How do we unit test this lambda expression?

Question

We have a REST end point (JAX-RS) that is invoked from the browser. We are passing around OutputStream so that we could have the browser display the result of the call.

Here is the method.

@Path("/mypath/{userId}")
@POST
public Response createUser(@PathParam("userId") final int userId ) {
    StreamingOutput stream = (outputStream) -> {
        User user = userHelper.findUser(userId);
        userHelper.updateUser(user,outputStream);
    };

    return Response.ok(stream).build();
}

Using Junit and Mockito, how do we verify if userHelper.findUser and userHelper.updateUser has been called ?

Basically we just want to verify the interactions.

Answer 1

To "unit" test this you should create your test class and create a new instance of the class this method belongs to in the test class. The userHelper is not defined in the lambda so it is a class member? If so it can be mocked:

• Create a mock userhelper object with Mockito
• inject the mock to your test class.
• Call the createUser method.
• verify on the mock to assert the updateUser method is called once.
• You can go a step further and verify what user and outputStream objects are passed using captors.