Скачать или смотреть Fixing NullPointerException When Instantiating Objects in @ Before in Your JUnit Tests

Learn how to resolve `NullPointerException` issues in JUnit tests when instantiating objects by using Mockito's annotations effectively.
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This video is based on the question https://stackoverflow.com/q/64337901/ asked by the user 'ela_ela' ( https://stackoverflow.com/u/14172234/ ) and on the answer https://stackoverflow.com/a/64338181/ provided by the user 'Deepak Patankar' ( https://stackoverflow.com/u/7003331/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Resolving the NullPointerException in JUnit Tests: A Guide for Java Developers

When you're developing and testing Java applications, unit tests play a crucial role. However, encountering java.lang.NullPointerException during testing can be frustrating. Let's dig deeper into this problem and explore a structured solution that will help you resolve this issue effectively.

Understanding the Problem

In your case, you're trying to instantiate an object in the @ Before setup method of your JUnit tests. You have a custom class HerePoiSearchTransformer that is extending AbstractHereTransformer, which contains a dependency on AddressDistrictCity through the @ Inject annotation. However, you noticed that the AddressDistrictCity instance is null when you run your test cases, leading to a NullPointerException.

Here's a simplified look at your unit test setup:

Class in Question: HerePoiSearchTransformer

Dependency: AddressDistrictCity (inherited from AbstractHereTransformer)

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The Mistake

While you created a local object with private final AddressDistrictCity addressDistrictCity = new AddressDistrictCity();, this does not replace the instance that AbstractHereTransformer expects. Therefore, when your tests run, they are trying to use a null instance, leading to the exception.

The Solution: Using Mockito Annotations

To properly inject the required dependencies, you can leverage Mockito's capabilities by using @ InjectMocks and @ Mock annotations. This will allow the JUnit framework to automatically provide the necessary dependencies during test execution.

Step-by-Step Approach

Import Mockito Annotations:
Start by ensuring you have the necessary Mockito and JUnit libraries in your project. You can import these at the top of your test class.

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Use @ InjectMocks on your Transformer:
Instead of creating a new instance of HerePoiSearchTransformer manually, annotate it with @ InjectMocks. This tells Mockito to inject the mock objects into this transformer class automatically.

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Mock the AddressDistrictCity class:
You will also need to create a mock instance of AddressDistrictCity using the @ Mock annotation.

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Initialize Mocks:
In your @ Before method, initialize the mock objects with MockitoAnnotations.openMocks(this);. This sets everything up for your tests.

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Run the Tests:
Now, with everything set up properly, you can run your tests without worrying about the NullPointerException. Mockito will take care of injecting the dependencies.

Updated JUnit Test Example

Here's how the updated test class might look after integrating Mockito:

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Conclusion

By using @ InjectMocks and @ Mock, you can effectively manage dependencies in your JUnit tests, thus resolving issues like the NullPointerException. This approach not only simplifies your setup but also improves code maintainability and readability in the long run. Don't forget to leverage mocking libraries like Mockito to enhance your testing strategy!

By following this guide, you can ensure that your unit tests are robust and free of null reference errors.