Are Java static initializers thread safe?

I'm using a static code block to initialize some controllers in a registry I have. My question is therefore, can I guarantee that this static code block will only absolutely be called once when the class is first loaded? I understand I cannot guarantee when this code block will be called, I'm guessing its when the Classloader first loads it. I realize I could synchronize on the class in the static code block, but my guess is this is actually what happens anyway?

Simple code example would be;

class FooRegistry {

    static {
        //this code must only ever be called once 
        addController(new FooControllerImpl());
    }

    private static void addController(IFooController controller) { 
        // ...
    }
}

or should I do this;

class FooRegistry {

    static {
        synchronized(FooRegistry.class) {
            addController(new FooControllerImpl());
        }
    }

    private static void addController(IFooController controller) {
        // ...
    }
}

Yes, Java static initializers are thread safe (use your first option).

However, if you want to ensure that the code is executed exactly once you need to make sure that the class is only loaded by a single class-loader. Static initialization is performed once per class-loader.

This is a trick you can use for lazy initialization

enum Singleton {
    INSTANCE;
}

or for pre Java 5.0

class Singleton {
   static class SingletonHolder {
      static final Singleton INSTANCE = new Singleton();
   }
   public static Singleton instance() {
      return SingletonHolder.INSTANCE;
   }
}

As the static block in SingletonHolder will run once in a thread safe manner you don't need any other locking. The class SingletonHolder will only get loaded when you call instance()

In usual circumstances everything in the static initialiser happens-before everything that uses that class, so synchronisation is not usually necessary. However, the class is accessible to anything that the static intiailiser calls (including causing other static initialisers to be invoked).

A class can be loaded by a class loaded but not necessarily initialised straight away. Of course, a class can be loaded by multiples instances of class loaders and thereby become multiple classes with the same name.

Yes, sort of

A static initializer only gets called once, so by that definition it's thread safe -- you'd need two or more invocations of the static initializer to even get thread contention.

That said, static initializers are confusing in many other ways. There's really no specified order in which they're called. This gets really confusing if you have two classes whose static initializers depend on each other. And if you use a class but don't use what the static initializer will set up, you're not guaranteed the class loader will invoke the static initializer.

Finally, keep in mind the objects you're synchronizing on. I realize this isn't really what you're asking, but make sure your question isn't really asking if you need to make addController() thread-safe.

Yes, Static initializers are run only once. Read this for more information.

So basically, since you want a singleton instance, you should do it more or less the old-fashioned way and make sure your singleton object is initialised once and only once.