BAEL-1029 - deep20jain@gmail.com - An Introduction to Atomic Variables in Java (#2269)

* Adding test classes for java atomic variables

* Updating counter with atomic integer

* Adding reason for ignoring test

core-java/src/main/java/com/baeldung/concurrent/atomic/SafeCounterWithLock.java

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+package com.baeldung.concurrent.atomic;
+
+public class SafeCounterWithLock {
+    int counter;
+    
+    public int getValue() {
+        return counter;
+    }
+    
+    public synchronized void increment() {
+        counter++;
+    }
+}

core-java/src/main/java/com/baeldung/concurrent/atomic/SafeCounterWithoutLock.java

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+package com.baeldung.concurrent.atomic;
+
+import java.util.concurrent.atomic.AtomicInteger;
+
+public class SafeCounterWithoutLock {
+    AtomicInteger counter = new AtomicInteger(0);
+    
+    public int getValue() {
+        return counter.get();
+    }
+    
+    public void increment() {
+        while(true) {
+            int existingValue = getValue();
+            int newValue = existingValue + 1;
+            if(counter.compareAndSet(existingValue, newValue)) {
+                return;
+            }
+        }
+    }
+}

core-java/src/main/java/com/baeldung/concurrent/atomic/UnsafeCounter.java

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+package com.baeldung.concurrent.atomic;
+
+public class UnsafeCounter {
+    int counter;
+    
+    public int getValue() {
+        return counter;
+    }
+    
+    public void increment() {
+        counter++;
+    }
+}

core-java/src/test/java/com/baeldung/concurrent/atomic/CounterTest.java

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+package com.baeldung.concurrent.atomic;
+
+import static org.junit.Assert.assertEquals;
+
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
+import java.util.stream.IntStream;
+
+import org.junit.Ignore;
+import org.junit.Test;
+
+public class CounterTest {
+
+    /**
+     * This test shows the behaviour of a thread-unsafe class in a multithreaded scenario. We are calling
+     * the increment methods 1000 times from a pool of 3 threads. In most of the cases, the counter will 
+     * less than 1000, because of lost updates, however, occasionally it may reach 1000, when no threads
+     * called the method simultaneously. This may cause the build to fail occasionally. Hence excluding this
+     * test by adding Ignore annotation.
+     */
+    @Test
+    @Ignore
+    public void givenMultiThread_whenUnsafeCounterIncrement() throws InterruptedException {
+        ExecutorService service = Executors.newFixedThreadPool(3);
+        UnsafeCounter unsafeCounter = new UnsafeCounter();
+
+        IntStream.range(0, 1000)
+          .forEach(count -> service.submit(unsafeCounter::increment));
+        service.awaitTermination(100, TimeUnit.MILLISECONDS);
+
+        assertEquals(1000, unsafeCounter.getValue());
+    }
+    
+    @Test
+    public void givenMultiThread_whenSafeCounterWithLockIncrement() throws InterruptedException {
+        ExecutorService service = Executors.newFixedThreadPool(3);
+        SafeCounterWithLock safeCounter = new SafeCounterWithLock();
+
+        IntStream.range(0, 1000)
+          .forEach(count -> service.submit(safeCounter::increment));
+        service.awaitTermination(100, TimeUnit.MILLISECONDS);
+
+        assertEquals(1000, safeCounter.getValue());
+    }
+    
+    @Test
+    public void givenMultiThread_whenSafeCounterWithoutLockIncrement() throws InterruptedException {
+        ExecutorService service = Executors.newFixedThreadPool(3);
+        SafeCounterWithoutLock safeCounter = new SafeCounterWithoutLock();
+
+        IntStream.range(0, 1000)
+          .forEach(count -> service.submit(safeCounter::increment));
+        service.awaitTermination(100, TimeUnit.MILLISECONDS);
+
+        assertEquals(1000, safeCounter.getValue());
+    }
+}