当服务器启动的时候，服务器的引擎就会创建一个Servlet的实例对象。系统只会创建一个直到服务区停止。每次请求都会开启一个新的线程进行处理，每次都会访问该对象的 service方法。当设计到共享资源的时候就会出现线程安全的问题。

/**
 * Servlet implementation class ServletDemo1
 */
public class ServletDemo1 extends HttpServlet {
    private static final long serialVersionUID = 1L;
    int  num=1;
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            num++;
            try {
                Thread.sleep(6000);
            } catch (InterruptedException e) {
            }
            System.out.println(num);
            response.getWriter().append(num+"");
    }
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

不同请求，不同线程都用到了 num 这个变量，所以会出现线程安全的问题。

解决办法1

/**
 * Servlet implementation class ServletDemo1
 */
public class ServletDemo1 extends HttpServlet {
    private static final long serialVersionUID = 1L
      int  num=1;
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //同步鎖   锁住这个对象  当
        synchronized (this) {
            num++;
            try {
                Thread.sleep(6000);
            } catch (InterruptedException e) {
                 
            }
            System.out.println(num);
            response.getWriter().append(num+"");
        }
        
    }    
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

加上锁就行了。当有人访问该对象的时候，锁住。执行完后进行释放次个锁，给别人用再 。就不会涉及到线程安全的问题了。

解决办法2

public class ServletThreadPoolDemo extends HttpServlet implements SingleThreadModel{
    
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
         
        response.getWriter().append("Served at: ").append(request.getContextPath());
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
         
        doGet(request, response);
    }
}

让 servlet去实现SingleThreadModel接口（标记性接口，不用实现任何东西） 就标注下次servlet 访问的方式是单线程的访问 ，不存在并发的现象了。但是次接口目前已经废除，不建议使用。