Junit Rules: Combining @Rule and @ClassRule

Really useful for when you need to repeat test setup/teardown in multiple test classes.

Imagine have a server and before we run the tests we want to call server.start() and after the tests are finished we want to call server.stop().

	public static final Scassandra SERVER
	= ScassandraFactory.createServer(binaryPort,
	adminPort);
	@BeforeClass
	public static void startScassandra() {
	SERVER.start();
	}
	@AfterClass
	public void stopCassandra() {
	SERVER.stop();
	}

view raw BeforeJunitRule.java hosted with ❤ by GitHub

We create a rule that implements junit's 'TestRule' class which starts and stops the server:

	public class ScassandraServerRule implements TestRule {
	private Scassandra scassandra;
	public ScassandraServerRule(Scassandra scassandra){
	this.scassandra = scassandra;
	}
	@Override
	public Statement apply(final Statement base,
	Description description) {
	return new Statement() {
	@Override
	public void evaluate() throws Throwable {
	scassandra.start();
	try {
	base.evaluate();
	}
	finally {
	scassandra.stop();
	}
	}
	};
	}
	}

view raw ExampleJunitRule.java hosted with ❤ by GitHub

Now, our test class doesn't need the two methods annotated with beforeclass and afterclass, only the rule:

	public static final Scassandra SERVER
	= ScassandraFactory.createServer(binaryPort,
	adminPort);
	@ClassRule
	public static ScassandraServerRule rule
	= new ScassandraServerRule(SERVER);

view raw AfterJunitRule.java hosted with ❤ by GitHub

Note the use of @ClassRule. An alternate annotation would be @Rule which would mean that the server was stopped and started between every test.

Combining Before and BeforeClass behaviour in one rule

Now imagine that we want to clear down server state between each test. Logically, this belongs in the rule we have already created. We want one rule to manage server state entirely, such that the rule starts and stops the server beforeclass and afterclass but clears the state between each test.

However, in the recent version of Junit, the rule variable in our test needs to be static.

The way round it is the following. Firstly, we modify our rule to behave so that if the server has been started it clears the state when the apply method is called:

	@Override
	public Statement apply(final Statement base, Description description) {
	return new Statement() {
	@Override
	public void evaluate() throws Throwable {
	if (!started) {
	started = true;
	scassandra.start();
	try {
	base.evaluate();
	} finally {
	scassandra.stop();
	}
	} else {
	scassandra.clearState();
	base.evaluate();
	}
	}
	};
	}

view raw ExampleRule2.java hosted with ❤ by GitHub

Then in our junit test we must use both rule annotations for a @ClassRule and @Rule.

	public static final Scassandra SERVER
	= ScassandraFactory.createServer(binaryPort,
	adminPort);
	@ClassRule
	public static ScassandraServerRule classRule
	= new ScassandraServerRule(SERVER);
	@Rule
	public ScassandraServerRule rule
	= classRule;

view raw AfterBothRules.java hosted with ❤ by GitHub