which part of the servlet runs in new threads

[vijaya saradhi]

hi all,

in Head First servlets and jsp book it was given like this .A seperate thread is created on every request, and before service() method called a seperate thread is created and control is transferred to service() method().

Does this mean remaining parts of servlet are shared by multiple threads
simultaniously ?

If I declare a HashMap in a servlet class as an instance variable then will it be shared by multiple threads .

I refer this HashMap in doPost()


eg

  
package com.example.web;
import com.example.model.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.util.*;
 
public class fileUploadServlet extends HttpServlet 
{
        HashMap h = new HashMap();
        
 
public void doPost(HttpServletRequest request,HttpServletResponse response)throws IOException,ServletException {
          
          String name = request.getParameter("filetitle");
 
          ...
          .... /* code to upload file in local file system and get rename
                file to a unique name with timestamp appended to name  
                and store uploaded physical filename in a String 
               */
          String physical_file_name = uploadedFile.getName();
               
          h.put(name,physical_file_name);
        
        }
 
public void doGet(HttpServletRequest request,HttpServletResponse response)throws IOException,ServletException {
          String filetitle = request.getParameter("title");
          Set s = h.KeySet();
 
          Iterator it = s.iterator();
 
          while(it.next()){
              out.print(h.get(filetitle));
          }
 
 
        }
 
 
 
 
}  

please tell in the above example if the HashMap h is shared by multiple threads .Is this the proper place to declare a HashMap in a servlet.



my application requirement after upload keep track of all uploaded file in a HashMap .So that when a user tries to delete using a link in the html page servlet shud delete that file only in the file system.
[ December 16, 2008: Message edited by: vijaya saradhi ]

[Ankit Garg]

you are right that the HashMap will be shared by multiple threads. I don't think you can face problems with the specification of your code. But I am thinking about a case where two files with the same name are uploaded at the same time. There might be some more cases which I am not able to figure out . So for safety, I think you must synchronize the doPost method.

[Ulf Dittmer]

Instance variables are shared by all threads. Since that HashMap is mutable (there's a "put" method), this code is not thread-safe. You need to synchronize access to it.

The doGet and doPost methods are called from the service method, so both can be run by multiple threads/requests simultaneously.
[ December 17, 2008: Message edited by: Ulf Dittmer ]