Merge pull request #263 from iamrichardjones/master

Update to singleton unit tests and lazy loading unit test

lazy-loading/src/test/java/com/iluwatar/lazy/loading/HolderThreadSafeTest.java

@@ -0,0 +1,42 @@
+package com.iluwatar.lazy.loading;
+
+import org.junit.Test;
+
+import java.lang.reflect.Field;
+
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertNull;
+
+/**
+ * Using reflection this test shows that the heavy field is not instantiated until the method getHeavy is called
+ *
+ * Created by jones on 11/10/2015.
+ */
+public class HolderThreadSafeTest {
+
+    @Test
+    public void test() throws IllegalAccessException {
+        HolderThreadSafe hts = new HolderThreadSafe();
+
+        {//first call is null
+            Field[] ff = HolderThreadSafe.class.getDeclaredFields();
+            for (Field f: ff) {
+                f.setAccessible(true);
+            }
+
+            assertNull(ff[0].get(hts));
+        }
+
+        // now it is lazily loaded
+        hts.getHeavy();
+
+        {//now it is not null - call via reflection so that the test is the same before and after
+            Field[] ff = HolderThreadSafe.class.getDeclaredFields();
+            for (Field f: ff) {
+                f.setAccessible(true);
+            }
+
+            assertNotNull(ff[0].get(hts));
+        }
+    }
+}

singleton/src/main/java/com/iluwatar/singleton/ThreadSafeLazyLoadedIvoryTower.java

@@ -4,6 +4,8 @@ package com.iluwatar.singleton;
  * Thread-safe Singleton class.
  * The instance is lazily initialized and thus needs synchronization
  * mechanism.
+ *
+ * Note: if created by reflection then a singleton will not be created but multiple options in the same classloader
  */
 public class ThreadSafeLazyLoadedIvoryTower {
 

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

@@ -2,73 +2,78 @@ package com.iluwatar.singleton;
 
 import org.junit.Test;
 
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.concurrent.*;
+
+import static org.junit.Assert.assertEquals;
+
 /**
+ * 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:
- * <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 List<ThreadSafeLazyLoadedIvoryTower> threadObjects = Collections.synchronizedList(new ArrayList<>());
+
+    //NullObject class so Callable has to return something
+    private class NullObject{private NullObject(){}}
 
     @Test
-	public void test() {
+    public void test_MultipleCallsReturnTheSameObjectInSameThread() {
-		SingletonThread runnable = new SingletonThread();
+        //Create several instances in the same calling thread
-		for (int j=0; j<NUM_THREADS; j++) {
+        ThreadSafeLazyLoadedIvoryTower instance1 = ThreadSafeLazyLoadedIvoryTower.getInstance();
-			Thread thread = new Thread(runnable);
+        ThreadSafeLazyLoadedIvoryTower instance2 = ThreadSafeLazyLoadedIvoryTower.getInstance();
-			thread.start();
+        ThreadSafeLazyLoadedIvoryTower instance3 = ThreadSafeLazyLoadedIvoryTower.getInstance();
+        //now check they are equal
+        assertEquals(instance1, instance1);
+        assertEquals(instance1, instance2);
+        assertEquals(instance2, instance3);
+        assertEquals(instance1, instance3);
+    }
+
+    @Test
+    public void test_MultipleCallsReturnTheSameObjectInDifferentThreads() throws InterruptedException, ExecutionException {
+        {//create several threads and inside each callable instantiate the singleton class
+            ExecutorService executorService = Executors.newSingleThreadExecutor();
+
+            List<Callable<NullObject>> threadList = new ArrayList<>();
+            for (int i = 0; i < NUM_THREADS; i++) {
+                threadList.add(new SingletonCreatingThread());
+            }
+
+            ExecutorService service = Executors.newCachedThreadPool();
+            List<Future<NullObject>> results = service.invokeAll(threadList);
+
+            //wait for all of the threads to complete
+            for (Future res : results) {
+                res.get();
+            }
+
+            //tidy up the executor
+            executorService.shutdown();
+        }
+        {//now check the contents that were added to threadObjects by each thread
+            assertEquals(NUM_THREADS, threadObjects.size());
+            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));
         }
     }
 
-	private static class SingletonThread implements Runnable {
+    private class SingletonCreatingThread implements Callable<NullObject> {
-
         @Override
-		public void run() {
+        public NullObject call() {
-			LazyLoadedIvoryTower instance = LazyLoadedIvoryTower.getInstance();
+            //instantiate the thread safety class and add to list to test afterwards
-			System.out.println("Thread=" + Thread.currentThread().getName() + " got instance=" + instance);
+            ThreadSafeLazyLoadedIvoryTower instance = ThreadSafeLazyLoadedIvoryTower.getInstance();
-		}
+            threadObjects.add(instance);
-	}
+            return new NullObject();//return null object (cannot return Void)
-	
-	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;
         }
     }
 }