Resolving NullPointerException in PdfService Test with Mockito and Spring Boot

Learn how to solve `NullPointerException` with MockitoJUnitRunner and Spring Boot by properly injecting mocked dependencies in your tests.
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This video is based on the question https://stackoverflow.com/q/76755307/ asked by the user 'Nuñito Calzada' ( https://stackoverflow.com/u/4450024/ ) and on the answer https://stackoverflow.com/a/76755734/ provided by the user 'changuk' ( https://stackoverflow.com/u/15566000/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Handling NullPointerException in PdfService Tests

When writing tests using Mockito and Spring Boot, developers often run into issues, one of which is the dreaded NullPointerException. Today, we’ll explore how to effectively set up your tests to avoid this common pitfall, specifically focusing on the scenario involving the PdfService class. Let's dive into the details and uncover a solution.

The Problem: Understanding the Error

You have the following service class called PdfService that relies on AsAPIService:

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You've written a test for the generatePdf() method, but when you run the test, you encounter the following error:

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This error occurs because when you use @ InjectMocks, if Mockito cannot find a suitable object to inject (like a mock of AsAPIService), it defaults to passing null.

The Solution: Setting Up Mocks

To resolve this issue, you need to ensure that every dependency used by the PdfService is properly mocked. This includes the AsAPIService. Here's how to adjust your test class so that it works correctly:

Step-by-Step Solution

Use Mock Annotations: Declare your service dependency as a mock.

Inject Mocks into Your Service: Ensure that Mockito injects the mock into your PdfService class.

Here’s how your test should look:

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Key Changes Explained

@ Mock Annotation: This is used to create a mock instance of AsAPIService. This mock will replace the real instance of AsAPIService when you initialize PdfService.

@ InjectMocks Annotation: This tells Mockito to create an instance of PdfService and inject the mocked AsAPIService into it. This is crucial to avoid getting null when generatePdf() tries to access the asAPIService.

Conclusion

By properly setting up your Mockito test framework with the appropriate @ Mock and @ InjectMocks annotations, you can effectively resolve the NullPointerException when testing your service layers. It's essential in unit testing to ensure that all dependencies are accounted for, primarily when dealing with multiple service classes in Spring Boot applications.

Next time you encounter a NullPointerException in your tests, remember to check if all your dependencies are correctly mocked and injected.