Java Design patterns

Java Design Patterns/creational/singleton/Singleton.java

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+package javadevjournal.design.creational.singleton;
+
+/**
+ * @author Kunwar
+ */
+public class Singleton {
+
+   /* private instance variable  */
+   private static Singleton instance = new Singleton();
+
+   /* private constructor */
+   private Singleton(){}
+
+   /* returns the same object */
+   public static Singleton getInstance(){
+      return instance;
+   }
+}

Java Design Patterns/creational/singleton/SingletonMultiThreaded.java

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+package javadevjournal.design.creational.singleton;
+
+public class SingletonMultiThreaded {
+
+    /* private instance variable  */
+    private static volatile SingletonMultiThreaded INSTANCE;
+  
+    /* private constructor */
+    private SingletonMultiThreaded() { }
+
+    public static SingletonMultiThreaded getInstance() {
+        /* double-checking lock */
+        if(null == INSTANCE){
+            /* synchronized block */
+            synchronized (SingletonMultiThreaded.class) {
+                if(null == INSTANCE){
+                    INSTANCE = new SingletonMultiThreaded();
+                }
+            }
+        }
+        return INSTANCE;
+    }
+}

Java Design Patterns/creational/singleton/SingletonPatternDemo.java

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+package javadevjournal.design.creational.singleton;
+
+/**
+ * @author Kunwar
+ */
+public class SingletonPatternDemo {
+   public static void main(String[] args) {
+      /* Let's create 3 objects and see their hashcode and they will be same. */
+      System.out.println("in single threaded environment");
+      Singleton singleton1 = Singleton.getInstance();
+      Singleton singleton2 = Singleton.getInstance();
+      Singleton singleton3 = Singleton.getInstance();
+      System.out.println(singleton1.hashCode() +" "+ singleton2.hashCode() +" "+ singleton3.hashCode());
+
+      System.out.println("in multi threaded environment");
+      Thread1 t1 = new Thread1();
+      t1.run();
+      Thread2 t2 = new Thread2();
+      t2.run();
+      Thread3 t3 = new Thread3();
+      t3.run();
+
+   }
+}

Java Design Patterns/creational/singleton/Thread1.java

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+package javadevjournal.design.creational.singleton;
+
+public class Thread1 implements  Runnable{
+    @Override
+    public void run() {
+        SingletonMultiThreaded singletonMultiThreaded = SingletonMultiThreaded.getInstance();
+        System.out.print(singletonMultiThreaded.hashCode() + " ");
+    }
+}

Java Design Patterns/creational/singleton/Thread2.java

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+package javadevjournal.design.creational.singleton;
+
+public class Thread2 implements  Runnable{
+    @Override
+    public void run() {
+        SingletonMultiThreaded singletonMultiThreaded = SingletonMultiThreaded.getInstance();
+        System.out.print(singletonMultiThreaded.hashCode() + " ");
+    }
+}

Java Design Patterns/creational/singleton/Thread3.java

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+package javadevjournal.design.creational.singleton;
+
+public class Thread3 implements  Runnable{
+    @Override
+    public void run() {
+        SingletonMultiThreaded singletonMultiThreaded = SingletonMultiThreaded.getInstance();
+        System.out.println(singletonMultiThreaded.hashCode());
+    }
+}

Java Design Patterns/creational/singleton/breakit/BreakSingletonUsingReflection.java

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+package javadevjournal.design.creational.singleton.breakit;
+
+import java.lang.reflect.Constructor;
+
+/**
+ * @author Kunwar
+ */
+public class BreakSingletonUsingReflection {
+    public static void main(String[] args) {
+        Singleton singleton1 = Singleton.instance;
+        Singleton singleton2 = null;
+        try {
+            Constructor[] constructors =
+                    Singleton.class.getDeclaredConstructors();
+            for (Constructor constructor : constructors) {
+                // Below code will destroy the singleton pattern
+                constructor.setAccessible(true);
+                singleton2 = (Singleton) constructor.newInstance();
+                break;
+            }
+        } catch (Exception e) {
+            e.printStackTrace();
+        }
+        System.out.println("instance1.hashCode(): " + singleton1.hashCode());
+        System.out.println("instance2.hashCode(): " + singleton2.hashCode());
+    }
+}

Java Design Patterns/creational/singleton/breakit/Singleton.java

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+package javadevjournal.design.creational.singleton.breakit;
+
+/**
+ * @author Kunwar
+ */
+class Singleton{
+    public static Singleton instance = new Singleton();
+    private Singleton() {
+    }
+}