Servlet单例模式（注意）

 package com.servlet;

 import java.io.IOException;

 import javax.servlet.ServletException;
 import javax.servlet.http.HttpServlet;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;

 /**
  * Servlet implementation class Test
  */
 public class Test extends HttpServlet {
     private static final long serialVersionUID = 1L;
     private long abc = 123;
     ThreadLocal<String> local = new ThreadLocal<String>();
     /**
      * @see HttpServlet#HttpServlet()
      */
     public Test() {
         super();
         // TODO Auto-generated constructor stub
     }

     /**
      * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
      */
     protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
         // TODO Auto-generated method stub
         System.out.println(abc);
         abc = 134;

     }

     /**
      * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
      */
     protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
         // TODO Auto-generated method stub
     }

 }

servlet是单例模式 其内部定义的变量是所有用户共享的（多线程共享） 当一个用户改变了这个变量的值，其他用户访问得到的是改变之后的值
servlet中的变量最好放在map中 线程安全的map ConcurrentHashMap(),以sessionid为主键存放，这样每个用户(线程)都会有单独属于自己的一个值
不会受到其他用户的影响，

使用ThreadLocal时 每次访问servlet 都是一个新的线程，ThreadLocal就是这个线程所特有的...