicationEvent.
     * @param source the component that published the event (never <code>null</code>)
     */
    public ApplicationEvent(Object source) {
        super(source);
        this.timestamp = System.currentTimeMillis();
    }


    /**
     * Return the system time in milliseconds when the event happened.
     */
    public final long getTimestamp() {
        return this.timestamp;
    }

}

ApplicationListener ： 觀察者，觀察和處理具體事件
源碼如下：

public interface ApplicationListener<E extends ApplicationEvent> extends EventListener {

    /**
     * Handle an application event.
     * @param event the event to respond to
     */
    void onApplicationEvent(E event);

}

ApplicationEventMulticaster ： 觀察者模式中的Subject ，負責管理觀察者，發佈事件

源碼如下：

public interface ApplicationEventMulticaster {

    /**
     * Add a listener to be notified of all events.
     * @param listener the listener to add
     */
    void addApplicationListener(ApplicationListener listener);

    /**
     * Add a listener bean to be notified of all events.
     * @param listenerBeanName the name of the listener bean to add
     */
    void addApplicationListenerBean(String listenerBeanName);

    /**
     * Remove a listener from the notification list.
     * @param listener the listener to remove
     */
    void removeApplicationListener(ApplicationListener listener);

    /**
     * Remove a listener bean from the notification list.
     * @param listenerBeanName the name of the listener bean to add
     */
    void removeApplicationListenerBean(String listenerBeanName);

    /**
     * Remove all listeners registered with this multicaster.
     * <p>After a remove call, the multicaster will perform no action
     * on event notification until new listeners are being registered.
     */
    void removeAllListeners();

    /**
     * Multicast the given application event to appropriate listeners.
     * @param event the event to multicast
     */
    void multicastEvent(ApplicationEvent event);

}

ApplicationEventMulticaster spring中提供了默認的實現SimpleApplicationEventMulticaster，在spring的上下文application中有該類的bean，調用application的publishEvent方法時，就是直接調用該類的multicastEvent方法
publishEvent如下：

    public void publishEvent(ApplicationEvent event) {
        Assert.notNull(event, "Event must not be null");
        if (logger.isTraceEnabled()) {
            logger.trace("Publishing event in " + getDisplayName() + ": " + event);
        }
        getApplicationEventMulticaster().multicastEvent(event);
        if (this.parent != null) {
            this.parent.publishEvent(event);
        }
    }

ApplicationEventMulticaster 的multicastEvent會一次調用註冊的觀察者的onApplicationEvent方法
multicastEvent實現如下：

    public void multicastEvent(final ApplicationEvent event) {
        for (final ApplicationListener listener : getApplicationListeners(event)) {
            Executor executor = getTaskExecutor();
            if (executor != null) {
                executor.execute(new Runnable() {
                    @SuppressWarnings("unchecked")
                    public void run() {
                        listener.onApplicationEvent(event);
                    }
                });
            }
            else {
                listener.onApplicationEvent(event);
            }
        }
    }

使用spring事件機制的例子 郵件發送提示
1.定義事件

public class MailSendEvent extends ApplicationEvent {

    public MailSendEvent(Object source, String fromAddress, String toAddress) {
        super(source);
        this.fromAddress = fromAddress;
        this.toAddress = toAddress;
    }

    private static final long serialVersionUID = 1L;

    private String fromAddress;
    private String toAddress;

    public String getFromAddress() {
        return fromAddress;
    }

    public void setFromAddress(String fromAddress) {
        this.fromAddress = fromAddress;
    }

    public String getToAddress() {
        return toAddress;
    }

    public void setToAddress(String toAddress) {
        this.toAddress = toAddress;
    }

    @Override
    public String toString() {
        return fromAddress + " Send Mail to " + toAddress;
    }

}

2.定義觀察者

public class MailSendEventListener implements ApplicationListener<MailSendEvent> {

    public void onApplicationEvent(MailSendEvent event) {
        System.out.println("發生一個郵件發送的事件：" + event);
    }

}

3.定義觸發事件發生的地方，即發送郵件的地方

public class MailBean implements ApplicationContextAware {

    private String fromAddress;
    private String toAddress;

    ApplicationContext applicationContext;

    public MailBean(String fromAddress, String toAddress) {
        this.fromAddress = fromAddress;
        this.toAddress = toAddress;
    }

    public void sendMail() {
        System.out.println(fromAddress + " Send Mail to " + toAddress);
        MailSendEvent event = new MailSendEvent(applicationContext, fromAddress, toAddress);
        applicationContext.publishEvent(event);

    }

    public String getFromAddress() {
        return fromAddress;
    }

    public void setFromAddress(String fromAddress) {
        this.fromAddress = fromAddress;
    }

    public String getToAddress() {
        return toAddress;
    }

    public void setToAddress(String toAddress) {
        this.toAddress = toAddress;
    }

    public ApplicationContext getApplicationContext() {
        return applicationContext;
    }

    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        this.applicationContext = applicationContext;

    }

}

4.配置文件

    <bean class="com.yang.spring.event.MailSendEventListener" />


    <bean id="mailTest" class="com.yang.spring.event.MailBean" init-method="sendMail">
        <constructor-arg index="0" value="ttt"></constructor-arg>
        <constructor-arg index="1" value="yyy"></constructor-arg>
    </bean>

5.測試

    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
    }

6.測試結果

ttt Send Mail to yyy
發生一個郵件發送的事件：ttt Send Mail to yyy