Question

I’m fairly new to GWT and have never worked with Java Servlets before. I know how to make RPCs but I was wondering if there are any concurrency issues with declaring member variables in my RPC’s ServiceImpl/RemoteServiceServlet class. I.e. From multiple “simultaneous” RPCs overwriting the same variable, similar to what happens with threads when a variable isn’t declared volatile.

I also need to use an extra thread in my server side code, so I was wondering if there’s any problems (outside of the usual thread safety problems) with declaring some of the servlet’s members as static so the other thread can access the variables without a reference to the servlet instance. Is it possible for more than one instance of the same RemoteServiceServlet class to be running at the same time?

E.g.

public class MyServiceImpl extends RemoteServiceServlet implements MyService {

    // Which of these variable declarations are a bad idea in a servlet?
    private String someVariable;
    private static String anotherVariable;
    volatile private static String multiThreadedVariable;

    public void init() { ... }
    ...
}

Thanks.

Answer 1

A Servlet is a singleton, therefore there is only one instance of the MyServiceImpl class. By introducing these state variables you will run into thread-safety issues not because there might be more than one MyServiceImpl instance, but because there is only one instance that will service ALL of your requests. Unless you synchronize access to these variables, you will have thread-safety issues, so I recommend removing them completely (most likely you don’t even need them).