Accessing Internal Variables in Java JUnit Testing

Описание: Learn how to access internal variables in your Java JUnit tests without mocking methods through effective refactoring techniques and best practices.
---
This video is based on the question https://stackoverflow.com/q/71658397/ asked by the user 'Sepehr Mollaei' ( https://stackoverflow.com/u/13804680/ ) and on the answer https://stackoverflow.com/a/71658559/ provided by the user 'daniu' ( https://stackoverflow.com/u/2837741/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: How can I access the internal variable of a method in Java junit?

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Accessing Internal Variables in Java JUnit Testing: A Practical Guide

When developing Java applications, especially when writing tests involving complex logic, you may find yourself in a situation where you need to access a variable generated within a method — in this case, a randomly generated number. This can be tricky, particularly if the random number is produced by a method call within the class you are testing. Fortunately, there are effective techniques to overcome this challenge without resorting to mocking the random method. In this guide, we’ll explore these techniques step by step.

Understanding the Problem

Consider the following method that generates a random number:

[[See Video to Reveal this Text or Code Snippet]]

The challenge arises when you need to test this method and access the variable number, as it's not straightforward to retrieve values from within the scope of a method. This is particularly true in Java where encapsulation limits access to internal states. So, how can you effectively test it?

Proposed Solution: Code Refactoring

The key to accessing internal variables in this context is refactoring your code to make your internal logic more testable. Below are the steps and methods you can employ.

Step 1: Inject Dependencies

You’ll want to refactor your class to accept the method generating the random number as a parameter. By doing this, you can control the output during your tests without using mocks. The refactored code looks like this:

[[See Video to Reveal this Text or Code Snippet]]

In this setup, Supplier<Integer> allows you to pass any number-generating logic, thereby making testing straightforward.

Step 2: Write Your Test

With the refactored class, you can now write a unit test that provides a specific value when the random method is called. Here’s how you can do it:

[[See Video to Reveal this Text or Code Snippet]]

In this test, the method() uses 3 instead of a truly random number, allowing you to test the behavior without randomness interfering with your assertions.

Alternative Solution: Using ArgumentCaptor

If you cannot refactor the code easily, another option is to capture the arguments passed to methods in your class using Mockito’s ArgumentCaptor. For example, consider the following setup:

[[See Video to Reveal this Text or Code Snippet]]

To capture the value of the random, you can do the following in your test:

[[See Video to Reveal this Text or Code Snippet]]

Here, ArgumentCaptor allows you to capture the integer passed to the mocked service during the test and helps you validate its value.

Conclusion

Accessing internal variables generated in a method can be challenging in Java JUnit testing but is not insurmountable. By adopting best practices like refactoring your code to use dependency injection or utilizing Mockito's ArgumentCaptor, you can write cleaner, more maintainable tests that yield accurate results. These techniques ensure your tests remain meaningful and that your code is designed with testability in mind.

With this understanding, you can tackle similar challenges in your testing strategies, paving the way for more robust and reliable Java applications.