Solving the Null PasswordEncoder Error in Spring Boot Unit Tests

Learn how to resolve the `Cannot invoke "PasswordEncoder.encode"` error when writing unit tests in your Spring Boot application. This guide breaks down the solution step-by-step!
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This video is based on the question stackoverflow.com/q/73205915/ asked by the user 'ReallyNicePerson' ( stackoverflow.com/u/19284258/ ) and on the answer stackoverflow.com/a/73207958/ provided by the user 'Dimitri Mestdagh' ( stackoverflow.com/u/1915448/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Solving the Null PasswordEncoder Error in Spring Boot Unit Tests

When writing unit tests in Spring Boot, it's not unusual to encounter issues with dependency injection, especially for components like PasswordEncoder. One common problem developers face is a null reference error, such as:

Cannot invoke "org.springframework.security.crypto.password.PasswordEncoder.encode(java.lang.CharSequence)" because the return value of "com.store.restAPI.user.UserConfig.passwordEncoder()" is null.

In this post, we’ll dive into why this error occurs and how to fix it effectively.

Background: Understanding the Problem

In the provided test class UserServiceTest, the test method addNewUser fails due to the UserConfig being mocked. A mock is a placeholder that simulates the behavior of real instances. However, when a service method in your application (like UserService.addNewUser) calls a method on a mock object (like userConfig.passwordEncoder()), it does not return a real instance or value unless explicitly instructed to do so. This leads to a null value being returned, resulting in the error you've encountered.

Step-by-Step Solution

Step 1: Modify Test Class to Define Mock Behavior

To resolve the issue, you need to specify the behavior of the mocked UserConfig. By using Mockito, you can instruct it to return a real instance of PasswordEncoder when passwordEncoder() is called. Here’s how to do that:

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

This line effectively tells Mockito that whenever passwordEncoder() is invoked on userConfig, it should return a new instance of BCryptPasswordEncoder, which is a commonly used implementation of the PasswordEncoder interface.

Step 2: Alternative Approach - Refactor the Dependency

While the above solution fixes the immediate problem, there's a better approach for improved design. Instead of depending on UserConfig in UserService, you can directly inject PasswordEncoder as a dependency. Here’s how to update your UserService:

Modify the Constructor: Change UserConfig to PasswordEncoder in the constructor.

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

Test Class Remains Unchanged: With this refactor, your test will automatically receive a real PasswordEncoder bean from the Spring context, making the unit tests simpler and more reliable.

Conclusion

In summary, the Null PasswordEncoder error in your Spring Boot tests can be resolved in two ways:

By defining the behavior of the mocked UserConfig using Mockito.

By refactoring your service to directly autowire PasswordEncoder, which enhances code clarity and robustness.

With these solutions, you should be able to run your tests without encountering the null reference error. Happy coding!