What is thread-safety and how to achieve it

Issue: BAEL-2416

core-java-concurrency-basic/src/main/java/com/baeldung/concurrent/threadsafety/application/Application.java

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+package com.baeldung.concurrent.threadsafety.application;
+
+import com.baeldung.concurrent.threadsafety.callables.AtomicCounterCallable;
+import com.baeldung.concurrent.threadsafety.mathutils.MathUtils;
+import com.baeldung.concurrent.threadsafety.callables.CounterCallable;
+import com.baeldung.concurrent.threadsafety.callables.ExtrinsicLockCounterCallable;
+import com.baeldung.concurrent.threadsafety.callables.MessageServiceCallable;
+import com.baeldung.concurrent.threadsafety.callables.ReentranReadWriteLockCounterCallable;
+import com.baeldung.concurrent.threadsafety.callables.ReentrantLockCounterCallable;
+import com.baeldung.concurrent.threadsafety.services.AtomicCounter;
+import com.baeldung.concurrent.threadsafety.services.Counter;
+import com.baeldung.concurrent.threadsafety.services.ExtrinsicLockCounter;
+import com.baeldung.concurrent.threadsafety.services.MessageService;
+import com.baeldung.concurrent.threadsafety.services.ReentrantLockCounter;
+import com.baeldung.concurrent.threadsafety.services.ReentrantReadWriteLockCounter;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ExecutionException;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+
+public class Application {
+    
+    public static void main(String[] args) throws InterruptedException, ExecutionException {
+
+        new Thread(() -> {
+            System.out.println(MathUtils.factorial(10));
+        }).start();
+        new Thread(() -> {
+            System.out.println(MathUtils.factorial(5));
+        }).start();
+        
+        ExecutorService executorService = Executors.newFixedThreadPool(10);
+        MessageService messageService = new MessageService("Welcome to Baeldung!");
+        Future<String> future1 = (Future<String>) executorService.submit(new MessageServiceCallable(messageService));
+        Future<String> future2 = (Future<String>) executorService.submit(new MessageServiceCallable(messageService));
+        System.out.println(future1.get());
+        System.out.println(future2.get());
+        
+        Counter counter = new Counter();
+        Future<Integer> future3 = (Future<Integer>) executorService.submit(new CounterCallable(counter));
+        Future<Integer> future4 = (Future<Integer>) executorService.submit(new CounterCallable(counter));
+        System.out.println(future3.get());
+        System.out.println(future4.get());
+        
+        ExtrinsicLockCounter extrinsicLockCounter = new ExtrinsicLockCounter();
+        Future<Integer> future5 = (Future<Integer>) executorService.submit(new ExtrinsicLockCounterCallable(extrinsicLockCounter));
+        Future<Integer> future6 = (Future<Integer>) executorService.submit(new ExtrinsicLockCounterCallable(extrinsicLockCounter));
+        System.out.println(future5.get());
+        System.out.println(future6.get());
+        
+        ReentrantLockCounter reentrantLockCounter = new ReentrantLockCounter();
+        Future<Integer> future7 = (Future<Integer>) executorService.submit(new ReentrantLockCounterCallable(reentrantLockCounter));
+        Future<Integer> future8 = (Future<Integer>) executorService.submit(new ReentrantLockCounterCallable(reentrantLockCounter));
+        System.out.println(future7.get());
+        System.out.println(future8.get());
+        
+        ReentrantReadWriteLockCounter reentrantReadWriteLockCounter = new ReentrantReadWriteLockCounter();
+        Future<Integer> future9 = (Future<Integer>) executorService.submit(new ReentranReadWriteLockCounterCallable(reentrantReadWriteLockCounter));
+        Future<Integer> future10 = (Future<Integer>) executorService.submit(new ReentranReadWriteLockCounterCallable(reentrantReadWriteLockCounter));
+        System.out.println(future9.get());
+        System.out.println(future10.get());
+        
+        AtomicCounter atomicCounter = new AtomicCounter();
+        Future<Integer> future11 = (Future<Integer>) executorService.submit(new AtomicCounterCallable(atomicCounter));
+        Future<Integer> future12 = (Future<Integer>) executorService.submit(new AtomicCounterCallable(atomicCounter));
+        System.out.println(future11.get());
+        System.out.println(future12.get());
+        
+        Collection<Integer> syncCollection = Collections.synchronizedCollection(new ArrayList<>());
+        Thread thread11 = new Thread(() -> syncCollection.addAll(Arrays.asList(1, 2, 3, 4, 5, 6)));
+        Thread thread12 = new Thread(() -> syncCollection.addAll(Arrays.asList(1, 2, 3, 4, 5, 6)));
+        thread11.start();
+        thread12.start();
+        
+        Map<String,String> concurrentMap = new ConcurrentHashMap<>();
+        concurrentMap.put("1", "one");
+        concurrentMap.put("2", "two");
+        concurrentMap.put("3", "three");
+    }
+}

