What is the purpose of nesting a unit test inside the class it tests, rather than in another (outside) class?

Question

I've been looking through a particular open source library in which all unit tests are defined as static nested class inside the class they test, for example:

public class Foo {

    public int bar() { ... }

    public static class UnitTest {
        @Test
        public void testBar() { ... }
    }
}

I've never seen a project or Java codebase that does this before and I am really curious about the thinking behind it.

• Is there any advantage over this pattern than having a separate FooTest class in another source folder src/test/java?

• Is this a convention of projects that use Gradle as their build tool?

Answer 1

As Jon Skeet suggested, I found the answer within the documentation of the project in question:

Hystrix has all of its unit tests as inner classes rather than in a separate /test/ folder.

Why?

• Low Friction

• Context
• Encapsulation
• Refactoring
• Self-documenting

More information on the reasoning can be found in this blog post: JUnit Tests as Inner Classes