Unit testing for void methods and threads in jUnit

Question

I'm new to unit testing. I understood the principles of it, but I still can't figure out how to test my current project. I need to test void methods, operating with java.nio.SocketChannel. These methods are:
- initSelector, where I open selector, bind new ServerSocketChannel and register it
- read, which reads data and puts it to a queue (should i write extra method for verifying, if that data actually exists in queue? and in that case, should i write tests for that methods?)
- write method, which takes data from a queue and writes it to a SocketChannel

I can test this methods for not throwing IOException, but what else?
And how should I test run() method of a Thread? Or is it not unit testing, but system or other?

Answer 1

Basically, you have two possibilities:

• if you want to thoroughly unit test these methods, you should hide the concrete (hardware dependent components like sockets etc. behind mockable interfaces, and use mocks in the unit tests to verify that the expected calls with the expected parameters are made to these objects
• or you can write integration / system tests using the real sockets within the whole component / app, to verify that the correct sockets are opened, data is transferred properly etc.

Ideally, you should do both, but in the real world, unit testing may not always be feasible. Especially for such low-level methods, which depend on some external software/hardware component like sockets, DBs, file system etc. Then the best approach is to leave as little logic (thus as little possibilities for failure) in these methods / layers as possible, and abstract out the logic into a higher layer, designed to be unit testable (using mockable interfaces as mentioned above).

To test threads, you can just run() them from your unit test as usual. Then most likely you need to wait for some time before trying to get and verify the results produced by the thread.

Again, if you abstract away the actual logic of the task into e.g. a Callable or Runnable, you can unit test it in isolation much easier. And this also enables you to use the Executor framework (now or later), which makes dealing with concurrency much easier and safer.