Listener监听器

不念不忘少年蓝@ 2022-04-16 05:05 262阅读 0赞

### 文章目录 ###

*  1. 引言
 *  2. 监听器接口和注册
 *   *  2.1 编写监听器格式
 *  3. ServletContext监听器
 *   *  3.1 ServletContextListener
     *  3.2 ServletContextAttributeListener
 *  4. SessionListener
 *   *  4.1 HttpSessionListener
     *  4.2 HttpSessionAttributeListener
     *  4.3 HttpSessionActivationListener
     *  4.4 HttpSessionBindingListener
 *  5. ServletRequestListener
 *   *  5.1 ServletRequestListener
     *  5.2 ServletRequestAttributeListener

# 1. 引言 #

Servlet API提供了一系列的事件和实践监听接口，监听器接口可以分为三类：ServletContext,HttpSession和ServletRequest,他们都继承自java.util.EventListener标记接口。

# 2. 监听器接口和注册 #

主要有以下这些接口

*  javax.servlet.ServletContextListener:能够响应ServletContext声明周期事件，提供了ServletContext创建之前和关闭之前调用的方法。
 *  javax.servlet.ServletContextAttributeListener:他能够响应ServletContext范围的属性添加、删除、替换事件。
 *  javax.servlet.http.HttpSessionListener:能够响应HttpSession创建、超时和失效时间。
 *  javax.servlet.http.HttpSessionAttributeListener:能响应HttpSession范围属性的添加、删除和替换事件
 *  javax.servlet.http.HttpSessionActivationListener:HttpSession激活或者失效时被调用
 *  javax.servlet.http.HttpSessionBindingListener：使对象在绑定到会话或从会话解除绑定时得到HttpSessionBindingEvent事件通知。
 *  javax.servlet.ServletRequestListener:响应ServletRequest的创建或删除
 *  javax.servlet.ServletRequestAttributeListener: 能响应ServletRequest范围的属性值添加、删除、修改事件

## 2.1 编写监听器格式 ##

有两种注册监听器的方法，第一种是使用WebListener注解

@WebListener
    public class ListenerClass implements ListenerInterface{
    
    }

第二种方式是在部署描述文档中增加一个listener元素

<listener>
      <listener-class>fully-qualified listener class</listener-class>
    </listener>

# 3. ServletContext监听器 #

ServletContext监听器接口有两个：ServletContextListener和ServletContextAttributeListener。

## 3.1 ServletContextListener ##

ServletContextListener能对ServletContext的创建和销毁作出响应

/** 
     * Interface for receiving notification events about ServletContext
     * lifecycle changes.
     *
     * <p>In order to receive these notification events, the implementation
     * class must be either declared in the deployment descriptor of the web
     * application, annotated with {@link javax.servlet.annotation.WebListener},
     * or registered via one of the addListener methods defined on
     * {@link ServletContext}.
     *
     * 他们的加载顺序是按照声明顺序确定的
     */
    @WebListener
    public class MyListener implements ServletContextListener {
    
        /**
         * Receives notification that the web application initialization
         * process is starting.
         *
         * 这个方法运行在filter和servlet初始化之前
         * that is being initialized
         */
        @Override
        public void contextInitialized(ServletContextEvent sce) {
            System.out.println("before initialization");
        }
        /**
         * Receives notification that the ServletContext is about to be
         * shut down.
         *
         * 在此方法收到通知前，所有的servlet和filter已经被销毁
         *
         */
        @Override
        public void contextDestroyed(ServletContextEvent sce) {
            System.out.println("after initialization");
        }
    }

ServletContextListener包含的两个方法都会从容器获得一个ServletContextEvent实例，他是EventObject的子类，定义了访问ServletContext的getServletContext()方法。

public class ServletContextEvent extends java.util.EventObject { 
    
        /** Construct a ServletContextEvent from the given context.
         *
         * @param source - the ServletContext that is sending the event.
         */
        public ServletContextEvent(ServletContext source) {
            super(source);
        }
        
        /**
         * Return the ServletContext that changed.
         *
         * @return the ServletContext that sent the event.
         */
        public ServletContext getServletContext () { 
            return (ServletContext) super.getSource();
        }
    }

## 3.2 ServletContextAttributeListener ##

当一个ServletContext范围的属性被添加、删除或者替换时，此接口的实现类会接收到消息，这个接口定义了如下三个方法

void attributeAdded(ServletContextAttributeEvent event);
    void attributeRemoved(ServletContextAttributeEvent event);
    void attributeReplaced(ServletContextAttributeEvent event);

ServletContextAttributeEvent继承自ServletContextEvent，并且增加了两个方法，分别用户获得该属性的名称和值

/** 
     * Event class for notifications about changes to the attributes of
     * the ServletContext of a web application.
      */
    
    public class ServletContextAttributeEvent extends ServletContextEvent { 
    
        private String name;
        private Object value;
    
        /**
         * @param source the ServletContext whose attribute changed
         * @param name the name of the ServletContext attribute that changed
         * @param value the value of the ServletContext attribute that changed
         */
        public ServletContextAttributeEvent(ServletContext source,
                String name, Object value) {
            super(source);
            this.name = name;
            this.value = value;
        }
    	
