If I have a method like this
public synchronized static foo()
{
}
this compiles fine.
So this leads me to two questions
for instance could you do this
synchronized(Foo) // where Foo is a class
{
}
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.
synchronized block; since Foo is not an expression that evaluates to an object, you should get a compile error.Yes, it is synchronizing on the Class object. You can synchronize on any Class object. You can either use its literal (Foo.class), or use the getClass() method of an instance of Foo.
For more information about synchronized (the second page in that trail contains the information about which lock synchronized methods use): http://docs.oracle.com/javase/tutorial/essential/concurrency/locksync.html
Foo, not the Class.synchronized block takes an object as its argument. Foo is not an object, but Foo.class is.class object, eg MyClass.classStatic methods use the class as the object for locking.
The relevant section on the Java Language Specification is 8.4.3.6, 'synchronized Methods':
A synchronized method acquires a monitor (§17.1) before it executes. For a class (static) method, the monitor associated with the Class object for the method's class is used. For an instance method, the monitor associated with this (the object for which the method was invoked) is used.
Classobject. I'm not sure if I understand your second question correctly, could you please restate it?Classobject. 2) It should really besynchronized(Foo.class)