JUnit: checking if a void method gets called

Question

I have a very simple filewatcher class which checks every 2 seconds if a file has changed and if so, the onChange method (void) is called. Is there an easy way to check ik the onChange method is getting called in a unit test?

code:

public class PropertyFileWatcher extends TimerTask {

private long timeStamp;
private File file;

public PropertyFileWatcher(File file) {
    this.file = file;
    this.timeStamp = file.lastModified();
}

public final void run() {
    long timeStamp = file.lastModified();

    if (this.timeStamp != timeStamp) {
        this.timeStamp = timeStamp;
        onChange(file);
    }
}

protected void onChange(File file) {
    System.out.println("Property file has changed");
}

}

@Test
 public void testPropertyFileWatcher() throws Exception {
    File file = new File("testfile");
    file.createNewFile();
    PropertyFileWatcher propertyFileWatcher = new PropertyFileWatcher(file);

    Timer timer = new Timer();
    timer.schedule(propertyFileWatcher, 2000);

    FileWriter fw = new FileWriter(file);
    fw.write("blah");
    fw.close();

    Thread.sleep(8000);
    // check if propertyFileWatcher.onChange was called

    file.delete();
 }

Answer 1

With Mockito, you can verify whether a method is called at least once/never.

See point 4 in this page:

http://mockito.googlecode.com/svn/branches/1.5/javadoc/org/mockito/Mockito.html

Answer 2

As I understand, your PropertyFileWatcher is meant to be subclassed. So, why not subclass it like this:

class TestPropertyFileWatcher extends PropertyFileWatcher
{
     boolean called = false;
     protected void onChange(File file) {
         called = true;
     }
}

...
TestPropertyFileWatcher watcher = new TestPropertyFileWatcher
...
assertTrue(watcher.called);

Answer 3

Here is a simple modification for your test.

@Test
 public void testPropertyFileWatcher() throws Exception {
    final File file = new File("testfile");
    file.createNewFile();

    final AtomicBoolean hasCalled = new AtomicBoolean( );
    PropertyFileWatcher propertyFileWatcher =
      new PropertyFileWatcher(file)
      {
        protected void onChange ( final File localFile )
        {
          hasCalled.set( true );
          assertEquals( file, localFile );
        }
      }


    Timer timer = new Timer();
    timer.schedule(propertyFileWatcher, 2000);

    FileWriter fw = new FileWriter(file);
    fw.write("blah");
    fw.close();

    Thread.sleep(8000);
    // check if propertyFileWatcher.onChange was called

    assertTrue( hasCalled.get() );
    file.delete();
 }