javaWeb监听器

javaWEB监听器

概述

Listener 表示监听器，是 JavaWeb 三大组件(Servlet、Filter、Listener)之一。
监听器可以监听就是在 application，session，request 三个对象创建、销毁或者往其中添加修改删除属性时自动
执行代码的功能组件。

监听器的组件：

监听器可以大致分为四个组件（方便理解）

事件源：事件发生的源头
监听器：监听事件发生
绑定：将监听器绑定到事件源
事件：能够触发监听器的事

监听器分类

Servlet规范中提供了8个监听器
按功能进行划分将其分成三类，如下：

一类：监听三个域对象的创建和销毁的监听器
二类：监听三个域对象的属性变更的监听器。(属性添加、属性移除、属性替换)
三类：监听HttpSession对象中的JavaBean的状态的改变。(绑定、解除绑定、钝化、活化)

监听器分类	监听器名称	作用
ServletContext监听	ServletContextListener	监听ServletContext对象的创建和销毁
	ServletContextAttributeListener	监听ServletContext对象中属性的增删改
Session监听	HttpSessionListener	监听Session对象的创建和销毁
	HttpSessionAttributeListener	监听Session对象中属性的增删改
	HttpSessionBindingListener	监听对象与Session的绑定与解除
	HttpSessionActivationListener	监听Session的激活
Request监听	ServletRequestListener	监听Request对象的创建和销毁
	ServletRequestAttributeListener	监听Request对象中属性的增删改

监听器的开发步骤

1.自定义类实现一类监听器
2.重写监听器中的方法
3.配置xml配置文件或注解配置

配置方式：

/**
 * 监听器开发步骤
 */
public class FirstListener implements ServletContextListener {
    @Override
    public void contextInitialized(ServletContextEvent servletContextEvent) 
{
        /**
         * 监听ServletContext域的初始化,随着服务器的启动
         */
        System.out.println("ServletContext初始化");
    }
    @Override
    public void contextDestroyed(ServletContextEvent servletContextEvent) {
        //监听ServletContext域的销毁，随着服务器的关闭
        System.out.println("ServletContext销毁");
    }
}

配置xml文件：

<listener>
    <listener-class>com.gx.listener.FirstListener</listener-class>
</listener>

注解方式：

@WebListener，加了该注解默认配置了监听器

/**
 * 监听器开发步骤
 */
@WebListener
public class FirstListener implements ServletContextListener {
    @Override
    public void contextInitialized(ServletContextEvent servletContextEvent) {
        /**
         * 监听ServletContext域的初始化,随着服务器的启动
         */
        System.out.println("ServletContext初始化");
    }
    @Override
    public void contextDestroyed(ServletContextEvent servletContextEvent) {
        /**
         * 监听ServletContext域的销毁，随着服务器的关闭
         */
        System.out.println("ServletContext销毁");
    }
}

一类监听器

监听三个域对象（ServletContext、Session、Request）的创建与销毁

监听ServletContext域对象

实现ServletContextListener接口并覆盖方法

1. 监听服务器启动的时候创建ServletContext对象时

2. 监听服务器关闭的时候ServletContext对象被销毁掉

/**
 * 监听ServletContext
 */
@WebListener
public class TestServletContextListener implements ServletContextListener {
    @Override
    public void contextInitialized(ServletContextEvent servletContextEvent) {
        /**
         * 监听ServletContext域的初始化,随着服务器的启动
         */
        System.out.println("ServletContext初始化");
    }
    @Override
    public void contextDestroyed(ServletContextEvent servletContextEvent) {
        /**
         * 监听ServletContext域的销毁，随着服务器的关闭
         */
        System.out.println("ServletContext销毁");
    }
}

监听Session域对象

实现HttpSessionListener接口并覆盖方法

1. 监听服务器端第一次调用getSession方法时
2. 监听session过期或调用了invalidate方法

/**
 * 监听HttpSession
 */
@WebListener
public class TestHttpSessionListener implements HttpSessionListener {
    @Override
    public void sessionCreated(HttpSessionEvent httpSessionEvent) {
        System.out.println("Session创建");
    }
    @Override
    public void sessionDestroyed(HttpSessionEvent httpSessionEvent) {
        System.out.println("Session销毁");
    }
}

Servlet测试

@WebServlet(name = "TestSessionServlet", value = "/tss")
public class TestSessionServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //创建Session对象
        HttpSession session = request.getSession();
        System.out.println(session.getId());
        //销毁Session对象
        session.invalidate();
    }
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request, response);
    }
}

监听Request域对象

实现ServletRequestListener接口并覆盖方法

1. 监听客户的向服务器发送了一次请求而创建这一个request对象
2. 监听当服务器为这次请求作出了响应之后的request对象销毁

/**
 * 监听ServletRequest
 */
@WebListener
public class TestServletRequestListener implements ServletRequestListener {
    @Override
    public void requestDestroyed(ServletRequestEvent servletRequestEvent) {
        System.out.println("Request对象创建");
    }
    @Override
    public void requestInitialized(ServletRequestEvent servletRequestEvent){
        System.out.println("Request对象销毁");
    }
}

Servlet测试

@WebServlet(name = "TestRequestServlet", value = "/trs")
public class TestRequestServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String requestURI = request.getRequestURI();
        System.out.println(requestURI);
    }
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request, response);
    }
}

二类监听器

监听三个域对象的属性变更的监听器(属性添加、属性移除、属性替换)

