In a recent project, I had the problem to check the correct format of a filename that it had to contain the creation timestamp; I easily overcame it with a nice test pattern made by Paolo “Nusco” Perrotta, well known Ruby Metaprogramming guru, in a version wrote by Bruno Bossola in a nice Open Source project.

In a nutshell, the trick is to adapt System Time with another object.

Let see a simple example.
This is the class to test:

package biz.scalzo.prod;

import java.text.SimpleDateFormat;
import java.util.Date;

public class TimeFormatter {
	public String now() {
		SimpleDateFormat sdf = new SimpleDateFormat(
				"yyyy/MM/dd HH:mm");
		return sdf.format(new Date(System.currentTimeMillis()));
	}
}

I know it mixes two responsibilities, it has a silly static dependecy and so on, but forgive my oversemplification used to show the pattern.

package biz.scalzo.test;

import junit.framework.Assert;
import org.junit.Test;

public class ATimeFormatter {
	@Test
	public void shouldFormatNowInFancyWay() {
		Assert.assertEquals("2009/10/13 15:29", new TimeFormatter().now());
	}

}

The behaviour to check is how well the Formatter formats current time in string; we have to find a way to set a well know time point so we can check te result of format.

To implement the VirtualClock pattern, we declare a simple Clock interface:

package biz.scalzo.prod;

public interface Clock {
	public long now();
}

and a repository where we can insert our fake clock

package biz.scalzo.prod;

import java.util.Date;

public class SystemTime {
	private static Clock clock;

	private static final Clock defaultClock = new Clock() {
		public long now() {
			return System.currentTimeMillis();
		}
	};

	public static long asMillis() {
		return getClock().now();
	}

	public static Date asDate() {
		return new Date(getClock().now());
	}

	public static void reset() {
		clock = null;
	}

	public static void setClock(Clock aClock) {
		clock = aClock;
	}

	static Clock getClock() {
		return (clock != null ? clock : defaultClock);
	}
}

Now we can change our class using SystemTime instead of System.currentTimeMillis():

package biz.scalzo.prod;

import java.text.SimpleDateFormat;
import java.util.Date;

public class TimeFormatter {
	public String now() {
		SimpleDateFormat sdf = new SimpleDateFormat(
				"yyyy/MM/dd HH:mm");
		return sdf.format(SystemTime.asDate());
	}
}

In test harness we can inject a fake clock

package biz.scalzo.test;

import junit.framework.Assert;

import org.junit.Before;
import org.junit.BeforeClass;
import org.junit.Test;

import biz.scalzo.prod.Clock;
import biz.scalzo.prod.SystemTime;
import biz.scalzo.prod.TimeFormatter;

public class ATimeFormatter {
	private static final long FAKED_TIME = 1255440571354L;

	private void fakeClock() {
		SystemTime.setClock(new Clock() {
			public long now() {
				return FAKED_TIME;
			}
		});
	}

	@Before
	public void setUp(){
		fakeClock();
	}

	@Test
	public void shouldFormatNowInFancyWay() {
		Assert.assertEquals("2009/10/13 15:29", new TimeFormatter().now());
	}

}

and we have green bar! Neat, isn’t it?

I know it’s a lot of code, but in legacy code it’s better than leave a feature without a test.

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