Singleton servlet?

Question

If I declare a singleton servlet like

public static class SomeServlet extends HttpServlet {.....

It gives the error indicating that modifier static not allowed here in NetBeans.

Please let me know how to create a singleton servlet which allows only one client at a time to execute.

Thanks in Advance, Mahes

Answer 1

• static has nothing to do with being executed by only one thread at any point in time.
• you can have static classes as inner classes, but this doesn't help your problem due to the first point
• If I recall correctly there's only one servlet instantiated ever, so there's no need to create singletons. Update: I didn't remember correctly as DefLog pointed out
• therefor synchronize on a method (doGet, doPost, service or your implementation method) update: will not do the trick.

Based on your demonstrated understanding of the problem (no pun intended) I doubt that this is the solution you should be choosing. Rather rephrase your question so that you state the problem you're trying to solve with the solution you have stated in your first revision of the question.

Update: See the comments to this answer for why you really don't want to fall for synchronized. You might (for example) want to have multiple servlets accessing the same functionality one at a time. Plus you'd introduce severe scaling problems if you synchronize on a servlet level. It's definitely the wrong place for solving concurrency problems.

Answer 2

Implement SingleThreadModel. But note that: it's deprecated, it may not do anything/make sense, it's a daft thing to do (much like singletons).

Answer 3

I really strongly oppose the idea of making a Servlet that can only handle one request at a time. You should probably make another (properly synchronized class) and have your servlet putting requests in some sort of execution queue.

Answer 4

A top-level declaration can never be static. static means that an element is declared on the class level, so it needs an enclosing class declaration.

With servlets there will be usually one but possibly more instances of that servlet which handle requests concurrently. So, maybe you don't really want to reduce concurrency at this level, or you would use servlet filters or a framework like Spring Security that can limit the number of concurrent sessions.

Answer 5

I tend to think that Singletons are overused, and often indicate a flaw with the basic architecture of a solution. Assuming you've done enough analysis to prove you need a Singleton, you can use an older Singleton pattern that doesn't rely on static initialization. Here's an example:

public SomeServlet extends HttpServlet {

    private instance = null;

    private SomeServlet() {
        // Construction code here
    }

    public synchronized SomeServlet getInstance() {
        if(instance == null) {
            instance = SomeServlet();
        }
        return instance;
    }

    // Servlet methods here

}

I see two potential problems though:

First, you'll need a wrapper Servlet (or maybe just chain from another servlet, since the Servlet container won't know how to deal with not having a private constructor.

Second, you'll still need to synchronize the servlet methods because you still can't guarantee that there's only one executor thread "in" the servlet at a time.