Resolving Null Pointer Exception in MockMvc: A Guide for Spring Boot Unit Testing

Overcome the `Null Pointer Exception` in MockMvc while unit testing your Spring Boot application. This guide provides a step-by-step solution to ensure smooth testing with Spock Framework.
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This video is based on the question https://stackoverflow.com/q/65198146/ asked by the user 'Hamza ATIF' ( https://stackoverflow.com/u/14779749/ ) and on the answer https://stackoverflow.com/a/65207704/ provided by the user 'cliff' ( https://stackoverflow.com/u/2585930/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Resolving Null Pointer Exception in MockMvc: A Guide for Spring Boot Unit Testing

When working with Spring Boot and Spock for unit testing, encountering a Null Pointer Exception can be both baffling and frustrating. This exception often surfaces during MockMvc tests, especially if the configuration and dependencies are not set up correctly. In this guide, we will explore a common scenario leading to this error and provide a clear, step-by-step solution.

The Problem: Null Pointer Exception Encountered

In the provided code, during the execution of the unit test for a Spring Boot controller using MockMvc, an error was thrown:

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This error indicates that some part of the request handling returned a null or empty value which wasn’t expected. Let's take a look at the relevant setup in the code:

Relevant Code Snippet

Below is the original code of the test which caused the issue:

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The Solution: Proper Configuration and Dependencies

To resolve the Null Pointer Exception, we need to ensure that we have the right dependencies and that the test is set up properly to handle the mocking.

Step 1: Add Necessary Dependencies

First, ensure that your build configuration file (like build.gradle) includes the necessary Spock and Spring dependencies. Here’s what you need to add:

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These dependencies ensure that you can leverage Spock's capabilities alongside Spring for your tests.

Step 2: Setup Test Class Correctly

Rather than manually setting up the MockMvc instance, you can streamline the process with annotations. Below is an improved test class setup using the @ Autowired annotation:

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Key Points of the Solution

Use of @ AutoConfigureMockMvc: This annotation automatically configures MockMvc for use in your tests.

Dependency Injection: Utilizing @ Autowired and @ SpringBean allows seamless mocking and testing of the service layer.

Proper Parameter Passing: Ensure that the required parameters are passed correctly in the request.

Conclusion

By following these steps, you should be able to resolve the Null Pointer Exception in MockMvc effectively. With the correct dependencies and a well-structured test setup, running unit tests on your Spring Boot application can be efficient and error-free.

Happy testing with Spock and Spring Boot!