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JavaWorld.com > Java Standard Edition >

Simply Singleton

Navigate the deceptively simple Singleton pattern

By David Geary, JavaWorld.com, 04/25/03

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Page 3 of 6

Example 2. A singleton test case

import org.apache.log4j.Logger;
import junit.framework.Assert;
import junit.framework.TestCase;
public class SingletonTest extends TestCase {
   private ClassicSingleton sone = null, stwo = null;
   private static Logger logger = Logger.getRootLogger();
   public SingletonTest(String name) {
      super(name);
   }
   public void setUp() {
      logger.info("getting singleton...");
      sone = ClassicSingleton.getInstance();
      logger.info("...got singleton: " + sone);
      logger.info("getting singleton...");
      stwo = ClassicSingleton.getInstance();
      logger.info("...got singleton: " + stwo);
   }
   public void testUnique() {
      logger.info("checking singletons for equality");
      Assert.assertEquals(true, sone == stwo);
   }
}

Example 2's test case invokes ClassicSingleton.getInstance() twice and stores the returned references in member variables. The testUnique() method checks to see that the references are identical. Example 3 shows that test case output:

Example 3. Test case output

Buildfile: build.xml
init:
     [echo] Build 20030414 (14-04-2003 03:08)
compile:
run-test-text:
     [java] .INFO main: getting singleton...
     [java] INFO main: created singleton: Singleton@e86f41
     [java] INFO main: ...got singleton: Singleton@e86f41
     [java] INFO main: getting singleton...
     [java] INFO main: ...got singleton: Singleton@e86f41
     [java] INFO main: checking singletons for equality
     [java] Time: 0.032
     [java] OK (1 test)

As the preceding listing illustrates, Example 2's simple test passes with flying colors—the two singleton references obtained with ClassicSingleton.getInstance() are indeed identical; however, those references were obtained in a single thread. The next section stress-tests our singleton class with multiple threads.

Multithreading considerations

Example 1's ClassicSingleton.getInstance() method is not thread-safe because of the following code: 

1: if(instance == null) {
2:    instance = new Singleton();
3: }

If a thread is preempted at Line 2 before the assignment is made, the instance member variable will still be null, and another thread can subsequently enter the if block. In that case, two distinct singleton instances will be created. Unfortunately, that scenario rarely occurs and is therefore difficult to produce during testing. To illustrate this thread Russian roulette, I've forced the issue by reimplementing Example 1's class. Example 4 shows the revised singleton class:

Example 4. Stack the deck

import org.apache.log4j.Logger;
public class Singleton {
  private static Singleton singleton = null;
  private static Logger logger = Logger.getRootLogger();
  private static boolean firstThread = true;
  protected Singleton() {
    // Exists only to defeat instantiation.
  }
  public static Singleton getInstance() {
     if(singleton == null) {
        simulateRandomActivity();
        singleton = new Singleton();
     }
     logger.info("created singleton: " + singleton);
     return singleton;
  }
  private static void simulateRandomActivity() {
     try {
        if(firstThread) {
           firstThread = false;
           logger.info("sleeping...");
           // This nap should give the second thread enough time
           // to get by the first thread.
             Thread.currentThread().sleep(50);
       }
     }
     catch(InterruptedException ex) {
        logger.warn("Sleep interrupted");
     }
  }
}

Example 4's singleton resembles Example 1's class, except the singleton in the preceding listing stacks the deck to force a multithreading error. The first time the getInstance() method is called, the thread that invoked the method sleeps for 50 milliseconds, which gives another thread time to call getInstance() and create a new singleton instance. When the sleeping thread awakes, it also creates a new singleton instance, and we have two singleton instances. Although Example 4's class is contrived, it stimulates the real-world situation where the first thread that calls getInstance() gets preempted.

Example 5 tests Example 4's singleton:

Example 5. A test that fails

import org.apache.log4j.Logger;
import junit.framework.Assert;
import junit.framework.TestCase;
public class SingletonTest extends TestCase {
   private static Logger logger = Logger.getRootLogger();
   private static Singleton singleton = null;
   public SingletonTest(String name) {
      super(name);
   }
   public void setUp() {
      singleton = null;
   }
   public void testUnique() throws InterruptedException {
      // Both threads call Singleton.getInstance().
      Thread threadOne = new Thread(new SingletonTestRunnable()),
             threadTwo = new Thread(new SingletonTestRunnable());
      threadOne.start();
      threadTwo.start();
      threadOne.join();
      threadTwo.join();
   }
   private static class SingletonTestRunnable implements Runnable {
      public void run() {
         // Get a reference to the singleton.
         Singleton s = Singleton.getInstance();
         // Protect singleton member variable from
         // multithreaded access.
         synchronized(SingletonTest.class) {
            if(singleton == null) // If local reference is null...
               singleton = s;     // ...set it to the singleton
         }
         // Local reference must be equal to the one and
         // only instance of Singleton; otherwise, we have two
                  // Singleton instances.
         Assert.assertEquals(true, s == singleton);
      }
   }
}

Example 5's test case creates two threads, starts each one, and waits for them to finish. The test case maintains a static reference to a singleton instance, and each thread calls Singleton.getInstance(). If the static member variable has not been set, the first thread sets it to the singleton obtained with the call to getInstance(), and the static member variable is compared to the local variable for equality.

Here's what happens when the test case runs: The first thread calls getInstance(), enters the if block, and sleeps. Subsequently, the second thread also calls getInstance() and creates a singleton instance. The second thread then sets the static member variable to the instance it created. The second thread checks the static member variable and the local copy for equality, and the test passes. When the first thread awakes, it also creates a singleton instance, but that thread does not set the static member variable (because the second thread has already set it), so the static variable and the local variable are out of synch, and the test for equality fails. Example 6 lists Example 5's test case output:

Example 6. Example 5's output

Buildfile: build.xml
init:
     [echo] Build 20030414 (14-04-2003 03:06)
compile:
run-test-text:
INFO Thread-1: sleeping...
INFO Thread-2: created singleton: Singleton@7e5cbd
INFO Thread-1: created singleton: Singleton@704ebb
junit.framework.AssertionFailedError: expected:<true> but was:<false>
   at junit.framework.Assert.fail(Assert.java:47)
   at junit.framework.Assert.failNotEquals(Assert.java:282)
   at junit.framework.Assert.assertEquals(Assert.java:64)
   at junit.framework.Assert.assertEquals(Assert.java:149)
   at junit.framework.Assert.assertEquals(Assert.java:155)
   at SingletonTest$SingletonTestRunnable.run(Unknown Source)
   at java.lang.Thread.run(Thread.java:554)
     [java] .
     [java] Time: 0.577
     [java] OK (1 test)

Now that we know Example 4's singleton is not thread-safe, let's see how we can fix it.

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