事件发布和监听机制

在一个完整的事件体系中，除了事件和监听器以外，还应该有3个概念；

1. 事件源：事件的产生者，任何一个event都必须有一个事件源；
2. 事件广播器：它是事件和事件监听器之间的桥梁，负责把事件通知给事件监听器；
3. 事件监听器注册表：就是spring框架为所有的监听器提供了一个存放的地方；

事件类：ApplicationEvent

ApplicationEvent唯一的构造函数是ApplicationEvent（Object source），通过source指定事件源

/**
 * Class to be extended by all application events. Abstract as it
 * doesn't make sense for generic events to be published directly.
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 */
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 object on which the event initially occurred (never {@code null})
     */
    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;
    }

}

abstract class ApplicationContextEvent 容器事件(事件源是ApplicationContext)

public abstract class ApplicationContextEvent extends ApplicationEvent {

    /**
     * Create a new ContextStartedEvent.
     * @param source the {@code ApplicationContext} that the event is raised for
     * (must not be {@code null})
     */
    public ApplicationContextEvent(ApplicationContext source) {
        super(source);
    }

    /**
     * Get the {@code ApplicationContext} that the event was raised for.
     */
    public final ApplicationContext getApplicationContext() {
        return (ApplicationContext) getSource();
    }

}

class RequestHandledEvent extends ApplicationEvent

一个与Web应用相关的事件，当一个请求被处理后，才会产生该事件

public class RequestHandledEvent extends ApplicationEvent {

    /** Session id that applied to the request, if any. */
    @Nullable
    private String sessionId;

    /** Usually the UserPrincipal. */
    @Nullable
    private String userName;

    /** Request processing time. */
    private final long processingTimeMillis;

    /** Cause of failure, if any. */
    @Nullable
    private Throwable failureCause;

    /**
     * Create a new RequestHandledEvent with session information.
     * @param source the component that published the event
     * @param sessionId the id of the HTTP session, if any
     * @param userName the name of the user that was associated with the
     * request, if any (usually the UserPrincipal)
     * @param processingTimeMillis the processing time of the request in milliseconds
     */
    public RequestHandledEvent(Object source, @Nullable String sessionId, @Nullable String userName,
            long processingTimeMillis) {

        super(source);
        this.sessionId = sessionId;
        this.userName = userName;
        this.processingTimeMillis = processingTimeMillis;
    }

    /**
     * Create a new RequestHandledEvent with session information.
     * @param source the component that published the event
     * @param sessionId the id of the HTTP session, if any
     * @param userName the name of the user that was associated with the
     * request, if any (usually the UserPrincipal)
     * @param processingTimeMillis the processing time of the request in milliseconds
     * @param failureCause the cause of failure, if any
     */
    public RequestHandledEvent(Object source, @Nullable String sessionId, @Nullable String userName,
            long processingTimeMillis, @Nullable Throwable failureCause) {

        this(source, sessionId, userName, processingTimeMillis);
        this.failureCause = failureCause;
    }

    public long getProcessingTimeMillis() {
        return this.processingTimeMillis;
    }

    @Nullable
    public String getSessionId() {
        return this.sessionId;
    }

    @Nullable
    public String getUserName() {
        return this.userName;
    }

    public boolean wasFailure() {
        return (this.failureCause != null);
    }

    @Nullable
    public Throwable getFailureCause() {
        return this.failureCause;
    }

    public String getShortDescription() {
        StringBuilder sb = new StringBuilder();
        sb.append("session=[").append(this.sessionId).append("]; ");
        sb.append("user=[").append(this.userName).append("]; ");
        return sb.toString();
    }

    public String getDescription() {
        StringBuilder sb = new StringBuilder();
        sb.append("session=[").append(this.sessionId).append("]; ");
        sb.append("user=[").append(this.userName).append("]; ");
        sb.append("time=[").append(this.processingTimeMillis).append("ms]; ");
        sb.append("status=[");
        if (!wasFailure()) {
            sb.append("OK");
        }
        else {
            sb.append("failed: ").append(this.failureCause);
        }
        sb.append(']');
        return sb.toString();
    }

    @Override
    public String toString() {
        return ("RequestHandledEvent: " + getDescription());
    }

}

事件监听器接口（ApplicationListener）

监听器都需要实现该接口，该接口只定义了一个方法:onApplicationEvent (E event)，该方法接收事件对象，在该方法中编写事件的响应处理逻辑。

@FunctionalInterface
public interface ApplicationListener<E extends ApplicationEvent> extends EventListener {

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

}

例子

• 具体事件，继承ApplicationContextEvent

public class MailSendEvent extends ApplicationContextEvent {

    private static final long serialVersionUID = 1L;

    private String to;  //目的地

    public MailSendEvent(ApplicationContext source, String to) {
        super(source);
        this.to = to;
    }

    public String getTo(){
        return this.to;
    }
}

• 构造事件实例，并发送事件

实现了ApplicationContextAware能获取到ApplicationContext，则能进行事件的发布

@Component
public class MailSender implements ApplicationContextAware {
    @Autowired
    private ApplicationContext applicationContext;  // 容器事件由容器触发

    public void sendMail(String to){
        System.out.println("...ApplicationContextAware");
        MailSendEvent event = new MailSendEvent(applicationContext, to);
        applicationContext.publishEvent(event);
    }

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

• 监听到具体事件

@Component
public class MailSendListener implements ApplicationListener<MailSendEvent> {
    @Override
    public void onApplicationEvent(MailSendEvent mailSendEvent) {
        System.out.println("...onApplicationEvent");
        MailSendEvent event = mailSendEvent;
        System.out.println("MailSender向"+ event.getTo()+ "发送了邮件");
    }
}