Question about working of ReentrantLock

[D Preethi]

Hi All,

Can someone tell me what I am doing wrong here? The program seems to execute eternally(and does not print DONE) though I am changing the condition of the boolean variable in the thread. Basically, I want the main thread to resume once a condition is satisfied in the thread. How do I achieve that?

 
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

public class TestReentrantLock {
	private static ReentrantLock lock = new ReentrantLock();
	private static Condition condition = lock.newCondition();
	private static boolean var = false;
	/**
	 * @param args
	 */
	public static void main(String[] args) throws Exception{
		Thread t = new Thread(){
			public void run(){
				try{
					Thread.sleep(10000);
				} catch(Exception e ){
					e.printStackTrace();
				}
				var = true;
			}
		};
		t.start();
		lock.tryLock();
		while (!var){
			System.out.println("Hey there");
			condition.await();
		}
		System.out.println("DONE!!!");
	}
}
 

Thanks in advance,
Preethi

[Adam Smolnik]

Hey.
First of all you must signal the resumed thread with condition.signal().
You remember about very important lock.unlock(s) in finally sections.

Adam

package adam.smolnik;

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

public class TestReentrantLock {
	private static ReentrantLock lock = new ReentrantLock();
	private static Condition condition = lock.newCondition();
	private static boolean var = false;

	/**
	 * @param args
	 */
	public static void main(String[] args) throws Exception {
		Thread t = new Thread() {
			public void run() {
				try {
					Thread.sleep(10000);
					var = true;
					lock.lock();
					condition.signal();
				} catch (Exception e) {
					e.printStackTrace();
				} finally {
					lock.unlock();
				}

			}
		};
		t.start();
		if (lock.tryLock()) {
			try {
				while (!var) {
					System.out.println("Hey there");
					condition.await();
				}
				System.out.println("DONE!!!");
			} finally {
				lock.unlock();
			}
		}
	}
}
 

[D Preethi]

Thanks a lot, Adam!! Currently, there is no way to get the list of all available conditions on a particular lock, right? I just see this API -

Condition c = lock.newCondition();

So, if I want to split the above mentioned code in different classes ( the sleep thread in one class, and the variable monitoring code in another class), I shall need to pass the references to both lock and the condition the first class created, right?

[Adam Smolnik]

Hey.

Essentially, I personally would NOT make access locks and conditions outside class without very justified reason.
Next, I would browse existing solutions in java.util.concurrent package like Executors, CyclicBarrier etc..

In case of need, you consider my somewhat other approach to your described problem(semantically should be equivalent):

package adam.smolnik;

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

public class MyExecutor {

	private final ReentrantLock lock = new ReentrantLock();

	private final Condition condition = lock.newCondition();

	private boolean var = false;

	private final Runnable task;

	public MyExecutor(Runnable task) {
		this.task = task;
	}

	public void startAndWait() {
		new Thread() {

			public void run() {
				try {
					lock.lock();
					task.run();
					while (!var) {
						condition.await();
						System.out.println("var = " + var);
					}
					System.out.println("DONE!!!");
				} catch (InterruptedException e) {
					Thread.currentThread().interrupt();
				} finally {
					lock.unlock();
				}
			};

		}.start();
	}

	public void changeCondition(boolean var) {
		try {
			lock.lock();
			this.var = var;
			condition.signal();
		} catch (Exception e) {
			e.printStackTrace();
		} finally {
			lock.unlock();
		}
	}

	public static void main(String[] args) throws Exception {
		MyExecutor executor = new MyExecutor(new Runnable() {

			@Override
			public void run() {
				System.out.println("Hey there");
			}
		});

		executor.startAndWait();
		Thread.sleep(2000);
		executor.changeCondition(false);
		Thread.sleep(2000);
		executor.changeCondition(true);
	}
}

Results:

Hey there
var = false
var = true
DONE!!!

Adam