BAEL-613 - Added workers which wait for all other workers to start

core-java/src/main/java/com/baeldung/concurrent/countdownlatch/WaitingWorker.java

@@ -0,0 +1,37 @@
+package com.baeldung.concurrent.countdownlatch;
+
+import java.util.List;
+import java.util.concurrent.CountDownLatch;
+
+public class WaitingWorker implements Runnable {
+
+    private final List<String> outputScraper;
+    private final CountDownLatch readyThreadCounter;
+    private final CountDownLatch callingThreadBlocker;
+    private final CountDownLatch completedThreadCounter;
+
+    public WaitingWorker(final List<String> outputScraper,
+                         final CountDownLatch readyThreadCounter,
+                         final CountDownLatch callingThreadBlocker,
+                         CountDownLatch completedThreadCounter) {
+
+        this.outputScraper = outputScraper;
+        this.readyThreadCounter = readyThreadCounter;
+        this.callingThreadBlocker = callingThreadBlocker;
+        this.completedThreadCounter = completedThreadCounter;
+    }
+
+    @Override
+    public void run() {
+        // Mark this thread as read / started
+        readyThreadCounter.countDown();
+        try {
+            callingThreadBlocker.await();
+            outputScraper.add("Counted down");
+        } catch (InterruptedException e) {
+            e.printStackTrace();
+        } finally {
+            completedThreadCounter.countDown();
+        }
+    }
+}

core-java/src/test/java/com/baeldung/concurrent/countdownlatch/CountdownLatchExampleTest.java

@@ -1,6 +1,5 @@
 package com.baeldung.concurrent.countdownlatch;
 
-import org.assertj.core.api.Assertions;
 import org.junit.Test;
 
 import java.util.ArrayList;
@@ -8,7 +7,6 @@ import java.util.Collections;
 import java.util.List;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.TimeUnit;
-import java.util.concurrent.TimeoutException;
 import java.util.stream.Stream;
 
 import static java.util.stream.Collectors.toList;
@@ -17,7 +15,6 @@ import static org.assertj.core.api.Assertions.assertThat;
 public class CountdownLatchExampleTest {
     @Test
     public void whenParallelProcessing_thenMainThreadWillBlockUntilCompletion() throws InterruptedException {
-
         // Given
         List<String> outputScraper = Collections.synchronizedList(new ArrayList<>());
         CountDownLatch countDownLatch = new CountDownLatch(5);
@@ -59,6 +56,41 @@ public class CountdownLatchExampleTest {
         final boolean result = countDownLatch.await(3L, TimeUnit.SECONDS);
 
         // Then
-        assertThat(result).isTrue();
+        assertThat(result).isFalse();
     }
+
+    @Test
+    public void whenDoingLotsOfThreadsInParallel_thenStartThemAtTheSameTime() throws InterruptedException {
+        // Given
+        List<String> outputScraper = Collections.synchronizedList(new ArrayList<>());
+        CountDownLatch readyThreadCounter = new CountDownLatch(5);
+        CountDownLatch callingThreadBlocker = new CountDownLatch(1);
+        CountDownLatch completedThreadCounter = new CountDownLatch(5);
+        List<Thread> workers = Stream
+          .generate(() -> new Thread(new WaitingWorker(outputScraper, readyThreadCounter, callingThreadBlocker, completedThreadCounter)))
+          .limit(5)
+          .collect(toList());
+
+        // When
+        workers.forEach(Thread::start);
+        readyThreadCounter.await(); // Block until workers start
+        outputScraper.add("Workers ready");
+        callingThreadBlocker.countDown(); // Start workers
+        completedThreadCounter.await(); // Block until workers finish
+        outputScraper.add("Workers complete");
+
+        // Then
+        outputScraper.forEach(Object::toString);
+        assertThat(outputScraper)
+          .containsExactly(
+            "Workers ready",
+            "Counted down",
+            "Counted down",
+            "Counted down",
+            "Counted down",
+            "Counted down",
+            "Workers complete"
+          );
+    }
+
 }