Tips and tricks with Jukito

After writing an enormous amount of tests for GWTP using Jukito, I ended up beating myself on many issues. Fortunately, many were common enough to have me noting them down.

This post will give you tips and tricks you can use with Jukito to make your life easier when testing things in GWTP. A few of the following tricks are also briefly shown in the previous posts of the series. I’ll try to give a little more detail for each of them.

Injecting concrete class

90% of the time, you want a class to depend on abstractions, and not concrete classes. For the other 10% if you have to inject a concrete class, don’t forget to forceMock the said class in your Jukito module:

Prevent onBind to be called twice

When you want to initialize something for a Presenter, a good practice is to do that costly work in the onBind method of a presenter. The problem with this, is that when you want to test your Presenter, Jukito calls onBind automatically.

If you call explicitly onBind() and do things like verify(someMock).someMethod() in your test, you will probably end up with a Mockito error, saying that the method was called twice instead of only once. To prevent that from happening, just add the following line in your Jukito module:

Mocking statically injected dependencies

This doesn’t happen often, but it happened in almost all my last projects.

One of the common use case for this is when you use REST-Dispatch with Resource Delegates, and you want to add a default behaviour in the onSuccess or onError/onFailure methods. You first have to implement your own RestCallBackImpl class, just like we did in the previous post. And then, for example, you decide that you want to fire an event from there.

To do so, you have to use static injection to inject an instance of EventBus into the class.

If you have a test that runs this code, you’ll need to request static injection for that RestCallBackImpl class in your module:

My system under test uses @Assisted injection

It happens a lot with PresenterWidgets, when you want to pass an entity or a value to build the Widget. Usually, you’ll have a Gin/Guice factory interface already in your code to create your instance. You can reuse this one to have a handle on the object that is passed using @Assisted injection.

In your JukitoModule, install the factory, and then in your test you can simply do this (I tend to do this in a @Before method):

And finally, you can write your test as you normally do.

Parameterized tests

When you get to test an algorithm, that is referentially transparent, it can be useful to write a test on a set of inputs and verify an expected output.

Let’s say you have plus(x, y) method you want to test, you could write a parameterized test like this:


That is all for the testing series. Hopefully, you should be ready to face most situations and challenges while developing and testing GWTP applications.

You’ve got no excuse for not writing tests now!

Happy coding, and have fun!