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

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

@@ -0,0 +1,38 @@
+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

@@ -0,0 +1,34 @@
+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

@@ -0,0 +1,31 @@
+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

@@ -0,0 +1,16 @@
+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

@@ -0,0 +1,41 @@
+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;
+	}
+
+}