how to mock a class used in the code

Question

public void sendMail() {  
    /* I have simplified the method for here */
    Transport t;
    t.send();
}

When I am writing unit test for this, since I don't want to actually send the mail, I want to mock the Transport class. Is it possible and if so, how to do it ?

Answer 1

'cglib' may fits.
Use 'Enhancer' to proxy the 'Transport' class. In order not to actually sending the mail, you need pass into the 'Enhancer' a 'MethodInterceptor' which will not invoke the 'send()' method in super class.

Answer 2

Unless you really want to use mockito, you can quite easily handcraft your own test double.

What you can do is create an interface that knows how to send mail:

public interface TransportInterface {
    public void send(Message msg);
}

Let the mail sending class use this interface to send the mail:

public class MailSender {
    private TransportInterface transport;

    public MailSender(TransportInterface transport) {
        this.transport = transport;
    }

    public void sendMail(Message msg) {
        /* This is the method from your question */
        this.transport.send(msg);
    }
}

In production you use an implemenation of TransportInterface that actually send the mail:

public class TransportAdapter implements TransportInterface {
    private Transport transport; // Created elsewhere...

    public void sendMail(Message msg) {
        transport.send(msg);
    }
}

And in your test code you can use a fake:

public class TransportFake implements TransportInterface {
    public void sendMail(Message msg) {
        // I don't send the mail!
    }
}

(It's been a while since I coded java. Hope there are not too many errors. Also, you can probably do a better job naming the classes than I have.)

Answer 3

You can try to use Mockito library: here is example code:

import org.mockito.Mock;
import org.junit.runner.RunWith;
import org.mockito.Mockito;
import org.mockito.runners.MockitoJUnitRunner;

@RunWith(MockitoJUnitRunner.class)
public class YourTestClass{
@Mock
Transport t;

@Test
public void someTest(){
t = Mockito.mock(Transport .class);
    Mockito.when(t.send()).thenReturn(true);
}
}

Answer 4

Here is a solution which works regardless of how the Transport t object is obtained:

@Test
public sendMail(
    // Mocks all current and future Transport instances, 
    // including those belonging to subclasses (if any).
    @Capturing final Transport t)
{
    new SUT().sendMail();

    // Verifies that the t.send() method was called:
    new Verifications() {{ t.send(); }};
}

The mocking API used above is JMockit (which I develop).