Mockito.any() pass Interface with Generics

java

mockito

generics

type-safety

byNikita Barsukov·Sep 13, 2024

To productively mock methods that utilize a generic interface parameter in Mockito, take advantage of any() combined with type literals. This way you neatly sidestep type erasure issues by noting down the generic type within angle brackets.

// Mocking method with generic interface parameter:
when(myMock.myMethod(Mockito.<MyGenericInterface<String>>any())).thenReturn(...);

With <MyGenericInterface<String>>, you're explicitly informing Mockito concerning the parameter's generic type, therefore assuring precise type matching throughout stubbing.

Ensuring Mockito.any() Type Safety

When mocking with generics, it is significant to retain type safety. This is achievable with the use of ArgumentMatchers.<AsyncCallback<ResponseX>>any(). By doing so, we are ascertaining that our mocks will align inevitably with the specified generic type for the interface under test.

Do remember, purely specifying class types isn't sufficient due to type erasure at runtime in Java. For instance, if you have an AsyncCallback interface that manages responses of type ResponseX:

interface AsyncCallback<T> {
  void onSuccess(T response);
  void onFailure(Throwable caught);
}

// An example of mocking AsyncCallback<ResponseX> using `any()` with generics
when(myService.executeAsync(Mockito.<AsyncCallback<ResponseX>>any())).thenReturn(...);

With ArgumentMatchers.<AsyncCallback<ResponseX>>any(), you've kickstarted efficient mocking interactions with generic types.

Leveraging Java 8 Type Inference Capabilities

With the advent of Java 8, type inference was improved tremendously, often letting us utilize Mockito.any() without the need to explicitly specify the type:

// In Java 8 and later
when(myMock.myMethod(any())).thenReturn(...); // Compiler: "Hold my beer, I got this!"

However, do provide context to ensure the compiler infers the correct method types.

Using Static Imports for Conciseness and Clarity

Import import static org.mockito.ArgumentMatchers.any; for simplified syntax and lesser verbosity when creating mocks. The key to maintaining type safety for generics is specifying within the Matchers.<>any() when required, especially in pre-Java 8 situations.

Mastering Mockito Generics: Tactics and Common Mistakes

When it comes to applying Mockito.any() with generics, there are some winning strategies and common mistakes to avoid:

Safeguarding Exact Type Matching

Aim always to provide the exact generic type. Accuracy saves you from subtle bugs:

when(repository.find(Mockito.<List<String>>any())).thenReturn(expectedResult);

Handling Generics in Java Versions Before 8

In Java versions prior to 8, type must be defined explicitly in the matcher:

when(repository.find(Mockito.<List<String>>any())).thenReturn(expectedResult);

Overcoming Type Erasure Woes

Remember, Java's runtime type erasure can be a headache, but Mockito's any() with correct generics usage provides the aspirin!

Navigating Complex Generics

When working with complex generic structures, like Map<Key, List<Value>>, be sure to roll up your sleeves and construct your mocks to mirror the expected types:

when(cache.retrieve(Mockito.<Map<Key, List<Value>>>any())).thenReturn(cacheEntry);

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