        /**
         * Gets the name of the ServletContext attribute that changed.
         *
         * @return the name of the ServletContext attribute that changed
         */
        public String getName() {
            return this.name;
        }
    	
        /**
         * Gets the value of the ServletContext attribute that changed.
         *
         * <p>If the attribute was added, this is the value of the attribute.
         * If the attribute was removed, this is the value of the removed
         * attribute. If the attribute was replaced, this is the old value of
         * the attribute.
         *
         * @return the value of the ServletContext attribute that changed
         */
        public Object getValue() {
            return this.value;   
        }
    }

# 4. SessionListener #

一共有四个HttpSession相关的监听器接口：HttpSessionListener,HttpSessionActivationListener,HttpSessionAttributeListener和HttpSessionBindingListener，他们都在javax.servlet.http包中

## 4.1 HttpSessionListener ##

当HttpSession创建或者销毁时，容器会通知所有的HttpSessionListener，这个接口有两个方法

void sessionCreated(HttpSessionEvent event);
    void sessionDestroy(HttpSessionEvent event);

HttpSessionEvent是继承自EventObject的类，可以调用HttpSessionEvent的getSession()方法获取当前的HttpSession

/**
     * This is the class representing event notifications for changes to
     * sessions within a web application.
     */
    public class HttpSessionEvent extends java.util.EventObject {
    
         /**
         * Construct a session event from the given source.
         */
        public HttpSessionEvent(HttpSession source) {
            super(source);
        }
    
        /**
         * Return the session that changed.
         */
        public HttpSession getSession () { 
            return (HttpSession) super.getSource();
        }
    }

## 4.2 HttpSessionAttributeListener ##

他响应的是HttpSession范围属性的添加、删除和替换，包含三个方法

void attributeAdded(HttpSessionBindingEvent event);
    void attributeRemoved(HttpSessionBindingEvent event);
    void attributeReplaced(HttpSeesionBindingEvent event);

HttpSessionBindingEvent是HttpSessionEvent的子类，可以获得HttpSession的同时实现了另外两个方法,可以获得对应属性的名称和值。

String getName();
    Object getValue();

## 4.3 HttpSessionActivationListener ##

在分布式环境下，会用多个容器来进行负载均衡，有可能需要将session保存起来，在容器之间传递，例如在一个容器内存不足时，会把很少用到的对象转存到其他容器上，这时候容器就会通知所有HttpSessionActivationListener接口实现类

/** 通知会话即将被钝化*/
        public void sessionWillPassivate(HttpSessionEvent se); 
        /** 通知会话刚刚被激活*/
        public void sessionDidActivate(HttpSessionEvent se);

## 4.4 HttpSessionBindingListener ##

在发生下面三种情况之一时，触发事件

*  执行session.invalidate()时。
 *  session超时，自动销毁时。
 *  执行session.setAttribute(“keyName”, Object)或session.removeAttribute(“keyName”)

这个接口的实现类可以用于统计实时登陆人数，包含两个方法：

public void valueBound(HttpSessionBindingEvent event);
     public void valueUnbound(HttpSessionBindingEvent event);

# 5. ServletRequestListener #

ServletRequest范围的监听器接口有三个：ServletRequestListener,ServletRequestAttributeListener和AsyncListener

## 5.1 ServletRequestListener ##

会对ServletRequest的创建和销毁事件进行响应，容器会通过一个池来存放并重复利用多个ServletRequest，ServletRequest的创建是从容器池里被分配出来的时刻开始，而他的销毁时刻是放回容器池里的时间，他包含两个方法

void requestInitialized(ServletRequestEvent event);
    void requestDestroyed(ServletRequestEvent event);

ServletRequestEvent接口有两个方法

public class ServletRequestEvent extends java.util.EventObject { 
    
        private final transient ServletRequest request;
    
        /** Construct a ServletRequestEvent for the given ServletContext
          * and ServletRequest.
          *
          * @param sc       the ServletContext of the web application.
          * @param request  the ServletRequest that is sending the event.
          */
        public ServletRequestEvent(ServletContext sc, ServletRequest request) {
            super(sc);
            this.request = request;
        }
    
        /**
          * 返回ServletRequest
          */
        public ServletRequest getServletRequest () { 
            return this.request;
        }
    
        /**
          * 返回上下文
          */
        public ServletContext getServletContext () { 
            return (ServletContext) super.getSource();
        }
    }

## 5.2 ServletRequestAttributeListener ##

当ServletRequest范围的属性被添加、删除或替换时，ServletRequestAttributeListener接口会被调用，他提供了三个方法：

void attributeAdded(ServletRequestAttributeEvent event);
    void attributeRemoved(ServletRequestAttributeEvent event);
    void attributeReplaced(ServletRequestAttributeEvent event);

ServletRequestAttributeEvent类继承自ServletRequestEvent，新增了两个方法

String getName()  //返回属性名
    Object getValue()  //返回属性的值