Servlet线程安全问题

测试Servlet是否线程安全

测试代码

package com.servlet.study;

import java.io.IOException;

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


public class ThreadSafeTestServlet extends HttpServlet{

	/**
	 * 
	 */
	private static final long serialVersionUID = -5678315968996207090L;
	
	private String name;
	
	@Override
	protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		resp.setContentType("text/html");
		resp.setCharacterEncoding("utf-8");
		// TODO Auto-generated method stub
		System.out.println(this+":"+Thread.currentThread());
		name = req.getParameter("user");
		try {
			Thread.sleep(2000);
		} catch (InterruptedException e1) {
			// TODO Auto-generated catch block
		}
		resp.getWriter().println("用户名："+name);;
		System.out.println("Counter = "+name);
	}
	
	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		// TODO Auto-generated method stub
		doPost(req, resp);
	}
	
}

页面请求：

请求1：http://localhost:8080/TESTWeb/thread?user=lisi

请求2：http://localhost:8080/TESTWeb/thread?user=wangw

输出结果：

com.servlet.study.ThreadSafeTestServlet@5dcc783c:Thread[http-nio-8080-exec-27,5,main]

com.servlet.study.ThreadSafeTestServlet@5dcc783c:Thread[http-nio-8080-exec-24,5,main]

Counter = wangw

请求1的输出，被请求2的name改写了，用户lisi的页面显示wangw的信息。

结论：

1、Servlet对象是一个无状态的单例对象（Singleton）,因为请求多次，打印出来的hashcode值都相同。

2、Servlet在不同的线程池中运行，如上的Thread[http-nio-8080-exec-27和 24 区分出不同的线程执行了同一段Servlet逻辑代码。

3、name变量在不同的线程中共享，且它的值被不同的线程修改。输出时，已被其它线程修改后的值。

也就是说，其它线程会篡改当前线程中实例变量的值，针对这些对象的访问不是线程安全的。

解决方式一：

使用局部变量，代替全局变量。

package com.servlet.study;

import java.io.IOException;

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


public class ThreadSafeTestServlet extends HttpServlet{

	/**
	 * 
	 */
	private static final long serialVersionUID = -5678315968996207090L;
	
	
	@Override
	protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		String name;
		resp.setContentType("text/html");
		resp.setCharacterEncoding("utf-8");
		// TODO Auto-generated method stub
		System.out.println(this+":"+Thread.currentThread());
		name = req.getParameter("user");
		try {
			Thread.sleep(2000);
		} catch (InterruptedException e1) {
			// TODO Auto-generated catch block
		}
		resp.getWriter().println("用户名："+name);;
		System.out.println("Counter = "+name);
	}
	
	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		// TODO Auto-generated method stub
		doPost(req, resp);
	}
	
}

输出：

com.servlet.study.ThreadSafeTestServlet@53a2686d:Thread[http-nio-8080-exec-44,5,main]

com.servlet.study.ThreadSafeTestServlet@53a2686d:Thread[http-nio-8080-exec-39,5,main]

Counter = wangw

Counter = lisi

解决方式二：

使用synchorized关键字，保证一次只有一个线程访问保护的区域块。

private String name;
	
	@Override
	protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		resp.setContentType("text/html");
		resp.setCharacterEncoding("utf-8");
		// TODO Auto-generated method stub
		System.out.println(this+":"+Thread.currentThread());
		synchronized(this){
			
			name = req.getParameter("user");
			try {
				Thread.sleep(2000);
			} catch (InterruptedException e1) {
				// TODO Auto-generated catch block
			}
			resp.getWriter().println("用户名："+name);;
			System.out.println("Counter = "+name);
		}
	}

页面请求：

http://localhost:8080/TESTWeb/thread?user=lisi

http://localhost:8080/TESTWeb/thread?user=wangw

输出结果：

信息: Reloading Context with name [/TESTWeb] is completed

com.servlet.study.ThreadSafeTestServlet@3fadc5ba:Thread[http-nio-8080-exec-8,5,main]

com.servlet.study.ThreadSafeTestServlet@3fadc5ba:Thread[http-nio-8080-exec-4,5,main]

Counter = wangw

Counter = lisi

Servlet线程安全问题

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