Java 监听器简介
监听器是Servlet规范定义的一种特殊类，用于监听ServletContext，HttpSession,ServletRequest等域对象的创建、销毁及其属性修改发生变化的事件。监听器可以在事件发生前后进行一些必要的处理操作。
常见的应用场景
- 统计在线人数和在线用户
- 应用启动时完成信息初始化工作
- 网站访问量的统计
- 与Spring结合
- Web系统防止用户重复登陆
监听器的实现步骤
1.编写java类实现监听器接口，并实现其接口方法
2.在web.xml文件中对实现的监听器类进行注册
Java监听器按监听事件分类
- 域对象自身的创建和销毁事件监听器
- 域对象中属性的创建、替换和消除事件监听器
- 绑定到session中某个对象的状态事件监听器（不需要在web.xml中注册）
app_name
Listener Web
version
1.0
com.listener.MyServletContextListener
public class MyServletContextListener implements ServletContextListener {
@Override
public void contextDestroyed(ServletContextEvent sce) {
String appName = (String)sce.getServletContext().getAttribute("app_name");
String version = (String)sce.getServletContext().getAttribute("version");
System.out.println("MyServletContextListener destroyed,appName:" + appName + ",version:" + version);
}
@Override
public void contextInitialized(ServletContextEvent sce) {
String appName = sce.getServletContext().getInitParameter("app_name");
String version = sce.getServletContext().getInitParameter("version");
sce.getServletContext().setAttribute("app_name", appName);
sce.getServletContext().setAttribute("version", version);
System.out.println("MyServletContextListener init,appName:" + appName + ",version:" + version);
}
}
public class MyHttpSessionListener implements HttpSessionListener {
@Override
public void sessionCreated(HttpSessionEvent hse) {
String sessionId = hse.getSession().getId();
Date createTime = new Date(hse.getSession().getCreationTime());
System.out.println("session id:"+sessionId+",createTime:"+createTime);
}
@Override
public void sessionDestroyed(HttpSessionEvent hse) {
String sessionId = hse.getSession().getId();
System.out.println("session destroyed,session id:"+sessionId);
}
}
com.listener.MyHttpSessionListener
1
访问index.jsp，创建一个会话，后台输出sessionId,再用另一个浏览器访问，就会再创建一个sessionId，一分钟后销毁。
public class MyServletRequestListener implements ServletRequestListener {
@Override
public void requestDestroyed(ServletRequestEvent sre) {
System.out.println("request listener,request destroyed... ...");
}
@Override
public void requestInitialized(ServletRequestEvent sre) {
HttpServletRequest request = (HttpServletRequest) sre.getServletRequest();
String path = request.getRequestURI();
String par = request.getParameter("par");
System.out.println("request listener,path:" + path + ",par is :" + par);
}
}
com.listener.MyServletRequestListener
首页
public class MyServletContextAttributeListener implements ServletContextAttributeListener {
@Override
public void attributeAdded(ServletContextAttributeEvent scae) {
System.out.println("ServletContext#attAdded#name:" + scae.getName() + "#value:" + scae.getValue());
}
@Override
public void attributeRemoved(ServletContextAttributeEvent scae) {
System.out.println("ServletContext#attRemoved#name:" + scae.getName() + "#value:" + scae.getValue());
}
@Override
public void attributeReplaced(ServletContextAttributeEvent scae) {
System.out.println("ServletContext#attReplaced#name:" + scae.getName() + "#value:" + scae.getValue());
}
}
com.listener.MyServletContextAttributeListener
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<%
application.setAttribute("servletcontextPar", "servletcontext_par");
//application.removeAttribute("servletcontextPar");
//application.setAttribute("servletcontextPar", "servletcontext_par1");
%>
attribute_add_listener_test
attribute_removed_listener_test
attribute_replaced_listener_test
public class User implements HttpSessionBindingListener {
private String username;
private String password;
@Override
public void valueBound(HttpSessionBindingEvent hsbe) {
String name = hsbe.getName();
System.out.println("value bound,name:"+name);
}
@Override
public void valueUnbound(HttpSessionBindingEvent hsbe) {
String name = hsbe.getName();
System.out.println("value unbound,name:"+name);
}
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;
}
}
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<%
session.setAttribute("user", new com.model.User());