core-java-concurrency-basic/src/main/java/com/baeldung/concurrent/threadsafety/callables/AtomicCounterCallable.java

@@ -0,0 +1,19 @@
+package com.baeldung.concurrent.threadsafety.callables;
+
+import com.baeldung.concurrent.threadsafety.services.AtomicCounter;
+import java.util.concurrent.Callable;
+
+public class AtomicCounterCallable implements Callable<Integer> {
+
+    private final AtomicCounter counter;
+    
+    public AtomicCounterCallable(AtomicCounter counter) {
+        this.counter = counter;
+    }
+
+    @Override
+    public Integer call() throws Exception {
+        counter.incrementCounter();
+        return counter.getCounter();
+    }
+}

core-java-concurrency-basic/src/main/java/com/baeldung/concurrent/threadsafety/callables/CounterCallable.java

@@ -0,0 +1,19 @@
+package com.baeldung.concurrent.threadsafety.callables;
+
+import com.baeldung.concurrent.threadsafety.services.Counter;
+import java.util.concurrent.Callable;
+
+public class CounterCallable implements Callable<Integer> {
+
+    private final Counter counter;
+    
+    public CounterCallable(Counter counter) {
+        this.counter = counter;
+    }
+
+    @Override
+    public Integer call() throws Exception {
+        counter.incrementCounter();
+        return counter.getCounter();
+    }
+}

core-java-concurrency-basic/src/main/java/com/baeldung/concurrent/threadsafety/callables/ExtrinsicLockCounterCallable.java

@@ -0,0 +1,19 @@
+package com.baeldung.concurrent.threadsafety.callables;
+
+import com.baeldung.concurrent.threadsafety.services.ExtrinsicLockCounter;
+import java.util.concurrent.Callable;
+
+public class ExtrinsicLockCounterCallable implements Callable<Integer> {
+
+    private final ExtrinsicLockCounter counter;
+    
+    public ExtrinsicLockCounterCallable(ExtrinsicLockCounter counter) {
+        this.counter = counter;
+    }
+    
+    @Override
+    public Integer call() throws Exception {
+        counter.incrementCounter();
+        return counter.getCounter();
+    }
+}

core-java-concurrency-basic/src/main/java/com/baeldung/concurrent/threadsafety/callables/MessageServiceCallable.java

@@ -0,0 +1,19 @@
+package com.baeldung.concurrent.threadsafety.callables;
+
+import com.baeldung.concurrent.threadsafety.services.MessageService;
+import java.util.concurrent.Callable;
+
+public class MessageServiceCallable implements Callable<String> {
+    
+    private final MessageService messageService;
+    
+    public MessageServiceCallable(MessageService messageService) {
+        this.messageService = messageService;
+    
+    }
+    
+    @Override
+    public String call() {
+        return messageService.getMesssage();
+    }
+}

core-java-concurrency-basic/src/main/java/com/baeldung/concurrent/threadsafety/callables/ReentranReadWriteLockCounterCallable.java

