Test singletons
Throughout the rest of this article, I use JUnit in concert with log4j to test singleton classes. If you are not familiar with JUnit or log4j, see Resources.

Example 2 lists a JUnit test case that tests Example 1's singleton:

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