How to use Synchronized Keyword

Question

Hi guys,
Sorry if this is a simple question, but I'm not sure about the following:
Suppose we have the class,
Class Foo
public synchronized void bar1() {...}
public synchronized void bar2() {...}
If I have two threads running concurrently, say T1 and T2.
I know that T1 and T2 can't both be in bar1() at the same time right? [It has to be in some serial order]
But, can T1 be in bar1() and T2 be in bar2() at the same time?
I guess the question is, does the synchronized apply to that specific method only? Or rather to the object (which basically limits at most one thread calling that object's synchronized methods)?
Thanks a bunch! :)
-Michael.

Answer 1

Hi Michael,
In general, when you syncronize a block of code, the execution of the block is syncronized ON an object. Any object can function as a lock, and only one thread can have a lock of one object at a time. If one thread already has a lock on an object and another thread tries to acquire lock on the same object, the latter thread will wait until the lock is released.
When you use 'synchronized' in method declaration, it means that the execution of the whole method is syncronized on the object itself (NOT class, if the method is non-static. Static method would be synchronized on the .class object.)
So, if you have more than one syncronized methods, only one of them is executed at a time ON THE SAME OBJECT. The same method can, of course, be executed simultaneously on other objects. Also, if some of the methods are synchronized, but other methods are not, the non-synchronized methods are not affected in any way.
It might be good to note that these two pieces of code have exactly the same meaning:
public synchronized doSomething() {
    // do something
}and
public doSomething() {
    synchronized (this) {
        // do something
}Hope this helps.
Cheers,
Vesa
p.s. It's a good idea to design your code so that one thread will have no more than one lock at a time. If it's possible for more than one thread to have lock on more than one object, there may be a risk of the threads dead-locking, which will crash any program...