Servlet的线程安全问题
2015-10-28 11:12
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我们知道当多个客户端并发访问同一个Servlet时，web服务器会为每一个客户端的访问请求创建一个线程，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，看下面代码：
如果是下面这种情况就会存在线程安全问题了
首先我们想到的是加synchronized，并发访问i时就不存在线程安全问题了。为什么加了synchronized后就没有线程安全问题了呢？假如现在有一个线程访问Servlet对象，那么它就先拿到了Servlet对象的那把锁等到它执行完之后才会把锁还给Servlet对象，由于是它先拿到了Servlet对象的那把锁，所以当有别的线程来访问这个Servlet对象时，由于锁已经被之前的线程拿走了，后面的线程只能排队等候了。这种做法虽然解决了线程安全问题，但是编写Servlet却万万不能用这种方式处理线程安全问题，假如有5000个人同时访问这个Servlet，那么这5000个人必须按先后顺序排队轮流访问。这样效率很低。
针对Servlet的线程安全问题，Sun公司是提供有解决方案的：让Servlet去实现一个SingleThreadModel接口，如果某个Servlet实现了SingleThreadModel接口，那么Servlet引擎将以单线程模式来调用其service方法。
对于实现了SingleThreadModel接口的Servlet，Servlet引擎仍然支持对该Servlet的多线程并发访问，其采用的方式是产生多个Servlet实例对象，并发的每个线程分别调用一个独立的Servlet实例对象。
实现SingleThreadModel接口并不能真正解决Servlet的线程安全问题，因为Servlet引擎会创建多个Servlet实例对象，而真正意义上解决多线程安全问题是指一个Servlet实例对象被多个线程同时调用的问题。事实上，在Servlet API 2.4中，已经将SingleThreadModel标记为Deprecated（过时的）。
ThreadLocal实现
执行结果：
1 服务器对每个Servlet只创建一个实例。flag不停增加
2 Session范围内的ThreadLocal中对象唯一。不同的请求，Object的hashCode相同。
3 不同的Session共享ThreadLocal，但内部对象不同
package com.oec;
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 Servlet extends HttpServlet {
/**
* Constructor of the object.
*/
public Servlet() {
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");
int i =1; //定义一个局部变量i
i++; //i增加
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(i); //输出i的值
out.println(", using the GET method");
out.println(" </BODY>");
out.println("</HTML>");
out.flush();
out.close();
/**
* 在这段代码中，多线程并发访问时，不会存在线程安全问题，因为i是doGet方法的局部变量。
* 当有多个线程并发访问doGet方法时，每一个线程里面都有自己的i变量，每一个线程都操作自己的i所以不存在
*/
}
/**
* 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
*/
public void doPost(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 POST method");
out.println(" </BODY>");
out.println("</HTML>");
out.flush();
out.close();
}
/**
* Initialization of the servlet. <br>
*
* @throws ServletException if an error occurs
*/
public void init() throws ServletException {
// Put your code here
}
}如果是下面这种情况就会存在线程安全问题了
package com.oec;
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 Servlet extends HttpServlet {
int i =1; //定义成全局变量
/**
* Constructor of the object.
