iluwatar#100 Added test case for demonstrating the thread safety issues of a

naive lazy loaded Singleton implementation.

Author: iluwatar

singleton/src/test/java/com/iluwatar/singleton/LazyLoadedSingletonThreadSafetyTest.java

@@ -0,0 +1,70 @@
+package com.iluwatar.singleton;
+
+import org.junit.Test;
+
+/**
+ * 
+ * This test case demonstrates thread safety issues of lazy loaded Singleton implementation.
+ * 
+ * Out of the box you should see the test output something like the following:
+ * 
+ * Thread=Thread-4 got instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@6fde356e
+ * Thread=Thread-2 creating instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@6fde356e
+ * Thread=Thread-0 creating instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@6fde356e
+ * Thread=Thread-0 got instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@6fde356e
+ * Thread=Thread-3 got instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@6fde356e
+ * Thread=Thread-1 got instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@60f330b0
+ * Thread=Thread-2 got instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@6fde356e
+ * 
+ * By changing the method signature of LazyLoadedIvoryTower#getInstance from
+ * 	 public static LazyLoadedIvoryTower getInstance()
+ * into
+ *   public synchronized static LazyLoadedIvoryTower getInstance()
+ * you should see the test output change to something like the following:
+ * 
+ * Thread=Thread-4 creating instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@3c688490
+ * Thread=Thread-4 got instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@3c688490
+ * Thread=Thread-0 got instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@3c688490
+ * Thread=Thread-3 got instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@3c688490
+ * Thread=Thread-2 got instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@3c688490
+ * Thread=Thread-1 got instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@3c688490
+ *
+ */
+public class LazyLoadedSingletonThreadSafetyTest {
+
+	private static final int NUM_THREADS = 5;
+	
+	@Test
+	public void test() {
+		SingletonThread runnable = new SingletonThread();
+		for (int j=0; j<NUM_THREADS; j++) {
+			Thread thread = new Thread(runnable);
+			thread.start();
+		}
+	}
+	
+	private static class SingletonThread implements Runnable {
+
+		@Override
+		public void run() {
+			LazyLoadedIvoryTower instance = LazyLoadedIvoryTower.getInstance();
+			System.out.println("Thread=" + Thread.currentThread().getName() + " got instance=" + instance);
+		}
+	}
+	
+	private static class LazyLoadedIvoryTower {
+
+		private static LazyLoadedIvoryTower instance = null;
+		
+		private LazyLoadedIvoryTower() {
+		}
+
+		public static LazyLoadedIvoryTower getInstance() {
+			if (instance == null) {
+				instance = new LazyLoadedIvoryTower();
+				System.out.println("Thread=" + Thread.currentThread().getName() + " creating instance=" + instance);
+			}
+			return instance;
+		}
+	}
+}