Java Thread wait() => blocked?

Question

According to http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Thread.State.html calling wait() will result a thread to go in BLOCKED state. However this piece of code will result (after being called) in a Thread in WAITING State.

class bThread extends Thread {
    public synchronized void run() {
        try {
            wait();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

}

Have I got something wrong? Can anybody explain this behaviour to me? Any help would be appreciated!

Answer 1

Waiting is when it's not doing anything at all. Blocked is when it's trying to start running again but hasn't been allowed to yet.

Answer 2

The thread is WAITING until it is notified. Then it becomes BLOCKED trying to reenter the synchronized region until all other threads have left.

Relevant parts from the link you posted (about WAITING):

For example, a thread that has called Object.wait() on an object is waiting for another thread to call Object.notify() or Object.notifyAll() on that object.

and (about BLOCKED):

A thread in the blocked state is waiting for a monitor lock to [...] reenter a synchronized block/method after calling Object.wait.

The last part occurs when the thread tries to return from wait(), but not until then.

Answer 3

Where did you see it say stuff like that?

In the same page you linked, thread.state, it clearly states that

WAITING will be after Object.wait()

BLOCKED will be before entering synchronized

Answer 4

The monitor executes one thread at a time. Assuming you have T1-T10 threads, 9 are BLOCKED and one is RUNNABLE. Every once in a while, the monitor picks a new thread to run. When that happens, the chosen/current thread, say T1, goes from RUNNABLE to BLOCKED. Then another thread, say, T2, goes from BLOCKED to RUNNABLE, becoming the current thread.

When one of the threads needs some information to be made available by another thread, you use wait(). In that case, the thread will be flagged as WAITING until it is notify()ed. So, a thread that is waiting will not be executed by the monitor until then. An example would be, wait until there are boxes to be unloaded. The guy loading boxes will notify me when that happens.

In other words, both BLOCKED and WAITING are status of inactive threads, but a WAITING thread cannot be RUNNABLE without going to BLOCKED first. WAITING threads "don't want" to become active, whereas BLOCKED threads "want" to, but can't, because it isn't their turn.

I think.

Answer 5

Just as a reminder, you should always call wait() inside a while loop waiting on the condition for entering the synchronized region/critical section. This is because Java has "spurious wakeups" (essentially, a thread can wakeup at any moment for no reason).