java-监听器(Listener)

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监听器：用于监听web应用中某些对象、信息的创建、销毁等动作，服务器会自动调用相应的方法进行处理。常用于统计在线人数，初始化系统参数等。

Javaweb监听器主要监听对象有ServletContext、HttpSession、ServletRequest。

下面是贴上一个统计登录人信息的栗子：

　　在web.xml 中配置监听器：

<!--用户登录监听器-->
    <listener>
        <listener-class>com.xxx.listener.SessionListener</listener-class>
    </listener>

　　创建监听器：

import javax.servlet.ServletRequestEvent;
import javax.servlet.ServletRequestListener;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
import java.util.*;

public class SessionListener implements HttpSessionListener,ServletRequestListener {
    // 定义监听类对象，用于存储所有用户的登录信息
    private static Map<String,String> userList = new HashMap<String, String>();
    // 定义session  map对象
    private static Map<String,HttpSession> sessionMap = new HashMap<String, HttpSession>();

    private HttpServletRequest request = null;

    @Override
    public void sessionCreated(HttpSessionEvent httpSessionEvent) {// 监听 session 创建

    }

    @Override
    public void sessionDestroyed(HttpSessionEvent httpSessionEvent) {// 监听 session 销毁
        HttpSession httpSession = httpSessionEvent.getSession();
        // 获取登录用户名
        String username = (String) httpSession.getAttribute("username");
        if(username != null){
            Iterator iterator = userList.entrySet().iterator();
            Map map = null;
            String logmsg = null;
            // 遍历 userList：将userlist中的用户信息删除
            while (iterator.hasNext()){
                map = (Map) iterator.next();
                if(map.get(username) != null){
                    logmsg = (String) map.get(username);
                    if(logmsg.indexOf(request.getLocalAddr()) > -1){
                        userList.remove(username);
                        sessionMap.remove(username);
                        break;
                    }
                }
            }
        }
    }

    @Override
    public void requestDestroyed(ServletRequestEvent servletRequestEvent) {// 监听 request 销毁

    }

    @Override
    public void requestInitialized(ServletRequestEvent servletRequestEvent) {// 监听 request 创建
        this.request = (HttpServletRequest) servletRequestEvent.getServletRequest();
        HttpSession httpSession = this.request.getSession();
        // 获取登录用户名
        String username = (String) httpSession.getAttribute("username");
        if(username != null){
            // 如果 userList为空直接 put
            if(userList.isEmpty()){
                userList.put(username,"IP: " + request.getLocalAddr() + " ,logtime:" + ProUtil.getStrDatetime(new Date()));
                sessionMap.put(username,httpSession);
                return;
            }
            String logmsg = null;
            if(userList.get(username) == null){
                userList.put(username,"IP: " + request.getLocalAddr() + " ,logtime:" + ProUtil.getStrDatetime(new Date()));
                sessionMap.put(username,httpSession);
            }else {
                logmsg = userList.get(username);
                if(logmsg.indexOf(request.getLocalAddr()) > -1){
                    userList.put(username,"IP: " + request.getLocalAddr() + " ,logtime:" + ProUtil.getStrDatetime(new Date()));
                    sessionMap.put(username,httpSession);
                }else {
                    userList.put(username,"IP: " + request.getLocalAddr() + " ,logtime:" + ProUtil.getStrDatetime(new Date()));
                    sessionMap.get(username).setAttribute("logError","该账号已经在其他地方登录!!!");
                    sessionMap.put(username,httpSession);
                }
            }
        }
    }
}