bestreplacement.java

Java Thread Programming (Source

// uses BooleanLock from chapter 17

public class BestReplacement extends Object {
	private Thread internalThread;
	private volatile boolean stopRequested;

	private BooleanLock suspendRequested;
	private BooleanLock internalThreadSuspended;

	public BestReplacement() {
		stopRequested = false;

		suspendRequested = new BooleanLock(false);
		internalThreadSuspended = new BooleanLock(false);

		Runnable r = new Runnable() {
				public void run() {
					try {
						runWork();
					} catch ( Exception x ) {
						// in case ANY exception slips through
						x.printStackTrace(); 
					}
				}
			};

		internalThread = new Thread(r);
		internalThread.start();
	}

	private void runWork() {
		int count = 0;

		while ( !stopRequested ) {
			try {
				waitWhileSuspended();
			} catch ( InterruptedException x ) {
				// Reassert interrupt so that remaining code
				// sees that an interrupt has been requested.
				Thread.currentThread().interrupt(); 

				// Reevaluate while condition --probably 
				// false now.
				continue;
			}

			System.out.println("Part I - count=" + count);

			try {
				Thread.sleep(1000);
			} catch ( InterruptedException x ) {
				Thread.currentThread().interrupt(); // reassert
				// continue on as if sleep completed normally
			}

			System.out.println("Part II - count=" + count);

			try {
				Thread.sleep(1000);
			} catch ( InterruptedException x ) {
				Thread.currentThread().interrupt(); // reassert
				// continue on as if sleep completed normally
			}

			System.out.println("Part III - count=" + count);

			count++;
		}
	}

	private void waitWhileSuspended() 
					throws InterruptedException {

		// only called by the internal thread - private method

		synchronized ( suspendRequested ) {
			if ( suspendRequested.isTrue() ) {
				try {
					internalThreadSuspended.setValue(true);
					suspendRequested.waitUntilFalse(0); 
				} finally {
					internalThreadSuspended.setValue(false);
				}
			}
		}
	}

	public void suspendRequest() {
		suspendRequested.setValue(true);
	}

	public void resumeRequest() {
		suspendRequested.setValue(false);
	}

	public boolean waitForActualSuspension(long msTimeout) 
					throws InterruptedException {

		// Returns 'true' if suspended, 'false' if the 
		// timeout expired.

		return internalThreadSuspended.waitUntilTrue(msTimeout);
	}

	public void stopRequest() {
		stopRequested = true;
		internalThread.interrupt();
	}

	public boolean isAlive() {
		return internalThread.isAlive();
	}

	public static void main(String[] args) {
		try {
			BestReplacement br = new BestReplacement();
			System.out.println(
					"--> just created, br.isAlive()=" + 
					br.isAlive());
			Thread.sleep(4200);

			long startTime = System.currentTimeMillis();
			br.suspendRequest();
			System.out.println(
					"--> just submitted a suspendRequest");

			boolean suspensionTookEffect = 
					br.waitForActualSuspension(10000);
			long stopTime = System.currentTimeMillis();

			if ( suspensionTookEffect ) {
				System.out.println(
					"--> the internal thread took " +
					(stopTime - startTime) + " ms to notice " +
					"\n    the suspend request and is now " +
					"suspended.");
			} else {
				System.out.println(
					"--> the internal thread did not notice " +
					"the suspend request " +
					"\n    within 10 seconds.");
			}

			Thread.sleep(5000);

			br.resumeRequest();
			System.out.println(
					"--> just submitted a resumeRequest");
			Thread.sleep(2200);

			br.stopRequest();
			System.out.println(
					"--> just submitted a stopRequest");
		} catch ( InterruptedException x ) {
			// ignore
		}
	}
}