ConcurrentTimer.java

// Simple framework for timing concurrent execution
package org.effectivejava.examples.chapter10.item69;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Executor;

public class ConcurrentTimer {
	private ConcurrentTimer() {
	} // Noninstantiable

	public static long time(Executor executor, int concurrency,
			final Runnable action) throws InterruptedException {
		final CountDownLatch ready = new CountDownLatch(concurrency);
		final CountDownLatch start = new CountDownLatch(1);
		final CountDownLatch done = new CountDownLatch(concurrency);

		for (int i = 0; i < concurrency; i++) {
			executor.execute(new Runnable() {
				public void run() {
					ready.countDown(); // Tell timer we're ready
					try {
						start.await(); // Wait till peers are ready
						action.run();
					} catch (InterruptedException e) {
						Thread.currentThread().interrupt();
					} finally {
						done.countDown(); // Tell timer we're done
					}
				}
			});
		}

		ready.await(); // Wait for all workers to be ready
		long startNanos = System.nanoTime();
		start.countDown(); // And they're off!
		done.await(); // Wait for all workers to finish
		return System.nanoTime() - startNanos;
	}
}