wait() and notify()

[Faraah Dabhoiwala]

which object should be marked synchronized when we want a thread to wait??

how does a thread know which object amongst those which are waiting to notify??

[Henry Wong]

For first question, see my response here...

http://www.coderanch.com/t/233995/Threads-Synchronization/java/synchronization


For second question, most modern JVMs pass the threading to the underlying OS libraries. So it is the OS libraries that choose the waiting thread. As for the Java Specification, it doesn't require any order or preference.

Henry

[Faraah Dabhoiwala]

didnt get what you wrote there

[Henry Wong]

Shadab Wadiwala wrote:didnt get what you wrote there

It looks like you reworded you question, after I answered it. I read the previous question as why synchronization was needed.

Henry

[Henry Wong]

which object should be marked synchronized when we want a thread to wait??

The object that will be used to wait, should be the object that needs to be synchronized.

Henry

[Balraj Momi]

First of all ,you should know that objects don't wait for threads, Its opposite. Threads actually wait for an notification so that they can continue their operation. One thread can own multiple locks but one object lock can only be owned by one thread.

Let me put it this way, following is an example where a machine is waiting for a for a job to work on. Here job is a object and as soon as a new job is build, machine will get notification to continue to work on that job.

  public class Machine{
	
	public static void main(String[] args) {
		MachineJob myJob = new MachineJob();
		Thread t = new Thread(myJob);
		t.start(); // 1
		synchronized (myJob) {
			try {
				myJob.wait(); //2
//				Start working on job once notified //4
			} catch (InterruptedException e) {
//				 Catch Interrupted Exception
			}
		}
	}
}
class MachineJob implements Runnable{
	public void run(){
		createJob();
	}
	void createJob(){
		synchronized (this) {
//			 doing complex logic to create a job
//			notify once job is created
			this.notify(); //3
		} 
	}
}  

Though all of you may agree that, this code is not foolproof, for example notify can happen even before wait starts. This is just to help Shadab understand how threads owns lock on an object and gets notified to reclaim their property .

Regards