Adv Java - [Servlet: Multithreaded]

♠ Posted by Unknown in Adv Java at 07:35

Multithreading into Servlet

The servlet container has a pool of threads that it will dispatch to handle client requests. It is quite likely that two clients arriving at the same time could be processing through your service() at the same time. Therefore the service() method must written in a thread-safe manner. Any access to common resources will need to be guarded by using the synchronized keyword.

A thread-safe servlet, like a thread safe JSP, is one that works correctly even if more than one servlet thread is running at the same time. To code a thread-safe servlet, you not only have to synchronize the use of any instance variables that could be corrupted, but also any methods that could cause problems if they were  used by two or more threads at the same time.

There are three different levels to limit the use of code to a single thread. First, you can synchronize a block of code by using the synchronized and this keywords. Second, you can synchronize an entire method. Third, you can use the Single ThreadModel interface to limit the use of an entire servlet to a single thread.

As you code thread-safe servlets, you must remember that one thread has to wait while another thread is using a synchronized block of code, a synchronized method, or a single-thread servlet. Since that can affect the performance of a Web application, your general goal should be to synchronize as little code as possible. As a result, synchronizing a block of code within a methods is best for performance, synchronizing a method is next best, and coding single-thread servlet should be usually avoided.

The following simple example puts a synchronized clause around the thread’s sleep() method. This will block all other threads until the allotted time (five second) is all used up. When testing this you should start several browser instances and hit this servlet as quickly as possible start several browser instances and hit this servlet as quickly as possible in each one – you’ll see that each one has to wait until its turn comes up.

Example:

import javax.servlet.*;

import javax.servlet.http.*;

import java.io.*;

public class ThreadServlet extends HttpServlet

{

            int i;

            public void service(HttpServletRequest req, HttpServletResponse res) throws IOException

            {

                        res.setContentType("text/html");

                        PrintWriter out = res.getWriter();

                        synchronized(this)

                        {

                                    try

                                    {

                                                Thread.currentThread().sleep(5000);

                                    }catch(InterruptedException e)

                                                {

                                                            System.err.println("Intrrupted");

                                                }

                        }

                        out.print("<H1> Finished " + i++ + "</H1>");

                        out.close();

            }

}

It is also possible to synchronize the entire servlet by putting the synchronized keyword in front of the service() method.