在登陸後顯示還有哪些使用者線上，用一個列表顯示出來，算是完善了之前寫的程式碼的一個小功能吧，以後還會繼續。

一個是在LoginCheck的servlet裡面，把登陸成功的使用者用List的形式存進application裡

package servlet;

import java.io.IOException;
import java.io.PrintWriter;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.util.ArrayList
 
;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import com.apple
 
.eawt.Application;

import pub.GetDataBaseConnection;
import userPOJO.UserPOJO;

public class LoginCheck extends HttpServlet{

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        this.doPost(request, response);
 


    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=utf-8");
        PrintWriter out = response.getWriter();//取得out物件
        String username = request.getParameter("username");
        String password = request.getParameter("password");
          boolean flag = false;
          Connection conn = null;
          PreparedStatement pstate = null;
          ResultSet res = null;
          try{
            conn = GetDataBaseConnection.getConnection();
            String sql = "select user_id,headicon from usert where username = ? and password = ?";
            pstate = conn.prepareStatement(sql);
            pstate.setString(1,username);
            pstate.setString(2,password);
            res = pstate.executeQuery();
            while(res.next()){  
                    Cookie c[] = request.getCookies();
                    if(null == c || c.length < 2){//如果cookie不存在或者小於2就新增使用者名稱和密碼的cookie用於一分鐘內免登陸
                    Cookie cook_name = new Cookie("username",username);
                    cook_name.setMaxAge(60);
                    Cookie cook_pass = new Cookie("password",password);
                    cook_pass.setMaxAge(60);
                    response.addCookie(cook_name);
                    response.addCookie(cook_pass);

                    int userId = res.getInt(1);
                    String headicon = res.getString(2);
                    UserPOJO userpojo = new UserPOJO(userId,username,password,headicon);
                    HttpSession session = request.getSession(true);
                    session.setAttribute("userpojo", userpojo);//設定使用者屬性範圍
                    out.print("<SCRIPT language=javascript>alert('登入成功');window.location='jsp/Main.jsp';</script>");    
                    flag = true;        

                    String userName = request.getParameter("username");  
                    ServletContext application = request.getServletContext();  
                    ArrayList loginList = (ArrayList)application.getAttribute("loginlist");  
                    if(loginList == null){  
                      loginList = new ArrayList();  
                      application.setAttribute("loginlist",loginList);  
                    }  
                    loginList.add(userName);  
                    request.getRequestDispatcher("/jsp/Main.jsp").forward(request,response);  
                    }
            }
            if(!flag){
            out.print("<SCRIPT language=javascript>alert('使用者名稱或密碼不正確，請重新輸入！'); window.location='jsp/Login.jsp';</script>");
            }
            }catch(Exception e){
              e.printStackTrace();
          }finally{
              try{
                  res.close();
                  pstate.close();
                  conn.close();
              }catch(Exception e){
                  e.printStackTrace();
              }
          }
    }

}

然後就是在main.jsp裡面把list給迭代出來了

<%@page contentType="text/html; charset=utf-8" %>
<%@page import="java.util.*" %>
<%Cookie c[] = request.getCookies();
if (null != c) {
} %>
<html>
<head><title><h1>主介面</h1></title></head>
<body>
    <img src="<%=request.getContextPath() %>/headicon/${userpojo.headicon}" name="headicon" alt="頭像" />
    <h1>${userpojo.userName},你好</h1>
    <h3><a href="<%=request.getContextPath() %>/jsp/Logout.jsp" onClick="{if(confirm('確定要登出嗎？')){return true;}return false;}">登出</a></h3>
    <a href="<%=request.getContextPath() %>/jsp/UpdateInfo.jsp">修改資料</a>
    <h1>線上人員列表</h1>
<hr/>
<%
    ArrayList<String> list = (ArrayList<String>)application.getAttribute("loginlist");
    Iterator<String> it = list.iterator();
    while(it.hasNext()){

%>
    <li><%=it.next()%></li>

<%
    }
 %>
</body>
</html>

在完善過程中，發現要把專案部署進tomcat裡面的server.xml裡面，不然上傳的圖片不能儲存進專案裡，而是儲存在了tomcat裡面的webapps裡。