// session.removeAttribute("user");
%>
binding_listener_test
unbinding_listener_test
Java监听器项目案例
- 通过过滤器实现登录控制，未登录用户不能访问系统首页
- 用户登录，将登录名存储到session里
- 登录监听器监听session属性中登录值属性变化
- 若登录用户用户名已登录系统，清除前次登录信息
- 记录该次登录信息至缓存中，用于下次登录判断
登陆权限过滤验证
public class SessionFilter implements Filter {
@Override
public void destroy() {
}
@Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
throws IOException, ServletException {
HttpServletRequest hrequest = (HttpServletRequest)request;
HttpServletResponse hresponse = (HttpServletResponse)response;
String loginUser = (String)hrequest.getSession().getAttribute("loginUser");//从session对象中获取登录用户名
if(loginUser==null){//登录用户名不存在，用户未登录，强制重定向至登陆页面
hresponse.sendRedirect(hrequest.getContextPath()+"/index.jsp?flag=1");
return;
}else{
chain.doFilter(request, response);//已登录，转入相应的请求处理
return;
}
}
@Override
public void init(FilterConfig arg0) throws ServletException {
}
}
用户信息存储实现
/** 这里用到了单例模式 **/
public class LoginCache {
private static LoginCache instance = new LoginCache();
/**servlet api 3.0版本中HttpSessionContext中的httpSession不支持直接通过sessionId获得Session对象**/
/** 因此通过用户名获取sessionId，然后通过sessionId获取session**/
private Map loginUserSession = new HashMap();// key值：登录用户登录名，value值：登录用户sessionId
private Map loginSession = new HashMap();//key值:登录用户sessionId,value值：登录用户session对象
private LoginCache(){
}
public static LoginCache getInstance(){
return instance;
}
/**
* 通过登录名获取对应登录用户的sessionId
* @param username
* @return
*/
public String getSessionIdByUsername(String username){
return loginUserSession.get(username);
}
/**
* 通过sessionId获取对应的session对象
* @param sessionId
* @return
*/
public HttpSession getSessionBySessionId(String sessionId){
return loginSession.get(sessionId);
}
/**
* 存储登录名与对应的登录sessionID至缓存对象
* @param username
* @param sessionId
*/
public void setSessionIdByUserName(String username,String sessionId){
loginUserSession.put(username, sessionId);
}
/**
* 存储sessionId与对应的session对象至缓存对象
* @param sessionId
* @param session
*/
public void setSessionBySessionId(String sessionId,HttpSession session){
loginSession.put(sessionId, session);
}
}
监听功能实现
public class LoginSessionListener implements HttpSessionAttributeListener {
private static final String LOGIN_USER="loginUser";
@Override
public void attributeAdded(HttpSessionBindingEvent hsbe) {
String attrName = hsbe.getName();//监听到session属性值发生添加操作，获取对应操作的属性名
if(LOGIN_USER.equals(attrName)){//若属性名为登录属性名，判定为用户登录操作
String attrVal = (String)hsbe.getValue();//获取添加的属性值，即用户登录名
HttpSession session = hsbe.getSession();//该次操作的session对象
String sessionId = session.getId();//该次操作的session对象ID
String sessionId2 = LoginCache.getInstance().getSessionIdByUsername(attrVal);//从缓存对象里面，获得该用户登录名对应的sessionID值
if(null == sessionId2){//未获得结果，不需要清理前次登录用户会话信息
}else{
HttpSession session2 = LoginCache.getInstance().getSessionBySessionId(sessionId2);//获取前次该用户登录对应的session对象
session2.invalidate();//清理前次登录用户会话存储信息，使得前次登录失效
}
//完成该次登录用户登录名、sessionID，session对象的缓存对象存储
LoginCache.getInstance().setSessionIdByUserName(attrVal, sessionId);
LoginCache.getInstance().setSessionBySessionId(sessionId, session);
}
}
@Override
public void attributeRemoved(HttpSessionBindingEvent arg0) {
// TODO Auto-generated method stub
}
@Override
public void attributeReplaced(HttpSessionBindingEvent arg0) {
// TODO Auto-generated method stub
}
}
web.xml
com.listener.LoginSessionListener
sessionFilter
com.filter.SessionFilter
sessionFilter
/main.jsp
index.jsp
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<%
String flag = request.getParameter("flag");
%>
Login
login.jsp
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<%
String username = request.getParameter("username");
String password = request.getParameter("password");
session.setAttribute("loginUser", username);//登录完成，将登录用户名存储至session对象
response.sendRedirect(request.getContextPath()+"/main.jsp");
%>
main.jsp
<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<%
String user = (String)session.getAttribute("loginUser");
%>
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