Simply Singleton -- part2 By David Geary

　　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