*/
public Servlet() {
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");
i++; //i增加
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(i); //输出i的值
out.println(", using the GET method");
out.println(" </BODY>");
out.println("</HTML>");
out.flush();
out.close();
/**
* 把i定义成全局变量，当多个线程并发访问变量i时，就会存在线程安全问题了，
* 同时开启两个浏览器模拟并发访问同一个Servlet，本来正常来说，第一个浏览器应该看到2，
* 而第二个浏览器应该看到3的，结果两个浏览器都看到了3
*/
}
/**
* 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
*/
public void doPost(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 POST method");
out.println(" </BODY>");
out.println("</HTML>");
out.flush();
out.close();
}
/**
* Initialization of the servlet. <br>
*
* @throws ServletException if an error occurs
*/
public void init() throws ServletException {
// Put your code here
}
}线程安全问题只存在多个线程并发操作同一个资源的情况下，所以在编写Servlet的时候，如果并发访问某一个资源(变量，集合等)，就会存在线程安全问题，那么该如何解决这个问题。首先我们想到的是加synchronized，并发访问i时就不存在线程安全问题了。为什么加了synchronized后就没有线程安全问题了呢？假如现在有一个线程访问Servlet对象，那么它就先拿到了Servlet对象的那把锁等到它执行完之后才会把锁还给Servlet对象，由于是它先拿到了Servlet对象的那把锁，所以当有别的线程来访问这个Servlet对象时，由于锁已经被之前的线程拿走了，后面的线程只能排队等候了。这种做法虽然解决了线程安全问题，但是编写Servlet却万万不能用这种方式处理线程安全问题，假如有5000个人同时访问这个Servlet，那么这5000个人必须按先后顺序排队轮流访问。这样效率很低。
针对Servlet的线程安全问题，Sun公司是提供有解决方案的：让Servlet去实现一个SingleThreadModel接口，如果某个Servlet实现了SingleThreadModel接口，那么Servlet引擎将以单线程模式来调用其service方法。
package com.oec;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.SingleThreadModel;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class Servlet extends HttpServlet implements SingleThreadModel{
int i =1; //定义成全局变量
/**
* Constructor of the object.
*/
public Servlet() {
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");
i++; //i增加
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(i); //输出i的值
out.println(", using the GET method");
out.println(" </BODY>");
out.println("</HTML>");
out.flush();
out.close();
/**
* 把i定义成全局变量，当多个线程并发访问变量i时，就会存在线程安全问题了，
* 同时开启两个浏览器模拟并发访问同一个Servlet，本来正常来说，第一个浏览器应该看到2，
* 而第二个浏览器应该看到3的，结果两个浏览器都看到了3
*/
}
/**
* 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
*/
public void doPost(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 POST method");
out.println(" </BODY>");
out.println("</HTML>");
out.flush();
out.close();
}
/**
* Initialization of the servlet. <br>
*
* @throws ServletException if an error occurs
*/
public void init() throws ServletException {
// Put your code here
}
} 让Servlet实现了SingleThreadModel接口，只要在Servlet类的定义中增加实现SingleThreadModel接口的声明即可。对于实现了SingleThreadModel接口的Servlet，Servlet引擎仍然支持对该Servlet的多线程并发访问，其采用的方式是产生多个Servlet实例对象，并发的每个线程分别调用一个独立的Servlet实例对象。
实现SingleThreadModel接口并不能真正解决Servlet的线程安全问题，因为Servlet引擎会创建多个Servlet实例对象，而真正意义上解决多线程安全问题是指一个Servlet实例对象被多个线程同时调用的问题。事实上，在Servlet API 2.4中，已经将SingleThreadModel标记为Deprecated（过时的）。
ThreadLocal实现
package com.oec;
import java.io.IOException;
import java.io.PrintWriter;
import java.net.InetSocketAddress;
import java.util.concurrent.Executor;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import com.sun.net.httpserver.HttpContext;
import com.sun.net.httpserver.HttpHandler;
public class TestThreadServlet extends HttpServlet{
private static ThreadLocal thread = new ThreadLocal();
private int flag = 0;
public void doGet( HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
flag++;
String str = "This is the first String." + new Object();
if (thread.get() == null)
thread.set(str);
PrintWriter out = response.getWriter();
out.println("<p>");
out.println("<BR>flag : " + flag);
out.println("<BR>sessionid : " + request.getSession().getId());
out.println("<BR>servlet : " + this.toString());
out.println("<BR>thread : " + thread.get());
out.println("</p>");
}
}执行结果：
Session 1: flag : 2 sessionid : amGeaiVwKvL9 servlet : test.other.TestThreadServlet@5f2db0 thread : This is the first String.java.lang.Object@1ad6b4b Session 1: flag : 3 sessionid : aR3GkcUQoXT- servlet : test.other.TestThreadServlet@5f2db0 thread : This is the first String.java.lang.Object@6214f5
1 服务器对每个Servlet只创建一个实例。flag不停增加
2 Session范围内的ThreadLocal中对象唯一。不同的请求，Object的hashCode相同。
3 不同的Session共享ThreadLocal，但内部对象不同
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