监听ServletContext属性

实现ServletContextAttributeListener接口并覆盖方法

• 监听ServletContext域中的属性变更

/**
 * 监听ServletContext域中的属性变更
 */
@WebListener
public class TestServletContextAttributeListener implements 
ServletContextAttributeListener {
    /**
     * 监听ServletContext域对象中属性的添加
     */
    @Override
    public void attributeAdded(ServletContextAttributeEvent servletContextAttributeEvent) {
        System.out.println("ServletContext Added");
    }
    /**
     * 监听ServletContext域对象中属性的移除
     */
    @Override
    public void attributeRemoved(ServletContextAttributeEvent servletContextAttributeEvent) {
        System.out.println("ServletContext Removed");
    }
    /**
     * 监听ServletContext域对象中属性的替换
     */
    @Override
    public void attributeReplaced(ServletContextAttributeEvent servletContextAttributeEvent) {
        System.out.println("ServletContext Replaced");
    }
}

servlet:测试

@WebServlet(name = "TestServletContextAttributeServlet", value = "/tscas")
public class TestServletContextAttributeServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletContext servletContext = getServletContext();
        //添加
        servletContext.setAttribute("name", "ziph");
        //替换
        servletContext.setAttribute("name", "join");
        //移除
        servletContext.removeAttribute("name");
    }
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request, response);
    }
}

监听Session属性

实现HttpSessionAttributeListener接口并覆盖方法

1. 监听HttpSession域中的属性变更

/**
 * 监听Session域中的属性变更
 */
@WebListener
public class TestHttpSessionAttributeListener implements 
HttpSessionAttributeListener {
    @Override
    public void attributeAdded(HttpSessionBindingEvent httpSessionBindingEvent) {
        System.out.println("Session Added");
    }
    @Override
    public void attributeRemoved(HttpSessionBindingEvent httpSessionBindingEvent) {
        System.out.println("Session Removed");
    }
    @Override
    public void attributeReplaced(HttpSessionBindingEvent httpSessionBindingEvent) {
        System.out.println("Session Replaced");
    }
}

Servlet测试

/**
 * 注意：销毁和移除都能触发监听器的attributeRemoved方法
 */
@WebServlet(name = "TestSessionAttributeServlet", value = "/tsas")
public class TestSessionAttributeServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        HttpSession session = request.getSession();
        StringBuffer buffer = new StringBuffer();
        buffer.append("ziph");
        //添加
        session.setAttribute("name", buffer);
        buffer.append(" is very good!");
        //替换
        session.setAttribute("name", buffer);
        //移除
        session.removeAttribute("name");
        //销毁
        session.invalidate();
    }
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request, response);
    }
}

监听Request属性

• 实现ServletRequestAttributeListener接口并覆盖方法
• 监听ServletRequest中的属性变更

/**
 * 监听Request域中的属性变更
 */
@WebListener
public class TestServletRequestAttributeListener implements 
ServletRequestAttributeListener {
    @Override
    public void attributeAdded(ServletRequestAttributeEvent servletRequestAttributeEvent) {
        System.out.println("Request Added");
    }
    @Override
    public void attributeRemoved(ServletRequestAttributeEvent servletRequestAttributeEvent) {
        System.out.println("Request Removed");
    }
    @Override
    public void attributeReplaced(ServletRequestAttributeEvent servletRequestAttributeEvent) {
        System.out.println("Request Replaced");
    }
}

Servlet测试

@WebServlet(name = "TestServletRequestAttributeServlet", value = "/tsras")
public class TestServletRequestAttributeServlet extends HttpServlet {
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        StringBuffer buffer = new StringBuffer();
        buffer.append("ziph");
        //添加
        request.setAttribute("name", buffer);
        buffer.append("marry");
        //替换
        request.setAttribute("name", buffer);
        //移除
        request.removeAttribute("name");
    }
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request, response);
    }
}

三类监听器

HttpSessionBindingListener监听HttpSession中的JavaBean的状态(绑定和解除绑定状态)
注意：该监听器不需要xml配置和注解配置

/**
 * 三类监听器HttpSessionBingingListener
 */
public class User implements HttpSessionBindingListener {
    private int id;
    private String username;
    private String password;
    public User() {
    }
    public User(int id, String username, String password) {
        this.id = id;
        this.username = username;
        this.password = password;
    }
    @Override
    public void valueBound(HttpSessionBindingEvent httpSessionBindingEvent) 
{
        System.out.println("User对象绑定");
    }
    @Override
    public void valueUnbound(HttpSessionBindingEvent 
httpSessionBindingEvent) {
System.out.println("User对象解绑");
    }
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getUsername() {
        return username;
    }
    public void setUsername(String username) {
        this.username = username;
    }
    public String getPassword() {
        return password;
    }
    public void setPassword(String password) {
        this.password = password;
    }
    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", username='" + username + '\'' +
                ", password\='" + password + '\'' +
                '}';
    }
}

Servlet测试

@WebServlet(name = "TestHttpSessionBingingListenerServlet", value = "/thsbls")
public class TestHttpSessionBingingListenerServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        HttpSession session = request.getSession();
        //绑定
        session.setAttribute("user", new User(1, "ziph", "123456"));
        //解绑
        session.removeAttribute("user");
        //销毁也是解绑的一种
//        session.invalidate();
    }
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request, response);
    }
}