servlet多线程安全问题

Servelet多线程安全问题

原因

一个servlet被实例化一次，当有多个用户访问时，多个线程会访问同一个实例，实例变量就会被不同的用户修改。

简单的案例

新建servlet,访问http://localhost:8080/serverlet/servlert1?u=aaa，即可看到aaa输出

public class servlert1 extends HttpServlet {
    String username;   //实例变量username
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        username=request.getParameter("u");
        
        //睡眠5s，模拟多并发访问
        try {
            Thread.sleep(5000);
        } catch (InterruptedException e) {  
            e.printStackTrace();
        }
        
        //输出u接收到的参数
        response.getOutputStream().write((username).getBytes());
    }
}

A访问http://localhost:8080/serverlet/servlert1?u=aaa
B立马访问http://localhost:8080/serverlet/servlert1?u=bbb

A输出bbb(A的数据被B修改了)，B也输出BBB

解决

避免实例变量

public class servlert1 extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
         String username;
        username=request.getParameter("u");
        
        //睡眠5s，模拟多并发访问
        try {
            Thread.sleep(5000);
        } catch (InterruptedException e) {  
            e.printStackTrace();
        }
        
        //输出u接收到的参数
        response.getOutputStream().write((username).getBytes());
    }
}

Servlet 实现SingleThreadModel接口

public class servlert1 extends HttpServlet implements SingleThreadModel{
    String username;   //实例变量username
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        username=request.getParameter("u");
        
        //睡眠5s，模拟多并发访问
        try {
            Thread.sleep(5000);
        } catch (InterruptedException e) {  
            e.printStackTrace();
        }
        
        //输出u接收到的参数
        response.getOutputStream().write((username).getBytes());
    }
}

同步化

public class servlert1 extends HttpServlet {
    String username;   //实例变量username
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        
        //同步访问
        synchronized (this) {
        username=request.getParameter("u");
        //睡眠5s，模拟多并发访问
        try {
            Thread.sleep(5000);
        } catch (InterruptedException e) {  
            e.printStackTrace();
        }
        
        //输出u接收到的参数
        response.getOutputStream().write((username).getBytes());
        }
    }
}