Mockito Matcher for a TypeReference: Resolving Issues with Testing Java Unit Tests

Introduction:

Unit testing is a crucial aspect of software development, ensuring that individual components of a program function as expected. Mockito, a popular Java testing framework, provides powerful features for creating mock objects and verifying interactions. However, sometimes issues arise when trying to test certain scenarios, such as matching a Type Reference in Mockito. In this article, we will explore how to overcome this challenge and successfully test a Java unit using Mockito’s matchers.

Understanding the Challenge

• Explaining the scenario: We have a CustomService class that throws a CustomException when a certain condition is met.
• Identifying the problem: The current unit test fails to detect the thrown CustomException, causing confusion and hindering the test’s effectiveness.

Analyzing the Code

• Examining the CustomService class: We delve into the CustomService class and its structure, highlighting the role of ObjectMapper and its readValue method.
• Understanding the CustomException class: Explaining the purpose and importance of the CustomException class.

The Mockito Solution

• Setting up the test environment: We establish the necessary mock objects and the CustomService instance for testing.
• Mocking ObjectMapper behavior: Demonstrating how to mock the readValue method of the ObjectMapper using Mockito’s given-when-then syntax.
• Crafting the test case: Constructing the test case that triggers the expected CustomException.
• Verifying the exception: Asserting that the CustomException is thrown successfully.

Handling Object Mapper Dependencies

• Adapting the constructor: Modifying the Custom Service class to accept an Object Mapper as a constructor parameter, enabling easier mocking in the test.
• Injecting the mock: Updating the test case to inject the mock ObjectMapper into the CustomService instance.
• Adjusting the mock definition: Fine-tuning the mock definition for readValue to match the new method signature.

Writing a Reliable Test

• Avoiding NullPointerExceptions: Addressing a potential issue with the null parameter passed to readValue and providing a solution.
• Finalizing the test case: Summarizing the modifications made to the test case and verifying its functionality.

Analyzing the Code

To understand the challenge better, let’s take a closer look at the code involved. The CustomService class is at the center of our testing efforts. It takes an instance of ApplicationProperties through its constructor and uses an ObjectMapper to perform operations.

The ObjectMapper is responsible for serializing and deserializing JSON data. In the method we want to test, named “method,” the ObjectMapper’s readValue method is used to deserialize a JSON string. This method expects a TypeReference parameter to correctly interpret the JSON structure. If an exception occurs during deserialization, a CustomException is thrown.

Additionally, we have the CustomException class, which extends the base Exception class and sets the response status to “INTERNAL_SERVER_ERROR.”

The Mockito Solution

Now, let’s explore how we can overcome the challenge of testing the CustomException in our unit test using Mockito. Here’s a step-by-step solution:

1. Setting up the test environment:
   • Use the @Mock annotation from Mockito to create a mock instance of the ApplicationProperties class.
   • Create an instance of the CustomService class, passing the mock ApplicationProperties object to its constructor.
2. Mocking ObjectMapper behavior:
   • Create a mock instance of the ObjectMapper class using the @Mock annotation.
   • Define the behavior of the readValue method using Mockito’s given-when-then syntax. In our case, we want the method to throw an IOException.
3. Crafting the test case:
   • Create a JSON string representing the required data for the test case.
   • Use the readTree method from the ObjectMapper class to convert the JSON string into a JsonNode object.
4. Verifying the exception:
   • Use the assertThrows method from JUnit to verify that the CustomException is thrown when invoking the method under test.
   • Ensure that the expected exception message or cause is correctly propagated.

Handling Object Mapper Dependencies

To improve the testability of the CustomService class, we can modify its constructor to accept the ObjectMapper as a parameter. This change allows us to inject a mock ObjectMapper during testing, simplifying the mocking process.

1. Adapting the constructor:
   • Update the constructor of the CustomService class to include an additional parameter of type ObjectMapper.
   • Assign the ObjectMapper parameter to the class field.
2. Injecting the mock:
   • Modify the test case to pass the mock ObjectMapper instance when creating the CustomService object.
3. Adjusting the mock definition:
   • Update the mock definition of the readValue method to match the new method signature in the CustomService class.
   • Consider using the RETURNS_DEEP_STUBS option while creating the mock ObjectMapper to avoid NullPointerExceptions.

Writing a Reliable Test

To ensure the reliability of our test case, we need to address a potential issue related to the null parameter passed to the readValue method.

1. Avoiding NullPointerExceptions:
   • Ensure that the JSON string passed to the readValue method is not null. If null, provide a valid JSON string for testing.
   • Consider using Mockito’s anyString() matcher to handle any input JSON string.
2. Finalizing the test case:
   • Recap the modifications made to the test case, including the constructor change and the usage of the mock ObjectMapper.
   • Verify that the test case now detects the expected CustomException and handles it appropriately.

Conclusion:

Explored a specific challenge related to testing Java units using Mockito. By understanding the code structure, analyzing the issues, and implementing the suggested solutions, you can improve the reliability and effectiveness of your unit tests. Mockito’s matchers and the ability to mock dependencies like the Object Mapper provide valuable tools for tackling testing challenges in Java applications. Remember to adapt the code and examples provided to your specific project needs and follow best practices for unit testing. By embracing a thorough testing approach, you can build robust and reliable software systems.