Code example: Using a Mutex Object in Java (#7587)

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

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

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

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

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

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+package com.baeldung.concurrent.mutex;
+
+import java.util.concurrent.locks.ReentrantLock;
+
+public class SequenceGeneratorUsingReentrantLock extends SequenceGenerator {
+
+    private ReentrantLock mutex = new ReentrantLock();
+
+    @Override
+    public int getNextSequence() throws InterruptedException {
+        try {
+            mutex.lock();
+            return super.getNextSequence();
+        } finally {
+            mutex.unlock();
+        }
+    }
+}

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

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+package com.baeldung.concurrent.mutex;
+
+import java.util.concurrent.Semaphore;
+
+public class SequenceGeneratorUsingSemaphore extends SequenceGenerator {
+
+    private Semaphore mutex = new Semaphore(1);
+
+    @Override
+    public int getNextSequence() throws InterruptedException {
+        try {
+            mutex.acquire();
+            return super.getNextSequence();
+        } finally {
+            mutex.release();
+        }
+    }
+}

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

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+package com.baeldung.concurrent.mutex;
+
+public class SequenceGeneratorUsingSynchronizedBlock extends SequenceGenerator {
+
+    @Override
+    public int getNextSequence() throws InterruptedException {
+        synchronized (this) {
+            return super.getNextSequence();
+        }
+    }
+
+}

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

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

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

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+package com.baeldung.concurrent.mutex;
+
+import java.util.ArrayList;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+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
+    public void givenUnsafeSequenceGenerator_whenRaceCondition_thenUnexpectedBehavior() throws Exception {
+        Set<Integer> uniqueSequences = getASetOFUniqueSequences(new SequenceGenerator());
+        Assert.assertNotEquals(RANGE, uniqueSequences.size());
+    }
+
+    @Test
+    public void givenSequenceGeneratorUsingSynchronizedMethod_whenRaceCondition_thenSuccess() throws Exception {
+        Set<Integer> uniqueSequences = getASetOFUniqueSequences(new SequenceGeneratorUsingSynchronizedMethod());
+        Assert.assertEquals(RANGE, uniqueSequences.size());
+    }
+
+    @Test
+    public void givenSequenceGeneratorUsingSynchronizedBlock_whenRaceCondition_thenSuccess() throws Exception {
+        Set<Integer> uniqueSequences = getASetOFUniqueSequences(new SequenceGeneratorUsingSynchronizedBlock());
+        Assert.assertEquals(RANGE, uniqueSequences.size());
+    }
+
+    @Test
+    public void givenSequenceGeneratorUsingReentrantLock_whenRaceCondition_thenSuccess() throws Exception {
+        Set<Integer> uniqueSequences = getASetOFUniqueSequences(new SequenceGeneratorUsingReentrantLock());
+        Assert.assertEquals(RANGE, uniqueSequences.size());
+    }
+
+    @Test
+    public void givenSequenceGeneratorUsingSemaphore_whenRaceCondition_thenSuccess() throws Exception {
+        Set<Integer> uniqueSequences = getASetOFUniqueSequences(new SequenceGeneratorUsingSemaphore());
+        Assert.assertEquals(RANGE, uniqueSequences.size());
+    }
+
+    @Test
+    public void givenSequenceGeneratorUsingMonitor_whenRaceCondition_thenSuccess() throws Exception {
+        Set<Integer> uniqueSequences = getASetOFUniqueSequences(new SequenceGeneratorUsingMonitor());
+        Assert.assertEquals(RANGE, uniqueSequences.size());
+    }
+
+    private Set<Integer> getASetOFUniqueSequences(SequenceGenerator generator) throws Exception {
+        ExecutorService executor = Executors.newFixedThreadPool(3);
+        Set<Integer> uniqueSequences = new HashSet<>();
+        List<Future<Integer>> futures = new ArrayList<>();
+
+        for (int i = 0; i < RANGE; i++) {
+            futures.add(executor.submit(generator::getNextSequence));
+        }
+
+        for (Future<Integer> future : futures) {
+            uniqueSequences.add(future.get());
+        }
+
+        executor.awaitTermination(15, TimeUnit.SECONDS);
+        executor.shutdown();
+
+        return uniqueSequences;
+    }
+
+}