Question

If I have a method like this

public synchronized static foo()
{

}

this compiles fine.

So this leads me to two questions

1. What is is syncrhonizing on? The Class, or something else?
2. Can you synchronize on a class, does that lock all object of that class as well.

for instance could you do this

synchronized(Foo) // where Foo is a class
{
}

Answer 1

Synchronized static method in MyClass is essentially the same as synchronized(MyClass.class) block. Your second example should be rewritten as

synchronized(Foo.class) {
}

to be correct.

If you would like to be defensive about your class synchronization, you should synchronize on a private static object not visible outside your class. This prevents malicious code from blocking your static methods by executing synchronized on their class object, thus blocking the legitimate method.

As far as “locking all objects” goes, non-static methods marked synchronized will not be locked by execution of a static synchronized method, because regular synchronized methods lock on an instance of the object, not on its class.