servlet：共享资源造成的线程冲突
一，由于servlet是单实例的类，在每个新客户端访问时创建一个新的线程，并在这个线程上调用Servlet的service方法，因此service方法内如果访问了同一个资源的话，就有可能引发线程安全问题。
例：
1，使用一个简单的登陆页面提交用户名，密码
<%@ page language="java" import="java.util.*" contentType="text/html; charset=utf-8"%> <% String path = request.getContextPath(); String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/"; %> <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <head> <base href="<%=basePath%>"> <title>My JSP 'login.jsp' starting page</title> <meta http-equiv="pragma" content="no-cache"> <meta http-equiv="cache-control" content="no-cache"> <meta http-equiv="expires" content="0"> <meta http-equiv="keywords" content="keyword1,keyword2,keyword3"> <meta http-equiv="description" content="This is my page"> <!-- <link rel="stylesheet" type="text/css" href="styles.css"> --> </head> <body> <h1>登陆</h1> <hr> <form action="test.do" name="loginform" method="post"> <table> <tr> <td>用户名</td> <td><input type="text" name="username"/></td> </tr> <tr> <td>密码</td> <td><input type="password" name="password"/></td> </tr> <tr> <td colspan="2"><input type="submit" value="登陆"/></td> </tr> </table> </form> </body> </html>
2，创建一个servlet文件
在servlet中创建两个全局变量用于接收用户名和密码
例：
package aaa; import java.io.IOException; import java.io.PrintWriter; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; public class Ddd extends HttpServlet { /** * Constructor of the object. */ public Ddd() { super(); } /** * Destruction of the servlet. <br> */ public void destroy() { super.destroy(); // Just puts "destroy" string in log // Put your code here } /** * The doGet method of the servlet. <br> * * This method is called when a form has its tag value method equals to get. * * @param request the request send by the client to the server * @param response the response send by the server to the client * @throws ServletException if an error occurred * @throws IOException if an error occurred */ public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html"); PrintWriter out = response.getWriter(); out.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">"); out.println("<HTML>"); out.println(" <HEAD><TITLE>A Servlet</TITLE></HEAD>"); out.println(" <BODY>"); out.print(" This is "); out.print(this.getClass()); out.println(", using the GET method"); out.println(" </BODY>"); out.println("</HTML>"); out.flush(); out.close(); } /** * The doPost method of the servlet. <br> * * This method is called when a form has its tag value method equals to post. * * @param request the request send by the client to the server * @param response the response send by the server to the client * @throws ServletException if an error occurred * @throws IOException if an error occurred */ String Username; String Password; public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { Username=request.getParameter("username"); Password=request.getParameter("password"); Thread a=new Thread(){ public void run(){ try { //延迟4秒突出线程冲突 Thread.sleep(4000); } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); } } }; request.setAttribute("user", this.Password+""+this.Username); request.getRequestDispatcher("index.jsp").forward(request, response); } /** * Initialization of the servlet. <br> * * @throws ServletException if an error occurs */ public void init() throws ServletException { // Put your code here } }
3，创建一个接收页面显示用户名和密码
例：
<%@ page language="java" import="java.util.*" contentType="text/html; charset=utf-8"%> <% String path = request.getContextPath(); String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/"; %> <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <head> <base href="<%=basePath%>"> <title>My JSP 'index.jsp' starting page</title> <meta http-equiv="pragma" content="no-cache"> <meta http-equiv="cache-control" content="no-cache"> <meta http-equiv="expires" content="0"> <meta http-equiv="keywords" content="keyword1,keyword2,keyword3"> <meta http-equiv="description" content="This is my page"> <!-- <link rel="stylesheet" type="text/css" href="styles.css"> --> </head> <body> <h1> <%String a=(String)request.getAttribute("user"); out.print(a); %> </h1><br> </body> </html>
如果同时打开多个登陆页面进行登陆会造成servlet的线程冲突
3，解决方法：
可以通过解决java多线程的冲突的方式进行解决
synchronized(this){ Username=request.getParameter("username"); Password=request.getParameter("password"); Thread a=new Thread(){ public void run(){ try { //延迟4秒突出线程冲突 Thread.sleep(4000); } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); } } }; }
通过在共享的代码块上锁解决
