Synchronize with static. Please see.

Question

Folks,
I have a question regarding a the synchronized statement
when used as a static method:
'When a synchronized static method is invoked,the lock is obtained
on the Class object for the class in which the method is defined'
Look at this code:
class Test{
  public synchronized static void m1(){
}My QUESTION is:
What does the statement mean by the lock is obtained
is associated with the Class Object in which the
method is defined?
Isnt a lock obtained on the Test object? (which is
basically a Class object isnt it?)
Can someone please guide?

Answer 1

What does the statement mean by the lock is obtained
is associated with the Class Object in which the
method is defined?Every class that is used by your program has an associated Class object. There's a class named (appropriately, but somewhat confusingly) java.lang.Class. You can use these objects for reflection--to get information about a class' methods, it's parent class, etc. The usual ways to get a class object are: class Foo {
Class c1 = Foo.class; // *1*
Foo foo = new Foo();
Class c2 = foo.getClass(); // *2**1* and *2* both give you the same object.
To understand what is meant by the statement you're asking about, you also need to understand that "synchronized" always locks on some object. You can name the object explicitly: synchronized(someFoo) {
} or the object can be implicitly specified: class Foo {
    public void synchronized m1() {
        // body
    public void m2() {
        synchronized (this) {
            // body
    public static void synchronized m3() {
        // body
    public static void m4() {
        synchronized Foo.class {
            //body
} m1 is equivalent to m2 -- both lock on the same object
m3 is equivalent to m4 -- both lock on the same object.
Isnt a lock obtained on the Test object? (which is
basically a Class object isnt it?)What do you mean "the Test object"? If you mean Test.class, then yes, that's what we lock on here, and yes, that is an instance of java.lang.Class.