Factory for Thread Safe Singleton in Java

Question

This is a sample of the basic pattern I've been using for a Factory that returns a thread-safe Singleton:

public class UserServiceFactory {

    private volatile static UserService userService;

    private UserServiceFactory() { }

    public static UserService getInstance() {
        if (userService == null) {
            synchronized(UserServiceImpl.class) {            
                if (userService == null) {
                    userService = new UserServiceImpl();
                }        
            }
        }

        return userService;
    }

}

It uses both volatile and the double check idiom to ensure that a single instance is created and is visible across threads.

Is there a less verbose and/or less expensive way to accomplish the same goal in 1.6+.

Answer 1

Use the Initialization On Demand Holder idiom, it's simpler and better readable:

public class UserServiceFactory {

    private UserServiceFactory () {}

    private static class UserServiceHolder {
        private static final UserService INSTANCE = new UserService();
    }

    public static UserService getInstance() {
        return UserServiceHolder.INSTANCE;
    }

}

However, I'd prefer Just Create One idiom.

---

Update: as your question history confirms, you're using Java EE. If your container supports it, you could also make it a @Singleton EJB and use @EJB to inject it (although @Stateless is preferable since @Singleton is by default read-locked).

@Singleton
public class UserService {}

with e.g. in a JSF managed bean

@EJB
private UserService userService;

This way you delegate the instantiation job to the container.