Alternative für 11.2

src/main/java/_11/_2/alternative/Barriers.java

@@ -0,0 +1,16 @@
+package _11._2.alternative;
+
+public class Barriers {
+
+	public static final int NUMBER_OF_RUNNERS = 3;
+
+	public static void main(String[] args) {
+		NumberRunner[] runners = new NumberRunner[NUMBER_OF_RUNNERS];
+		for(int i = 0; i < NUMBER_OF_RUNNERS; i++) {
+			runners[i] = new NumberRunner(i);
+		}
+		for(int i = 0; i < NUMBER_OF_RUNNERS; i++) {
+			runners[i].start();
+		}
+	}
+}

src/main/java/_11/_2/alternative/Checkpoint.java

@@ -0,0 +1,31 @@
+package _11._2.alternative;
+
+import java.util.concurrent.atomic.AtomicInteger;
+
+public class Checkpoint {
+
+	private static AtomicInteger GATHERED = new AtomicInteger(0);
+
+	private final Object lock = new Object();
+	private int groupSize;
+
+	public Checkpoint(int groupSize) {
+		this.groupSize = groupSize;
+	}
+
+	/**
+	 * Blocks until called by {@link #groupSize} different Threads,
+	 * then wakes all of them up again.
+	 */
+	public void gather() throws InterruptedException {
+		synchronized(this.lock) {
+			if(GATHERED.incrementAndGet() != this.groupSize) {
+				this.lock.wait();
+			} else {
+				GATHERED.set(0);
+				System.out.println("Group gathered");
+				this.lock.notifyAll();
+			}
+		}
+	}
+}

src/main/java/_11/_2/alternative/NumberRunner.java

@@ -0,0 +1,27 @@
+package _11._2.alternative;
+
+public class NumberRunner extends Thread {
+
+	private static final Checkpoint CHECKPOINT = new Checkpoint(Barriers.NUMBER_OF_RUNNERS);
+
+	private final int id;
+
+	public NumberRunner(int id) {
+		this.id = id;
+	}
+
+	@Override
+	public void run() {
+		for(int i = 0; i < 1000; i++) {
+			if(i % 10 == 0) {
+				try {
+					CHECKPOINT.gather();
+				} catch(InterruptedException e) {
+					throw new IllegalStateException("Group can never be complete again, because one of the members got lost", e);
+				}
+			}
+
+			System.out.println("Thread " + this.id + ": " + i);
+		}
+	}
+}