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...