BAEL 1639: added singleton examples and test. (#3977)

core-java/src/main/java/com/baeldung/designpatterns/singleton/synchronization/DclSingleton.java

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+package com.baeldung.designpatterns.singleton.synchronization;
+
+/**
+ * Double-checked locking design pattern applied to a singleton.
+ * 
+ * @author Donato Rimenti
+ *
+ */
+public class DclSingleton {
+
+	/**
+	 * Current instance of the singleton.
+	 */
+	private static volatile DclSingleton instance;
+
+	/**
+	 * Private constructor to avoid instantiation.
+	 */
+	private DclSingleton() {
+	}
+
+	/**
+	 * Returns the current instance of the singleton.
+	 * 
+	 * @return the current instance of the singleton
+	 */
+	public static DclSingleton getInstance() {
+		if (instance == null) {
+			synchronized (DclSingleton.class) {
+				if (instance == null) {
+					instance = new DclSingleton();
+				}
+			}
+		}
+		return instance;
+	}
+
+}

core-java/src/main/java/com/baeldung/designpatterns/singleton/synchronization/DraconianSingleton.java

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+package com.baeldung.designpatterns.singleton.synchronization;
+
+/**
+ * Draconian singleton. The method to get the instance is synchronized.
+ * 
+ * @author Donato Rimenti
+ *
+ */
+public class DraconianSingleton {
+
+	/**
+	 * Current instance of the singleton.
+	 */
+	private static DraconianSingleton instance;
+
+	/**
+	 * Private constructor to avoid instantiation.
+	 */
+	private DraconianSingleton() {
+	}
+
+	/**
+	 * Returns the current instance of the singleton.
+	 * 
+	 * @return the current instance of the singleton
+	 */
+	public static synchronized DraconianSingleton getInstance() {
+		if (instance == null) {
+			instance = new DraconianSingleton();
+		}
+		return instance;
+	}
+
+}

core-java/src/main/java/com/baeldung/designpatterns/singleton/synchronization/EarlyInitSingleton.java

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+package com.baeldung.designpatterns.singleton.synchronization;
+
+/**
+ * Singleton with early initialization. Inlines the singleton instance
+ * initialization.
+ * 
+ * @author Donato Rimenti
+ *
+ */
+public class EarlyInitSingleton {
+
+	/**
+	 * Current instance of the singleton.
+	 */
+	private static final EarlyInitSingleton INSTANCE = new EarlyInitSingleton();
+
+	/**
+	 * Private constructor to avoid instantiation.
+	 */
+	private EarlyInitSingleton() {
+	}
+
+	/**
+	 * Returns the current instance of the singleton.
+	 * 
+	 * @return the current instance of the singleton
+	 */
+	public static EarlyInitSingleton getInstance() {
+		return INSTANCE;
+	}
+}

core-java/src/main/java/com/baeldung/designpatterns/singleton/synchronization/EnumSingleton.java

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+package com.baeldung.designpatterns.singleton.synchronization;
+
+/**
+ * Enum singleton pattern. Uses an enum to hold a reference to the singleton
+ * instance.
+ * 
+ * @author Donato Rimenti
+ *
+ */
+public enum EnumSingleton {
+
+	/**
+	 * Current instance of the singleton.
+	 */
+	INSTANCE;
+}

core-java/src/main/java/com/baeldung/designpatterns/singleton/synchronization/InitOnDemandSingleton.java

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+package com.baeldung.designpatterns.singleton.synchronization;
+
+/**
+ * Initialization on demand singleton pattern. Uses a nested static class to
+ * hold a reference to the singleton instance.
+ * 
+ * @author Donato Rimenti
+ *
+ */
+public class InitOnDemandSingleton {
+
+	/**
+	 * Holder for a singleton instance.
+	 * 
+	 * @author Donato Rimenti
+	 *
+	 */
+	private static class InstanceHolder {
+
+		/**
+		 * Current instance of the singleton.
+		 */
+		private static final InitOnDemandSingleton INSTANCE = new InitOnDemandSingleton();
+	}
+
+	/**
+	 * Private constructor to avoid instantiation.
+	 */
+	private InitOnDemandSingleton() {
+	}
+
+	/**
+	 * Returns the current instance of the singleton.
+	 * 
+	 * @return the current instance of the singleton
+	 */
+	public static InitOnDemandSingleton getInstance() {
+		return InstanceHolder.INSTANCE;
+	}
+
+}

core-java/src/test/java/com/baeldung/designpatterns/singleton/synchronization/SingletonSynchronizationUnitTest.java

