An example of using a factory class to improve testability over a static method call.

MetadataTableUtilTesting.java

/*
 * Current getMetadataTable() method. This is hard to test because it requires a real HdfsZooInstance object.
 */
public synchronized static Writer getMetadataTable(Credentials credentials) {
  Writer metadataTable = metadata_tables.get(credentials);
  if (metadataTable == null) {
    metadataTable = new Writer(HdfsZooInstance.getInstance(), credentials, MetadataTable.ID);
    metadata_tables.put(credentials, metadataTable);
  }
  return metadataTable;
}

/*
 * A more testable getMetadataTable() method using an instance factory.
 */
private static InstanceFactory ifactory = new HdfsZooInstanceFactory();  // default for production
static InstanceFactory getInstanceFactory() {
  return ifactory;
}
static void setInstanceFactory(InstanceFactory ifactory) {
  MetadataTableUtil.ifactory = ifactory;
}

public synchronized static Writer getMetadataTable(Credentials credentials) {
  Writer metadataTable = metadata_tables.get(credentials);
  if (metadataTable == null) {
    // now the factory provides the instance instead of a static method call
    metadataTable = new Writer(ifactory.getInstance(), credentials, MetadataTable.ID);
    metadata_tables.put(credentials, metadataTable);
  }
  return metadataTable;
}

/*
 * Test code for the method.
 */
@BeforeClass public static void setUpClass() {
  // replace the instance factory in the class under test with one that returns a mock instance
  priorInstanceFactory = MetadataTableUtil.getInstanceFactory();
  mockInstanceFactory = new MockInstanceFactory();
  MetadataTableUtil.setInstanceFactory(mockInstanceFactory);
}
@AfterClass public static void tearDownClass() {
  // clean up: put back the "real" instance factory
  MetadataTableUtil.setInstanceFactory(priorInstanceFactory);
}
@Before public void setUp() {
  // create a mock instance and put it in place to be returned by the factory
  instance = createMock(Instance.class);
  mockInstanceFactory.setInstance(instance);
  // now I can tell the mock instance to behave in ways I need it to
  expect(instance.getInstanceID()).andReturn(INSTANCE_ID);
  replay(instance);
}
@Test public void testGetMetadataTable() {
  Credentials credentials = createMock(Credentials.class);
  Writer writer = MetadataTableUtil.getMetadataTable(credentials);
  // assertions about returned writer go here
}