@@ -0,0 +1,20 @@
+package com.baeldung.concurrent.threadsafety.callables;
+
+import com.baeldung.concurrent.threadsafety.services.ReentrantReadWriteLockCounter;
+import java.util.concurrent.Callable;
+
+public class ReentranReadWriteLockCounterCallable implements Callable<Integer> {
+
+    private final ReentrantReadWriteLockCounter counter;
+
+    public ReentranReadWriteLockCounterCallable(ReentrantReadWriteLockCounter counter) {
+        this.counter = counter;
+    }
+    
+    @Override
+    public Integer call() throws Exception {
+        counter.incrementCounter();
+        return counter.getCounter();
+    }
+
+}

core-java-concurrency-basic/src/main/java/com/baeldung/concurrent/threadsafety/callables/ReentrantLockCounterCallable.java

@@ -0,0 +1,19 @@
+package com.baeldung.concurrent.threadsafety.callables;
+
+import com.baeldung.concurrent.threadsafety.services.ReentrantLockCounter;
+import java.util.concurrent.Callable;
+
+public class ReentrantLockCounterCallable implements Callable<Integer> {
+
+    private final ReentrantLockCounter counter;
+    
+    public ReentrantLockCounterCallable(ReentrantLockCounter counter) {
+        this.counter = counter;
+    }
+    
+    @Override
+    public Integer call() throws Exception {
+        counter.incrementCounter();
+        return counter.getCounter();
+    }
+}

core-java-concurrency-basic/src/main/java/com/baeldung/concurrent/threadsafety/mathutils/MathUtils.java

@@ -0,0 +1,14 @@
+package com.baeldung.concurrent.threadsafety.mathutils;
+
+import java.math.BigInteger;
+
+public class MathUtils {
+    
+    public static BigInteger factorial(int number) {
+        BigInteger f = new BigInteger("1");
+        for (int i = 2; i <= number; i++) {
+            f = f.multiply(BigInteger.valueOf(i));
+        }
+        return f;
+    }
+}

core-java-concurrency-basic/src/main/java/com/baeldung/concurrent/threadsafety/services/AtomicCounter.java

@@ -0,0 +1,18 @@
+package com.baeldung.concurrent.threadsafety.services;
+
+import java.util.concurrent.atomic.AtomicInteger;
+
+public class AtomicCounter {
+    
+    private final AtomicInteger counter = new AtomicInteger();
+    
+    public AtomicCounter() {}
+    
+    public void incrementCounter() {
+        counter.incrementAndGet();
+    }
+    
+    public synchronized int getCounter() {
+        return counter.get();
+    }
+}

core-java-concurrency-basic/src/main/java/com/baeldung/concurrent/threadsafety/services/Counter.java

@@ -0,0 +1,18 @@
+package com.baeldung.concurrent.threadsafety.services;
+
+public class Counter {
+    
+    private volatile int counter;
+    
+    public Counter() {
+        this.counter = 0;
+    }
+    
+    public synchronized void incrementCounter() {
+        counter += 1;
+    }
+    
+    public int getCounter() {
+        return counter;
+    }
+}

core-java-concurrency-basic/src/main/java/com/baeldung/concurrent/threadsafety/services/ExtrinsicLockCounter.java

@@ -0,0 +1,23 @@
+package com.baeldung.concurrent.threadsafety.services;
+
+public class ExtrinsicLockCounter {
+
+    private int counter;
+    private final Object lock = new Object();
+
+    public ExtrinsicLockCounter() {
+        this.counter = 0;
+    }
+
+    public void incrementCounter() {
+        synchronized (lock) {
+            counter += 1;
+        }
+    }
+
+    public int getCounter() {
+        synchronized (lock) {
+            return counter;
+        }
+    }
+}

core-java-concurrency-basic/src/main/java/com/baeldung/concurrent/threadsafety/services/MessageService.java

@@ -0,0 +1,14 @@
+package com.baeldung.concurrent.threadsafety.services;
+
+public class MessageService {
+    
+    private final String message;
+    
+    public MessageService(String message) {
+        this.message = message;
+    }
+    
+    public String getMesssage() {
+        return message;
+    }
+}

core-java-concurrency-basic/src/main/java/com/baeldung/concurrent/threadsafety/services/ReentrantLockCounter.java

