Java Servlet源码走读二：Listener接口

在走读Java Servlet源码前，我的思路是有些混乱的，没有计划也不知道从哪里读起，所以直接选了Servlet接口作为突破口，但实际上选择Listener-Filter-Servlet-Request这条路线去走读源码，逻辑上会更清晰一点，因为它和web中信息流动的路线是一致的，也有助于回答我在上一篇Java Servlet源码走读一：Servlet接口中最后遗留的问题：web容器是在什么时候，如何去调用Servlet的方法的？

当然，在意识到思路混乱后，我首先将servlet3.1的规范文档通读了一遍，也是通过规范文档，我大致了解了应当从哪里作为突破口去更好的走读Servlet源码

Listener的定义

Listener一般翻译为监听器，所有的Listener接口都需要继承java.util.EventListener接口，用于监听Servlet中发生的关键事件并进行处理。

在Servlet中定义了3类Listener：

• Servlet上下文事件（2个Listener接口）:

• Http会话事件（5个Listener接口）:

• Servlet请求事件（3个Listener接口）:

选了3个监听器接口的定义源码，大致了解下定义了哪些接口，后面去看具体的实现类的时候，不至于一抹黑便可

• ServletRequestListener

/**
 * A ServletRequestListener can be implemented by the developer
 * interested in being notified of requests coming in and out of
 * scope in a web component. A request is defined as coming into
 * scope when it is about to enter the first servlet or filter
 * in each web application, as going out of scope when it exits
 * the last servlet or the first filter in the chain.
 *
 * @since Servlet 2.4
 */
public interface ServletRequestListener extends EventListener {

    /**
     * The request is about to go out of scope of the web application.
     * @param sre Information about the request
     */
    public void requestDestroyed (ServletRequestEvent sre);

    /**
     * The request is about to come into scope of the web application.
     * @param sre Information about the request
     */
    public void requestInitialized (ServletRequestEvent sre);
}

注释已经讲解的很明白了，通过实现ServletRequestListener接口可以监听到请求进/出事件
定义了2个回调接口：
requestDestroyed：request从web应用中返回的时候调用
requestInitialized：request进入web应用中的时候调用

• ServletContextListener

/**
 * Implementations of this interface receive notifications about changes to the
 * servlet context of the web application they are part of. To receive
 * notification events, the implementation class must be configured in the
 * deployment descriptor for the web application.
 *
 * @see ServletContextEvent
 * @since v 2.3
 */
public interface ServletContextListener extends EventListener {

    /**
     ** Notification that the web application initialization process is starting.
     * All ServletContextListeners are notified of context initialization before
     * any filter or servlet in the web application is initialized.
     * @param sce Information about the ServletContext that was initialized
     */
    public void contextInitialized(ServletContextEvent sce);

    /**
     ** Notification that the servlet context is about to be shut down. All
     * servlets and filters have been destroy()ed before any
     * ServletContextListeners are notified of context destruction.
     * @param sce Information about the ServletContext that was destroyed
     */
    public void contextDestroyed(ServletContextEvent sce);
}

通过实现ServletContextListener接口可以监听到Servlet上下文初始化和销毁事件
定义了2个回调接口：
contextInitialized：Servlet上下文初始化时调用
contextDestroyed：Servlet上下文销毁时调用

• HttpSessionListener

/**
 * Implementations of this interface are notified of changes to the list of
 * active sessions in a web application. To receive notification events, the
 * implementation class must be configured in the deployment descriptor for the
 * web application.
 *
 * @see HttpSessionEvent
 * @since v 2.3
 */
public interface HttpSessionListener extends EventListener {

    /**
     * Notification that a session was created.
     *
     * @param se
     *            the notification event
     */
    public void sessionCreated(HttpSessionEvent se);

    /**
     * Notification that a session is about to be invalidated.
     *
     * @param se
     *            the notification event
     */
    public void sessionDestroyed(HttpSessionEvent se);
}

通过实现HttpSessionListener接口可以监听到Servlet上下文初始化和销毁事件
定义了2个回调接口：
sessionCreated：会话创建时调用
sessionDestroyed：会话销毁时调用

总结：

1、用户可以通过自定义Listener实现对于Servlet的一些功能增强，比如在request进入web应用时候统一记录日志！
2、对于Servlet来说，它更多的是定义了一种规范（接口）告诉要使用Servlet的人如何去做，实际的处理是依赖具体实现类去做的（比如spring mvc里面大量对Servlet接口的扩展）
3、想完整的了解Servlet如何产生作用（也就是Servlet是如何初始化的，在什么时机初始化的，定义的方法是在什么时候调用的），需要了解Servlet容器比如tomcat的原理和机制（可以结合tomcat的一些书籍走读下源码）