Question

I want to understand how locking is done on static methods in Java.

let’s say I have the following class:

class Foo {
    private static int bar = 0;
    public static synchronized void inc() { bar++; }
    public synchronized int get() { return bar; }

It’s my understanding that when I call f.get(), the thread acquires the lock on the object f and when I do Foo.inc() the thread acquires the lock on the class Foo.

My question is how are the two calls synchronized in respect to each other?
Is calling a static method also acquires a lock on all instantiations, or the other way around (which seems more reasonable)?

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EDIT:

My question isn’t exactly how static synchronized works, but how does static and non-static methods are synchronized with each other.
i.e., I don’t want two threads to simultaneously call both f.get() and Foo.inc(), but these methods acquire different locks. My question is how is this preventable and is it prevented in the above code.

Answer 1

Static and instance synchronized methods are not related to each other, therefore you need to apply some additional synchronization between them, like this:

class Foo {
    private static int bar = 0;
    public static synchronized void inc() { bar++; }
    public synchronized int get() { 
        synchronized (Foo.class) { // Synchronizes with static synchronized methods
            return bar; 
        }
    }
}

(though in this case leaving synchronized on get() doesn’t make sense, since it doesn’t do anything that requires synchronization on instance).

Beware of deadlocks – since this code aquires multiple locks, it should do it in consistent order, i.e. other synchronized static methods shouldn’t try to acquire instance locks.

Also note that this particular task can be solved without synchronization at all, using atomic fields:

class Foo {
    private static AtomicInteger bar = new AtomicInteger(0);
    public static void inc() { bar.getAndIncrement(); }
    public int get() { return bar.get(); }
}