Software testing is one of the most essential parts of any project. Testing evaluates the stability, usability, quality assurance, functionality of components, and ensures that the software is ready to publish in the market. It also helps to find out the errors, missing requirements of a project, and so on. Testing uses techniques to execute some processes in an application or program with the intent to find bugs.

In Chapter 6, Accessing the Database and Chapter 9, Creating an Application, we created Spring applications and implemented REST APIs. Then we tested with a third-party tool called Insomnia. After this, we mentioned the URL path with the HTTP CRUD request functions and checked the output. This system was fine and we could see the output directly. However, it can often be difficult to find the errors and bugs, as it can't show you the errors or any abnormal behavior. Though this project is running, it's not necessarily safe to release to the market. Therefore, we need to further test its stability. Let's run through two popular testing tools and frameworks, JUnit and Espresso.