Junit @Runwith - with different Runners ; how does it work

Question

I am trying to understand how @RunWith can accept different types of Runners. How does Junit understand or accept a different Runner and flex to run with a different runners (like SpringRunner , MockitoRunner), which are written by different vendors altogether.

Answer 1

The class must extend the abstract class Runner.

From the javadoc

Element Detail
value
public abstract Class<? extends Runner> value
Returns:
a Runner class (must have a constructor that takes a single Class to run)

Any vendor can write a class which extends Runner and Junit 4 can use it. Junit 4 depends only on the abstract class.