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+package com.baeldung.designpatterns.singleton.synchronization;
+
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.Set;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+/**
+ * Unit tests for the singleton synchronization package with the same name.
+ * 
+ * @author Donato Rimenti
+ *
+ */
+public class SingletonSynchronizationUnitTest {
+
+	/**
+	 * Size of the thread pools used.
+	 */
+	private static final int POOL_SIZE = 1_000;
+	
+	/**
+	 * Number of tasks to submit.
+	 */
+	private static final int TASKS_TO_SUBMIT = 1_000_000;
+
+	/**
+	 * Tests the thread-safety of {@link DraconianSingleton}.
+	 */
+	@Test
+	public void givenDraconianSingleton_whenMultithreadInstancesEquals_thenTrue() {
+		ExecutorService executor = Executors.newFixedThreadPool(POOL_SIZE);
+		Set<DraconianSingleton> resultSet = Collections.synchronizedSet(new HashSet<DraconianSingleton>());
+
+		// Submits the instantiation tasks.
+		for (int i = 0; i < TASKS_TO_SUBMIT; i++) {
+			executor.submit(() -> resultSet.add(DraconianSingleton.getInstance()));
+		}
+
+		// Since the instance of the object we inserted into the set is always
+		// the same, the size should be one.
+		Assert.assertEquals(1, resultSet.size());
+	}
+	
+	/**
+	 * Tests the thread-safety of {@link DclSingleton}.
+	 */
+	@Test
+	public void givenDclSingleton_whenMultithreadInstancesEquals_thenTrue() {
+		ExecutorService executor = Executors.newFixedThreadPool(POOL_SIZE);
+		Set<DclSingleton> resultSet = Collections.synchronizedSet(new HashSet<DclSingleton>());
+
+		// Submits the instantiation tasks.
+		for (int i = 0; i < TASKS_TO_SUBMIT; i++) {
+			executor.submit(() -> resultSet.add(DclSingleton.getInstance()));
+		}
+
+		// Since the instance of the object we inserted into the set is always
+		// the same, the size should be one.
+		Assert.assertEquals(1, resultSet.size());
+	}
+
+	/**
+	 * Tests the thread-safety of {@link EarlyInitSingleton}.
+	 */
+	@Test
+	public void givenEarlyInitSingleton_whenMultithreadInstancesEquals_thenTrue() {
+		ExecutorService executor = Executors.newFixedThreadPool(POOL_SIZE);
+		Set<EarlyInitSingleton> resultSet = Collections.synchronizedSet(new HashSet<EarlyInitSingleton>());
+
+		// Submits the instantiation tasks.
+		for (int i = 0; i < TASKS_TO_SUBMIT; i++) {
+			executor.submit(() -> resultSet.add(EarlyInitSingleton.getInstance()));
+		}
+
+		// Since the instance of the object we inserted into the set is always
+		// the same, the size should be one.
+		Assert.assertEquals(1, resultSet.size());
+	}
+
+	/**
+	 * Tests the thread-safety of {@link InitOnDemandSingleton}.
+	 */
+	@Test
+	public void givenInitOnDemandSingleton_whenMultithreadInstancesEquals_thenTrue() {
+		ExecutorService executor = Executors.newFixedThreadPool(POOL_SIZE);
+		Set<InitOnDemandSingleton> resultSet = Collections.synchronizedSet(new HashSet<InitOnDemandSingleton>());
+
+		// Submits the instantiation tasks.
+		for (int i = 0; i < TASKS_TO_SUBMIT; i++) {
+			executor.submit(() -> resultSet.add(InitOnDemandSingleton.getInstance()));
+		}
+
+		// Since the instance of the object we inserted into the set is always
+		// the same, the size should be one.
+		Assert.assertEquals(1, resultSet.size());
+	}
+
+	/**
+	 * Tests the thread-safety of {@link EnumSingleton}.
+	 */
+	@Test
+	public void givenEnumSingleton_whenMultithreadInstancesEquals_thenTrue() {
+		ExecutorService executor = Executors.newFixedThreadPool(POOL_SIZE);
+		Set<EnumSingleton> resultSet = Collections.synchronizedSet(new HashSet<EnumSingleton>());
+
+		// Submits the instantiation tasks.
+		for (int i = 0; i < TASKS_TO_SUBMIT; i++) {
+			executor.submit(() -> resultSet.add(EnumSingleton.INSTANCE));
+		}
+
+		// Since the instance of the object we inserted into the set is always
+		// the same, the size should be one.
+		Assert.assertEquals(1, resultSet.size());
+	}
+}