is this thread safe?

[ranjani krishnaswamy]

Could you please tell me whether this servlet would be thread-safe?

import javax.servlet.*;
import javax.servlet.http.*:
import java.io.*;
public class TestServ extends HttpServlet implements SingleThreadModel
{
String index;
public void service(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException
{
response.setContentType("text/plain");
PrintWriter out = res.getWriter();
myName = req.getParameter("value");
doFunction(out);
out.close();
}
public void doFunction(PrintWriter out)
{
out.println(index);
}
}
-----------
I think it would be because although a class variable index is used, it is called and initialized in the service method.
However, if there was another function in the class, and the index variable is maipulated, the servlet would no longer be thread safe.
is my logic correct?
please reply

[g madhava]

Hello,
By the spec, if the Servlet implements the SingleThreadModel interface, for every access of the servlet a seperate instance is being created and this instance would service the client.
Ideally the answer for your question is yes.But a word of caution is that it actually depends on the implementation of the server.

NOTE: You shouldn't try to overload the service() method if the class extends HttpServlet.
thanks,

[Chintan Rajyaguru]

According to my understanding this servlet is thread safe because it implements SingleThreadModel. This comes from the specification.
"For a servlet implementing the SingleThreadModel interface, the servlet container may instantiate multiple instances to handle a heavy request load and serialize requests to a particular instance."
******
What I get out of this is, if a servlet implements SingleThreadModel interface, the servlet container must either create a new instance for every other request or if the container wants single instance to handle multiple requests, it must maintain a queue of incoming requests and handover the request to the instance only after it has finished servicing previous request.
******
I am not 100% sure about what I said between ****** and ****** but I strongly believe that the given example is threadsafe.
Chintan

[Ashik Uzzaman]

javax.servlet.SingleThreadModel is a tag interface that ensures a servlet is thread-safe or the servlet�s service(), doGet(), doPost() methods can�t be executed by more than one thread at a time. So the above index String is thread-safe as that will be accessed thru service() which becomes thread-safe due to implementation of ThreadSafe interface....
But if another object is able somehow to call doFunction() which is not automatically synchronized even after implementing SingleThreadModel interface then this class is not fully thread-safe. But we know there is no way to skip the init(),service(),destroy() life cycle's service() which must be used if u wanna call a servlet object where we are initializing index String. So happily safe....
------------------
Muhammad Ashikuzzaman (Fahim)
Sun Certified Programmer for the Java� 2 Platform
--When you learn something, learn it by heart!

[This message has been edited by Ashik uzzaman (edited December 30, 2001).]

[jyothi ve]

According to Servlet API documentation SingleThreadModel Servlet must guarantee "that no two threads will execute concurrently in the servlet's service method." To accomplish this, each thread uses a free servlet instance from the pool. Thus, any servlet implementing SingleThreadModel can be considered thread safe & isn't required to synchronize access to its instance variables.
-Jyothi

[Ashik Uzzaman]

Yah, Jhyoti that's what i understand abt it...

------------------
Muhammad Ashikuzzaman (Fahim)
Sun Certified Programmer for the Java� 2 Platform
--When you learn something, learn it by heart!

[ajay khandelwal]

Hello Guys, i know this is too late to post any question related to this topoic.
Ofcourse SingleThreadModel guarantees thread safety for above example.
What if i have a static veriable in the servlet class ? Is that variable still thread safe under SingleThreadedMedel ?? will there be any issue for this variable value ??
Thanks
Ajay

[Thomas Hubschman]

No the static variable will not be thread safe as it belongs to the class and can be changed by any member of the class. In this case that would be any instantiated servlet object.
To make it thread safe you can of course declare it synchronized. Which would then allow only one thread to "own" the object at any time.
Hope this helps.
Tom