jMock 1: Getting Started

This guide assumes you are familiar with unit-testing and JUnit¹.

For a simple example we are going to test a publish/subscribe message system. A Publisher sends objects to zero or more Subscribers. We want to test the Publisher, which involves testing its interactions with its Subscribers.

The Subscriber interface looks like this:

interface Subscriber {
    void receive(String message);
}

We will test that a Publisher sends a message to a single registered Subscriber. To test interactions between the Publisher and the Subscriber we will use a mock Subscriber object.

First we must import the jMock classes, define our test fixture class and define a test case method.

import org.jmock.*;

class PublisherTest extends MockObjectTestCase {
    public void testOneSubscriberReceivesAMessage() {
    }
}

We will now write the body of the testOneSubscriberReceivesAMessage method.

We first set up the context in which our test will execute. We create a Publisher to test. We create a mock Subscriber that should receive the message. We then register the Subscriber with the Publisher. Finally we create a message object to publish.

Mock mockSubscriber = mock(Subscriber.class);
Publisher publisher = new Publisher();
publisher.add( (Subscriber)mockSubscriber.proxy() );

final String message = "message";

Next we define expectations on the mock Subscriber that specify the methods that we expect to be called upon it during the test run. We expect the receive method to be called with a single argument, the message that will be sent. The eq method is defined in the MockObjectTestCase class and specifies a "constraint²" on the value of the argument passed to the subscriber: we expect the argument to be the equal to the message, but not necessarily the same object. (jMock provides several constraint types³ that can be used to precisely specify expected argument values). We don't need to specify what will be returned from the receive method because it has a void return type.

mockSubscriber.expects(once()).method("receive").with( eq(message) );

We then execute the code that we want to test.

publisher.publish(message);

After the test has finished, jMock will verify that the mock Subscriber was called as expected. If the expected calls were not made, the test will fail.

Here is the complete test.

import org.jmock.*;

class PublisherTest extends MockObjectTestCase {
    public void testOneSubscriberReceivesAMessage() {
        // set up
        Mock mockSubscriber = mock(Subscriber.class);
        Publisher publisher = new Publisher();
        publisher.add((Subscriber) mockSubscriber.proxy());
        
        final String message = "message";
        
        // expectations
        mockSubscriber.expects(once()).method("receive").with( eq(message) );
        
        // execute
        publisher.publish(message);
    }
}

That concludes this quick introduction. More advanced topics are covered in other tutorials⁴.