Updated code example and test case for mutex (#7642)

core-java-modules/core-java-concurrency-basic/src/main/java/com/baeldung/concurrent/mutex/SequenceGenerator.java

@@ -1,11 +1,12 @@
 package com.baeldung.concurrent.mutex;
 
 public class SequenceGenerator {
+
     private int currentValue = 0;
 
-    public int getNextSequence() throws InterruptedException {
+    public int getNextSequence() {
         currentValue = currentValue + 1;
-        Thread.sleep(500);
         return currentValue;
     }
+
 }

core-java-modules/core-java-concurrency-basic/src/main/java/com/baeldung/concurrent/mutex/SequenceGeneratorUsingMonitor.java

@@ -4,15 +4,16 @@ import com.google.common.util.concurrent.Monitor;
 
 public class SequenceGeneratorUsingMonitor extends SequenceGenerator {
 
-    private Monitor monitor = new Monitor();
+    private Monitor mutex = new Monitor();
 
     @Override
-    public int getNextSequence() throws InterruptedException {
-        monitor.enter();
+    public int getNextSequence() {
+        mutex.enter();
         try {
             return super.getNextSequence();
         } finally {
-            monitor.leave();
+            mutex.leave();
         }
     }
+
 }

core-java-modules/core-java-concurrency-basic/src/main/java/com/baeldung/concurrent/mutex/SequenceGeneratorUsingReentrantLock.java

@@ -7,7 +7,7 @@ public class SequenceGeneratorUsingReentrantLock extends SequenceGenerator {
     private ReentrantLock mutex = new ReentrantLock();
 
     @Override
-    public int getNextSequence() throws InterruptedException {
+    public int getNextSequence() {
         try {
             mutex.lock();
             return super.getNextSequence();
@@ -15,4 +15,5 @@ public class SequenceGeneratorUsingReentrantLock extends SequenceGenerator {
             mutex.unlock();
         }
     }
+
 }

core-java-modules/core-java-concurrency-basic/src/main/java/com/baeldung/concurrent/mutex/SequenceGeneratorUsingSemaphore.java

@@ -7,12 +7,15 @@ public class SequenceGeneratorUsingSemaphore extends SequenceGenerator {
     private Semaphore mutex = new Semaphore(1);
 
     @Override
-    public int getNextSequence() throws InterruptedException {
+    public int getNextSequence() {
         try {
             mutex.acquire();
             return super.getNextSequence();
+        } catch (InterruptedException e) {
+            throw new RuntimeException("Exception in critical section.", e);
         } finally {
             mutex.release();
         }
     }
+
 }

core-java-modules/core-java-concurrency-basic/src/main/java/com/baeldung/concurrent/mutex/SequenceGeneratorUsingSynchronizedBlock.java

@@ -2,9 +2,11 @@ package com.baeldung.concurrent.mutex;
 
 public class SequenceGeneratorUsingSynchronizedBlock extends SequenceGenerator {
 
+    private Object mutex = new Object();
+
     @Override
-    public int getNextSequence() throws InterruptedException {
-        synchronized (this) {
+    public int getNextSequence() {
+        synchronized (mutex) {
             return super.getNextSequence();
         }
     }

core-java-modules/core-java-concurrency-basic/src/main/java/com/baeldung/concurrent/mutex/SequenceGeneratorUsingSynchronizedMethod.java

@@ -3,7 +3,7 @@ package com.baeldung.concurrent.mutex;
 public class SequenceGeneratorUsingSynchronizedMethod extends SequenceGenerator {
 
     @Override
-    public synchronized int getNextSequence() throws InterruptedException {
+    public synchronized int getNextSequence() {
         return super.getNextSequence();
     }
 

core-java-modules/core-java-concurrency-basic/src/test/java/com/baeldung/concurrent/mutex/MutexUnitTest.java

@@ -1,7 +1,7 @@
 package com.baeldung.concurrent.mutex;
 
 import java.util.ArrayList;
-import java.util.HashSet;
+import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Set;
 import java.util.concurrent.ExecutorService;
@@ -12,59 +12,58 @@ import java.util.concurrent.TimeUnit;
 import org.junit.Assert;
 import org.junit.Test;
 
-import com.baeldung.concurrent.mutex.SequenceGenerator;
-import com.baeldung.concurrent.mutex.SequenceGeneratorUsingMonitor;
-import com.baeldung.concurrent.mutex.SequenceGeneratorUsingReentrantLock;
-import com.baeldung.concurrent.mutex.SequenceGeneratorUsingSemaphore;
-import com.baeldung.concurrent.mutex.SequenceGeneratorUsingSynchronizedBlock;
-import com.baeldung.concurrent.mutex.SequenceGeneratorUsingSynchronizedMethod;
-
 public class MutexUnitTest {
 
