Add additional unit tests to show that singletons can be created in single thread environment and multithread environment. Also add a test to demonstrate a whole with Singleton when instantiating using reflection

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

@@ -2,73 +2,109 @@ package com.iluwatar.singleton;
 
 import org.junit.Test;
 
+import java.lang.reflect.Constructor;
+import java.lang.reflect.Field;
+import java.lang.reflect.InvocationTargetException;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.*;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNull;
+
 /**
+ * This class provides several test case that test singleton construction.
  *
- * This test case demonstrates thread safety issues of lazy loaded Singleton implementation.
+ * The first proves that multiple calls to the singleton getInstance object are the same when called in the SAME thread.
- * <p>
+ * The second proves that multiple calls to the singleton getInstance object are the same when called in the DIFFERENT thread.
- * Out of the box you should see the test output something like the following:
+ * The third proves that there is a hole if the singleton is created reflectively
- * <p>
- * 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$LazyLoadedIvoryTower@6fde356e
- * <p>
- * By changing the method signature of LazyLoadedIvoryTower#getInstance from
- * <p><blockquote><pre>
- * 	 public static LazyLoadedIvoryTower getInstance()
- * </pre></blockquote><p>
- * into
- * <p><blockquote><pre>
- *   public synchronized static LazyLoadedIvoryTower getInstance()
- * </pre></blockquote><p>
- * you should see the test output change to something like the following:
- * <p>
- * Thread=Thread-4 creating instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$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;
+    private static final int NUM_THREADS = 5;
+    private List<ThreadSafeLazyLoadedIvoryTower> threadObjects = new ArrayList<>();
 
-	@Test
+    //NullObject class so Callable has to return something
-	public void test() {
+    private class NullObject{private NullObject(){}}
-		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 {
+    @Test
+    public void test_MultipleCallsReturnTheSameObjectInSameThread() {
+        //Create several instances in the same calling thread
+        ThreadSafeLazyLoadedIvoryTower instance1 = ThreadSafeLazyLoadedIvoryTower.getInstance();
+        ThreadSafeLazyLoadedIvoryTower instance2 = ThreadSafeLazyLoadedIvoryTower.getInstance();
+        ThreadSafeLazyLoadedIvoryTower instance3 = ThreadSafeLazyLoadedIvoryTower.getInstance();
+        //now check they are equal
+        assertEquals(instance1, instance1);
+        assertEquals(instance1, instance2);
+        assertEquals(instance2, instance3);
+        assertEquals(instance1, instance3);
+    }
 
-		@Override
+    @Test
-		public void run() {
+    public void test_MultipleCallsReturnTheSameObjectInDifferentThreads() throws InterruptedException, ExecutionException {
-			LazyLoadedIvoryTower instance = LazyLoadedIvoryTower.getInstance();
+        {//create several threads and inside each callable instantiate the singleton class
-			System.out.println("Thread=" + Thread.currentThread().getName() + " got instance=" + instance);
+            ExecutorService executorService = Executors.newSingleThreadExecutor();
-		}
-	}
 
-	private static class LazyLoadedIvoryTower {
+            List<Callable<NullObject>> threadList = new ArrayList<>();
+            for (int i = 0; i < NUM_THREADS; i++) {
+                threadList.add(new SingletonCreatingThread());
+            }
 
-		private static LazyLoadedIvoryTower instance = null;
+            ExecutorService service = Executors.newCachedThreadPool();
+            List<Future<NullObject>> results = service.invokeAll(threadList);
 
-		private LazyLoadedIvoryTower() {
+            //wait for all of the threads to complete
-		}
+            for (Future res : results) {
+                res.get();
+            }
 
-		public static LazyLoadedIvoryTower getInstance() {
+            //tidy up the executor
-			if (instance == null) {
+            executorService.shutdown();
-				instance = new LazyLoadedIvoryTower();
+        }
-				System.out.println("Thread=" + Thread.currentThread().getName() + " creating instance=" + instance);
+        {//now check the contents that were added to threadObjects by each thread
-			}
+            assertEquals(NUM_THREADS, threadObjects.size());
-			return instance;
+            assertEquals(threadObjects.get(0), threadObjects.get(1));
-		}
+            assertEquals(threadObjects.get(1), threadObjects.get(2));
-	}
+            assertEquals(threadObjects.get(2), threadObjects.get(3));
+            assertEquals(threadObjects.get(3), threadObjects.get(4));
+        }
+    }
+
+    @Test
+    @SuppressWarnings("unchecked")
+    public void test_HoleInSingletonCreationIfUsingReflection() throws ClassNotFoundException, NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
+        Field[] f = ThreadSafeLazyLoadedIvoryTower.class.getDeclaredFields();
+        assertEquals("One field only in ThreadSafeLazyLoadedIvoryTower", 1, f.length);
+        f[0].setAccessible(true);
+
+        {//reflectively create an object - the singleton field is null
+            Class lazyIvoryTowerClazz = Class.forName("com.iluwatar.singleton.ThreadSafeLazyLoadedIvoryTower");
+            Constructor<ThreadSafeLazyLoadedIvoryTower> constructor = lazyIvoryTowerClazz.getDeclaredConstructor();
+            constructor.setAccessible(true);
+            ThreadSafeLazyLoadedIvoryTower instance = constructor.newInstance();
+            assertNull(f[0].get(instance));
+        }
+
+        //instantiate the singleton but when we do the below code we are creating a new object where it is set to null still
+        IvoryTower.getInstance();
+
+        {//reflectively create an object - the singleton field is null as a new object is created
+            Class lazyIvoryTowerClazz = Class.forName("com.iluwatar.singleton.ThreadSafeLazyLoadedIvoryTower");
+            Constructor<ThreadSafeLazyLoadedIvoryTower> constructor = lazyIvoryTowerClazz.getDeclaredConstructor();
+            constructor.setAccessible(true);
+            ThreadSafeLazyLoadedIvoryTower instance = constructor.newInstance();
+            assertNull(f[0].get(instance));
+        }
+    }
+
+    private class SingletonCreatingThread implements Callable<NullObject> {
+        @Override
+        public NullObject call() {
+            //instantiate the thread safety class and add to list to test afterwards
+            ThreadSafeLazyLoadedIvoryTower instance = ThreadSafeLazyLoadedIvoryTower.getInstance();
+            threadObjects.add(instance);
+            return new NullObject();//return null object (cannot return Void)
+        }
+    }
 }