@@ -0,0 +1,26 @@
+package com.baeldung.concurrent.threadsafety.services;
+
+import java.util.concurrent.locks.ReentrantLock;
+
+public class ReentrantLockCounter {
+
+    private int counter;
+    private final ReentrantLock reLock = new ReentrantLock(true);
+
+    public ReentrantLockCounter() {
+        this.counter = 0;
+    }
+
+    public void incrementCounter() {
+        reLock.lock();
+        try {
+            counter += 1;
+        } finally {
+            reLock.unlock();
+        }
+    }
+
+    public int getCounter() {
+        return counter;
+    }
+}

core-java-concurrency-basic/src/main/java/com/baeldung/concurrent/threadsafety/services/ReentrantReadWriteLockCounter.java

@@ -0,0 +1,34 @@
+package com.baeldung.concurrent.threadsafety.services;
+
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
+
+public class ReentrantReadWriteLockCounter {
+    
+    private int counter;
+    private final ReentrantReadWriteLock rwLock = new ReentrantReadWriteLock();
+    private final Lock readLock = rwLock.readLock();
+    private final Lock writeLock = rwLock.writeLock();
+    
+    public ReentrantReadWriteLockCounter() {
+        this.counter = 0;
+    }
+    
+    public void incrementCounter() {
+        writeLock.lock();
+        try {
+            counter += 1;
+        } finally {
+            writeLock.unlock();
+        }
+    }
+    
+    public int getCounter() {
+        readLock.lock();
+        try {
+            return counter;
+        } finally {
+            readLock.unlock();
+        }
+    }
+}

core-java-concurrency-basic/src/main/java/com/baeldung/concurrent/threadsafety/services/StateHolder.java

@@ -0,0 +1,14 @@
+package com.baeldung.concurrent.threadsafety.services;
+
+public class StateHolder {
+    
+    private final String state;
+
+    public StateHolder(String state) {
+        this.state = state;
+    }
+    
+    public String getState() {
+        return state;
+    }
+}

core-java-concurrency-basic/src/test/com/baeldung/concurrent/threadsafety/tests/CounterTest.java

@@ -0,0 +1,23 @@
+package com.baeldung.concurrent.threadsafety.tests;
+
+import org.junit.Test;
+import static org.assertj.core.api.Assertions.assertThat;
+import com.baeldung.concurrent.threadsafety.callables.CounterCallable;
+import com.baeldung.concurrent.threadsafety.services.Counter;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+
+public class CounterTest {
+
+    @Test
+    public void whenCalledIncrementCounter_thenCorrect() throws Exception {
+        ExecutorService executorService = Executors.newFixedThreadPool(2);
+        Counter counter = new Counter();
+        Future<Integer> future1 = (Future<Integer>) executorService.submit(new CounterCallable(counter));
+        Future<Integer> future2 = (Future<Integer>) executorService.submit(new CounterCallable(counter));
+        
+        assertThat(future1.get()).isEqualTo(1);
+        assertThat(future2.get()).isEqualTo(2);
+    }
+}

core-java-concurrency-basic/src/test/com/baeldung/concurrent/threadsafety/tests/ExtrinsicLockCounterTest.java

@@ -0,0 +1,23 @@
+package com.baeldung.concurrent.threadsafety.tests;
+
+import org.junit.Test;
+import static org.assertj.core.api.Assertions.assertThat;
+import com.baeldung.concurrent.threadsafety.callables.ExtrinsicLockCounterCallable;
+import com.baeldung.concurrent.threadsafety.services.ExtrinsicLockCounter;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+
+public class ExtrinsicLockCounterTest {
+
+    @Test
+    public void whenCalledIncrementCounter_thenCorrect() throws Exception {
+        ExecutorService executorService = Executors.newFixedThreadPool(2);
+        ExtrinsicLockCounter counter = new ExtrinsicLockCounter();
+        Future<Integer> future1 = (Future<Integer>) executorService.submit(new ExtrinsicLockCounterCallable(counter));
+        Future<Integer> future2 = (Future<Integer>) executorService.submit(new ExtrinsicLockCounterCallable(counter));
+        
+        assertThat(future1.get()).isEqualTo(1);
+        assertThat(future2.get()).isEqualTo(2);
+    }
+}

core-java-concurrency-basic/src/test/com/baeldung/concurrent/threadsafety/tests/MathUtilsTest.java

