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

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import org.junit.Test;
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import java.lang.reflect.Constructor;
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import java.lang.reflect.Field;
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import java.lang.reflect.InvocationTargetException;
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import java.util.ArrayList;
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import java.util.List;
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import java.util.concurrent.*;
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import static org.junit.Assert.assertEquals;
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import static org.junit.Assert.assertNull;
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/**
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*
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* This test case demonstrates thread safety issues of lazy loaded Singleton implementation.
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* <p>
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* Out of the box you should see the test output something like the following:
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* <p>
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* Thread=Thread-4 got instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@6fde356e
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* Thread=Thread-2 creating instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@6fde356e
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* Thread=Thread-0 creating instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@6fde356e
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* Thread=Thread-0 got instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@6fde356e
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* Thread=Thread-3 got instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@6fde356e
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* Thread=Thread-1 got instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@60f330b0
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* Thread=Thread-2 got instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@6fde356e
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* <p>
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* By changing the method signature of LazyLoadedIvoryTower#getInstance from
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* <p><blockquote><pre>
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* public static LazyLoadedIvoryTower getInstance()
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* </pre></blockquote><p>
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* into
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* <p><blockquote><pre>
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* public synchronized static LazyLoadedIvoryTower getInstance()
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* </pre></blockquote><p>
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* you should see the test output change to something like the following:
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* <p>
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* Thread=Thread-4 creating instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@3c688490
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* Thread=Thread-4 got instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@3c688490
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* Thread=Thread-0 got instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@3c688490
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* Thread=Thread-3 got instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@3c688490
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* Thread=Thread-2 got instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@3c688490
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* Thread=Thread-1 got instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@3c688490
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* This class provides several test case that test singleton construction.
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*
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* The first proves that multiple calls to the singleton getInstance object are the same when called in the SAME thread.
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* The second proves that multiple calls to the singleton getInstance object are the same when called in the DIFFERENT thread.
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* The third proves that there is a hole if the singleton is created reflectively
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*
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*/
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public class LazyLoadedSingletonThreadSafetyTest {
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private static final int NUM_THREADS = 5;
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@Test
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public void test() {
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SingletonThread runnable = new SingletonThread();
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for (int j=0; j<NUM_THREADS; j++) {
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Thread thread = new Thread(runnable);
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thread.start();
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}
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}
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private static class SingletonThread implements Runnable {
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@Override
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public void run() {
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LazyLoadedIvoryTower instance = LazyLoadedIvoryTower.getInstance();
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System.out.println("Thread=" + Thread.currentThread().getName() + " got instance=" + instance);
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}
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}
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private static class LazyLoadedIvoryTower {
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private static LazyLoadedIvoryTower instance = null;
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private LazyLoadedIvoryTower() {
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}
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public static LazyLoadedIvoryTower getInstance() {
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if (instance == null) {
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instance = new LazyLoadedIvoryTower();
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System.out.println("Thread=" + Thread.currentThread().getName() + " creating instance=" + instance);
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}
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return instance;
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}
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}
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private static final int NUM_THREADS = 5;
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private List<ThreadSafeLazyLoadedIvoryTower> threadObjects = new ArrayList<>();
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//NullObject class so Callable has to return something
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private class NullObject{private NullObject(){}}
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@Test
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public void test_MultipleCallsReturnTheSameObjectInSameThread() {
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//Create several instances in the same calling thread
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ThreadSafeLazyLoadedIvoryTower instance1 = ThreadSafeLazyLoadedIvoryTower.getInstance();
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ThreadSafeLazyLoadedIvoryTower instance2 = ThreadSafeLazyLoadedIvoryTower.getInstance();
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ThreadSafeLazyLoadedIvoryTower instance3 = ThreadSafeLazyLoadedIvoryTower.getInstance();
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//now check they are equal
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assertEquals(instance1, instance1);
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assertEquals(instance1, instance2);
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assertEquals(instance2, instance3);
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assertEquals(instance1, instance3);
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}
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@Test
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public void test_MultipleCallsReturnTheSameObjectInDifferentThreads() throws InterruptedException, ExecutionException {
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{//create several threads and inside each callable instantiate the singleton class
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ExecutorService executorService = Executors.newSingleThreadExecutor();
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List<Callable<NullObject>> threadList = new ArrayList<>();
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for (int i = 0; i < NUM_THREADS; i++) {
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threadList.add(new SingletonCreatingThread());
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}
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ExecutorService service = Executors.newCachedThreadPool();
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List<Future<NullObject>> results = service.invokeAll(threadList);
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//wait for all of the threads to complete
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for (Future res : results) {
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res.get();
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}
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//tidy up the executor
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executorService.shutdown();
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}
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{//now check the contents that were added to threadObjects by each thread
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assertEquals(NUM_THREADS, threadObjects.size());
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assertEquals(threadObjects.get(0), threadObjects.get(1));
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assertEquals(threadObjects.get(1), threadObjects.get(2));
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assertEquals(threadObjects.get(2), threadObjects.get(3));
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assertEquals(threadObjects.get(3), threadObjects.get(4));
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}
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}
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@Test
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@SuppressWarnings("unchecked")
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public void test_HoleInSingletonCreationIfUsingReflection() throws ClassNotFoundException, NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
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Field[] f = ThreadSafeLazyLoadedIvoryTower.class.getDeclaredFields();
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assertEquals("One field only in ThreadSafeLazyLoadedIvoryTower", 1, f.length);
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f[0].setAccessible(true);
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{//reflectively create an object - the singleton field is null
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Class lazyIvoryTowerClazz = Class.forName("com.iluwatar.singleton.ThreadSafeLazyLoadedIvoryTower");
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Constructor<ThreadSafeLazyLoadedIvoryTower> constructor = lazyIvoryTowerClazz.getDeclaredConstructor();
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constructor.setAccessible(true);
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ThreadSafeLazyLoadedIvoryTower instance = constructor.newInstance();
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assertNull(f[0].get(instance));
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}
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//instantiate the singleton but when we do the below code we are creating a new object where it is set to null still
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IvoryTower.getInstance();
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{//reflectively create an object - the singleton field is null as a new object is created
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Class lazyIvoryTowerClazz = Class.forName("com.iluwatar.singleton.ThreadSafeLazyLoadedIvoryTower");
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Constructor<ThreadSafeLazyLoadedIvoryTower> constructor = lazyIvoryTowerClazz.getDeclaredConstructor();
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constructor.setAccessible(true);
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ThreadSafeLazyLoadedIvoryTower instance = constructor.newInstance();
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assertNull(f[0].get(instance));
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}
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}
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private class SingletonCreatingThread implements Callable<NullObject> {
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@Override
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public NullObject call() {
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//instantiate the thread safety class and add to list to test afterwards
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ThreadSafeLazyLoadedIvoryTower instance = ThreadSafeLazyLoadedIvoryTower.getInstance();
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threadObjects.add(instance);
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return new NullObject();//return null object (cannot return Void)
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}
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}
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}

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