TestNG is a testing framework inspired by JUnit and NUnit but introducing many new functionalities that make it more powerful and easier to use, such as annotations, Running your tests in arbitrarily big thread pools with various policies available (all methods in their own thread, one thread per test class, etc).

The gap between JUnit and TestNG has reduced because of using annotations from JUnit 4 and integrating the Hamcrest matchers as well but it's up to you.

If you decide to learn TestNG for unit testing your Java code then TestNG Complete Bootcamp For Beginners - Novice To Ninja is a good course to start with.

It provides step by step guide for learning TestNG Framework (Next Generation) to speed up your automation goals

Read more: https://javarevisited.blogspot.com/2018/01/10-unit-testing-and-integration-tools-for-java-programmers.html#ixzz81f5xAK39