@@ -0,0 +1,13 @@
+package com.baeldung.concurrent.threadsafety.tests;
+
+import com.baeldung.concurrent.threadsafety.mathutils.MathUtils;
+import org.junit.Test;
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class MathUtilsTest {
+    
+    @Test
+    public void whenCalledFactorialMethod_thenCorrect() {
+        assertThat(MathUtils.factorial(2)).isEqualTo(2);
+    }
+}

core-java-concurrency-basic/src/test/com/baeldung/concurrent/threadsafety/tests/MessageServiceTest.java

@@ -0,0 +1,23 @@
+package com.baeldung.concurrent.threadsafety.tests;
+
+import org.junit.Test;
+import static org.assertj.core.api.Assertions.assertThat;
+import com.baeldung.concurrent.threadsafety.callables.MessageServiceCallable;
+import com.baeldung.concurrent.threadsafety.services.MessageService;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+
+public class MessageServiceTest {
+
+    @Test
+    public void whenCalledgetMessage_thenCorrect() throws Exception {
+        ExecutorService executorService = Executors.newFixedThreadPool(2);
+        MessageService messageService = new MessageService("Welcome to Baeldung!");
+        Future<String> future1 = (Future<String>) executorService.submit(new MessageServiceCallable(messageService));
+        Future<String> future2 = (Future<String>) executorService.submit(new MessageServiceCallable(messageService));
+
+        assertThat(future1.get()).isEqualTo("Welcome to Baeldung!");
+        assertThat(future2.get()).isEqualTo("Welcome to Baeldung!");
+    }
+}

core-java-concurrency-basic/src/test/com/baeldung/concurrent/threadsafety/tests/ReentrantLockCounterTest.java

@@ -0,0 +1,23 @@
+package com.baeldung.concurrent.threadsafety.tests;
+
+import com.baeldung.concurrent.threadsafety.callables.ReentrantLockCounterCallable;
+import com.baeldung.concurrent.threadsafety.services.ReentrantLockCounter;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+import static org.assertj.core.api.Assertions.assertThat;
+import org.junit.Test;
+
+public class ReentrantLockCounterTest {
+    
+    @Test
+    public void whenCalledIncrementCounter_thenCorrect() throws Exception {
+        ExecutorService executorService = Executors.newFixedThreadPool(2);
+        ReentrantLockCounter counter = new ReentrantLockCounter();
+        Future<Integer> future1 = (Future<Integer>) executorService.submit(new ReentrantLockCounterCallable(counter));
+        Future<Integer> future2 = (Future<Integer>) executorService.submit(new ReentrantLockCounterCallable(counter));
+        
+        assertThat(future1.get()).isEqualTo(1);
+        assertThat(future2.get()).isEqualTo(2);
+    }
+}

core-java-concurrency-basic/src/test/com/baeldung/concurrent/threadsafety/tests/ReentrantReadWriteLockCounterTest.java

@@ -0,0 +1,24 @@
+package com.baeldung.concurrent.threadsafety.tests;
+
+import com.baeldung.concurrent.threadsafety.callables.ReentranReadWriteLockCounterCallable;
+import com.baeldung.concurrent.threadsafety.services.ReentrantReadWriteLockCounter;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+import static org.assertj.core.api.Assertions.assertThat;
+import org.junit.Test;
+
+public class ReentrantReadWriteLockCounterTest {
+    
+    @Test
+    public void whenCalledIncrementCounter_thenCorrect() throws Exception {
+        ExecutorService executorService = Executors.newFixedThreadPool(2);
+        ReentrantReadWriteLockCounter counter = new ReentrantReadWriteLockCounter();
+        Future<Integer> future1 = (Future<Integer>) executorService.submit(new  ReentranReadWriteLockCounterCallable(counter));
+        Future<Integer> future2 = (Future<Integer>) executorService.submit(new  ReentranReadWriteLockCounterCallable(counter));
+        
+        assertThat(future1.get()).isEqualTo(1);
+        assertThat(future2.get()).isEqualTo(2);
+    }
+    
+}