I have a very basic question, I'm new to unit testing and I have a test class in which I'm testing Spring WebClient using MockWebServer. All the tests run fine but why don't we need a runner when using MockWebServer? When using Mockito we annotate the class with @RunWith(MockitoJUnitRunner.class).
A JUnit 4 Runner or Rule or JUnit Jupiter Extension conveniently takes care (among other things) of the test setup, e.g. starting and stopping the MockWebServer for you in the background.
You don't have to use a Runner/Rule or Extension for your tests and can start/stop your test infrastructure manually as part of @BeforeAll/@AfterAll or inside the test.
However, this adds boilerplate code to every test that requires this kind of infrastructure and you can delegate this task to a Runner/Rule or Extension if your test library provides one.
In the particular example for MockWebServer, you can use their JUnit 4 rule, their JUnit 5 integration, or perform the start/stop operations for the server on your own.