-    private final int RANGE = 30;
-
-    @Test
+    // @Test
+    // This test verifies the race condition use case, it may pass or fail based on execution environment
+    // Uncomment @Test to run it
     public void givenUnsafeSequenceGenerator_whenRaceCondition_thenUnexpectedBehavior() throws Exception {
-        Set<Integer> uniqueSequences = getASetOFUniqueSequences(new SequenceGenerator());
-        Assert.assertNotEquals(RANGE, uniqueSequences.size());
+        int count = 1000;
+        Set<Integer> uniqueSequences = getUniqueSequences(new SequenceGenerator(), count);
+        Assert.assertNotEquals(count, uniqueSequences.size());
     }
 
     @Test
     public void givenSequenceGeneratorUsingSynchronizedMethod_whenRaceCondition_thenSuccess() throws Exception {
-        Set<Integer> uniqueSequences = getASetOFUniqueSequences(new SequenceGeneratorUsingSynchronizedMethod());
-        Assert.assertEquals(RANGE, uniqueSequences.size());
+        int count = 1000;
+        Set<Integer> uniqueSequences = getUniqueSequences(new SequenceGeneratorUsingSynchronizedMethod(), count);
+        Assert.assertEquals(count, uniqueSequences.size());
     }
 
     @Test
     public void givenSequenceGeneratorUsingSynchronizedBlock_whenRaceCondition_thenSuccess() throws Exception {
-        Set<Integer> uniqueSequences = getASetOFUniqueSequences(new SequenceGeneratorUsingSynchronizedBlock());
-        Assert.assertEquals(RANGE, uniqueSequences.size());
+        int count = 1000;
+        Set<Integer> uniqueSequences = getUniqueSequences(new SequenceGeneratorUsingSynchronizedBlock(), count);
+        Assert.assertEquals(count, uniqueSequences.size());
     }
 
     @Test
     public void givenSequenceGeneratorUsingReentrantLock_whenRaceCondition_thenSuccess() throws Exception {
-        Set<Integer> uniqueSequences = getASetOFUniqueSequences(new SequenceGeneratorUsingReentrantLock());
-        Assert.assertEquals(RANGE, uniqueSequences.size());
+        int count = 1000;
+        Set<Integer> uniqueSequences = getUniqueSequences(new SequenceGeneratorUsingReentrantLock(), count);
+        Assert.assertEquals(count, uniqueSequences.size());
     }
 
     @Test
     public void givenSequenceGeneratorUsingSemaphore_whenRaceCondition_thenSuccess() throws Exception {
-        Set<Integer> uniqueSequences = getASetOFUniqueSequences(new SequenceGeneratorUsingSemaphore());
-        Assert.assertEquals(RANGE, uniqueSequences.size());
+        int count = 1000;
+        Set<Integer> uniqueSequences = getUniqueSequences(new SequenceGeneratorUsingSemaphore(), count);
+        Assert.assertEquals(count, uniqueSequences.size());
     }
 
     @Test
     public void givenSequenceGeneratorUsingMonitor_whenRaceCondition_thenSuccess() throws Exception {
-        Set<Integer> uniqueSequences = getASetOFUniqueSequences(new SequenceGeneratorUsingMonitor());
-        Assert.assertEquals(RANGE, uniqueSequences.size());
+        int count = 1000;
+        Set<Integer> uniqueSequences = getUniqueSequences(new SequenceGeneratorUsingMonitor(), count);
+        Assert.assertEquals(count, uniqueSequences.size());
     }
 
-    private Set<Integer> getASetOFUniqueSequences(SequenceGenerator generator) throws Exception {
+    private Set<Integer> getUniqueSequences(SequenceGenerator generator, int count) throws Exception {
         ExecutorService executor = Executors.newFixedThreadPool(3);
-        Set<Integer> uniqueSequences = new HashSet<>();
+        Set<Integer> uniqueSequences = new LinkedHashSet<>();
         List<Future<Integer>> futures = new ArrayList<>();
 
-        for (int i = 0; i < RANGE; i++) {
+        for (int i = 0; i < count; i++) {
             futures.add(executor.submit(generator::getNextSequence));
         }
 
@@ -72,7 +71,7 @@ public class MutexUnitTest {
             uniqueSequences.add(future.get());
         }
 
-        executor.awaitTermination(15, TimeUnit.SECONDS);
+        executor.awaitTermination(1, TimeUnit.SECONDS);
         executor.shutdown();
 
         return uniqueSequences;