Testing with Thread.sleep

Question

What are the recommended approaches to using Thread.sleep() to speed up tests.

I am testing a network library with a retry functionality when connections are dropped or timeout errors occur, etc. The library however, uses a Thread.sleep() between the retries (so it won't connect thousands times while the server is restarting). The call is slowing the unit tests significantly, and I wonder what the options are to override it.

Note, I'm open to actually changing the code, or using a mocking framework to mock Thread.sleep(), but would like to hear your opinions/recommendation first.

Answer 1

It is usually a good idea to delegate time-related functionality to a separate component. That include getting the current time, as well as delays like Thread.sleep(). This way it is easy to substitute this component with mock during testing, as well as switch to a different implementation.

Answer 2

I just faced a similar issue and I created a Sleeper interface to abstract this away:

public interface Sleeper
{
    void sleep( long millis ) throws InterruptedException;
}

The default implementation uses Thread.sleep():

public class ThreadSleeper implements Sleeper
{
    @Override
    public void sleep( long millis ) throws InterruptedException
    {
        Thread.sleep( millis );
    }
}

In my unit tests, I inject a FixedDateTimeAdvanceSleeper:

public class FixedDateTimeAdvanceSleeper implements Sleeper
{
    @Override
    public void sleep( long millis ) throws InterruptedException
    {
        DateTimeUtils.setCurrentMillisFixed( DateTime.now().getMillis() + millis );
    }
}

This allows me to query the time in a unit test:

assertThat( new DateTime( DateTimeUtils.currentTimeMillis() ) ).isEqualTo( new DateTime( "2014-03-27T00:00:30" ) );

Note that you need to fix the time first using DateTimeUtils.setCurrentMillisFixed( new DateTime( "2014-03-26T09:37:13" ).getMillis() ); at the start of your test and restore the time again after the test using DateTimeUtils.setCurrentMillisSystem();