jMock Argument Interceptor
by Tyler Jennings, with Dave Hoover
May 5, 2006
Dave Hoover : Home

Motivation

An Object you need to test is constructing another complex object internally which you cannot access and this object is passed to a collaborator you can replace with a mock.

Solution

Write an Interceptor (a custom jMock Stub ) to intercept the argument passed to the mocked method. The Interceptor exposes the argument, allowing for standard jUnit assertions.

Alternatives

Example

public void testInterceptArgument() {
	List arguments = new ArrayList();

	Mock mockSubmissionTracker = mock(SubmissionTracker.class);
	mockSubmissionTracker.expects(once()).method("record").will(captureArgumentsIn(arguments));

	SecretLottery lotto = new SecretLottery( (SubmissionTracker)
	mockSubmissionTracker.proxy());

	lotto.createTicket();

	LotteryTicket ticket = (LotteryTicket) arguments.get(0);
	assertEquals("Secret Number", 12345, ticket.number);
}

private Stub captureArgumentsIn(List argumentList) {
	return new ArgumentInterceptor(argumentList);
}

class ArgumentInterceptor implements Stub {
	List arguments;

	public ArgumentInterceptor(List argumentList) {
		arguments = argumentList;
	}

	public Object invoke(Invocation invocation) throws Throwable {
		arguments.addAll(invocation.parameterValues);
		return null;
	}

	public StringBuffer describeTo(StringBuffer buffer) {
		return buffer;
	}
}

The Traditional jMock approach

The traditional way to handle this with jMock is to create a custom Constraint to verify the argument passed into the mocked method. Unfortunatly, this technique can produce a less explict test and a lot of supporting code. The above example handled with a traditional jMock Constraint would look like the following...
public void testInterceptArgument() {
	Mock mockSubmissionTracker = mock(SubmissionTracker.class);
	mockSubmissionTracker.expects(once()).method("record").with(ticketNumber(12345));
		
	SecretLottery lotto = new SecretLottery( (SubmissionTracker) mockSubmissionTracker.proxy());
		
	lotto.createTicket();
}

private Constraint ticketNumber(final int ticketNumber) {
	return new Constraint() {
		public boolean eval(Object arg) {
			LotteryTicket ticket = (LotteryTicket) arg;		
			return ticketNumber == ticket.number;
		}

		public StringBuffer describeTo(StringBuffer buffer) {
			return buffer.append(ticketNumber);
		}
	}
}
While this example works fairly well, each new test method would likely require its own Constraint. When the verification in the eval method becomes complex, it can become difficult to determine why a test is failing. An Argument Interceptor allows for traditional jUnit assertions, providing clearer failures for complex verfications.


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