using Object as a mutex in java

Question

Hello there good poeple, I need some help.

I'm writing a music player which streams music from the web. If I pres the play button before the music is done buffering I want it to wait.

I tried doing something like this:

Object mutex = new Object();

public void main() {
    startStreaming();
    mutex.notify();
}

private void onClickPlayButton() {
    mutex.wait();
}

The problem is that is the playButton is not pressed the mutex.notify() if throws a "llegalMonitorStateException". How do you normally solve problems like this?

EDIT To make clear. My question is: How do I make the button wait for the "startStreamning" method to finish?

this might help (see accepted answer)... http://stackoverflow.com/questions/5291041/is-there-a-mutex-in-java — Ahmed Kotb, May 19 '11 at 18:46
Do you really need a mutex for this? I doubt you want to block the UI thread while the download finishes. I would think you could accomplish "don't play yet" by just checking a flag for `isDownloadComplete()` — matt b, May 19 '11 at 19:02

Saurabh Gokhale · Accepted Answer · 2011-05-19T19:02:26.263

According to the JavaDoc,

IllegalMonitorStateException is thrown "to indicate that a thread has attempted to wait on an object's monitor or to notify other threads waiting on an object's monitor without owning the specified monitor."

In order to call mutex.wait() or mutex.notify(), the calling thread must own a lock on object mutex.

This exception is thrown if you call it without a preceding synchronized (mutex) { }

Check out the nice animation of wait and notify in this link : How do wait and notify really work?

score 5 · Answer 2 · answered May 19 '11 at 18:47

5

for wait(), notify() call, you need a synchronized code. try this:

synchronized (this) {
  try {
    this.wait();
   } catch (InterruptedException e) {
     e.printStackTrace();
   }
}


synchronized (this) {
   notify();
}

answered May 19 '11 at 18:47

Erhan Bagdemir

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To be more like the OP's code, the synchronized statements and the calls to `wait()` and `notify()` should be on `mutex` instead of `this`. – John Flatness May 19 '11 at 18:51
well, you're right. i would like to notice that the methods must be called in a sync. code. – Erhan Bagdemir May 19 '11 at 19:21

score 4 · Answer 3 · answered May 19 '11 at 18:52

4

Try use Semaphore with 0 initial permit. Semaphore mutex = new Semaphore(0);

in main mutex.release();

in on click mutex.acquire();

answered May 19 '11 at 18:52

Mikita Belahlazau

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+1 for pointing to the concurrent.* package instead of error-prone fiddling with low-level synchronization primitives – Waldheinz May 25 '11 at 15:19
It doesn't have to be set to a 1 permit? – Joe Plante Oct 08 '12 at 17:32

score 2 · Answer 4 · answered May 19 '11 at 18:44

from javadoc wait This method should only be called by a thread that is the owner of this object's monitor and for notify This method should only be called by a thread that is the owner of this object's monitor.

this means htat you have to synchronize using the mutex when using notify and wait

score 0 · Answer 5 · answered May 19 '11 at 18:43

0

You have to wait before you notify.

answered May 19 '11 at 18:43

Charlie Martin

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I'm sorry if I was unclear. I know why I get the exception, I don't know how to solve the problem. – Martin Hansen May 19 '11 at 18:44

score 0 · Answer 6 · answered May 19 '11 at 18:47

0

You have to synchronize on the mutex to call notify and wait

answered May 19 '11 at 18:47

AdrianRM

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score 0 · Answer 7 · answered May 19 '11 at 18:47

You can either look at using more complex lock objects, or simply munch the exception in a try/catch block. The later is definitely "quick and dirty".

For a more advance locking objects, take a look at http://download.oracle.com/javase/tutorial/essential/concurrency/newlocks.html

using Object as a mutex in java

7 Answers7