Handling mutable collections when verifying method calls with Mockito can be challenging because Mockito holds references to the collections, not their state at the time of method invocation. This means that if the collection is modified after the method call, Mockito will verify against the updated state, not the original state. Here are some strategies to handle this issue:
1. Use `ArgumentCaptor` with Caution**
While `ArgumentCaptor` can capture the arguments passed to a method, it also captures the reference to the collection. Therefore, if the collection is modified after the method call, the captured argument will reflect these changes. However, you can use it to verify the initial state by capturing the collection and then immediately checking its state.java
// Example usage
ArgumentCaptor> captor = ArgumentCaptor.forClass(Collection.class);
verify(myObject).removeAll(captor.capture());
// Immediately verify the captured collection's state
Collection capturedCollection = captor.getValue();
// Check the state of capturedCollection here
2. Implement a Custom Answer**
You can use Mockito's `thenAnswer` feature to create a custom answer that captures the state of the collection at the time of method invocation. This approach allows you to verify the collection's state as it was when the method was called.java
// Example implementation
Answer recordFirstArgIn = new Answer() {
private Collection capturedCollection;
@Override
public Object answer(InvocationOnMock invocation) throws Throwable {
capturedCollection = (Collection) invocation.getArgument(0);
return null; // Return value depends on the method being mocked
}
public Collection getCapturedCollection() {
return capturedCollection;
}
};
when(myObject.removeAll(anyCollectionOf(Object.class))).thenAnswer(recordFirstArgIn);
// After invoking the method
Collection capturedCollection = ((Answer) recordFirstArgIn).getCapturedCollection();
// Verify the state of capturedCollection here
3. Refactor to Use Immutable Collections**
If possible, refactor your code to use immutable collections. This approach ensures that once a collection is passed to a method, its state cannot be altered, making it easier to verify the method calls.4. Clone the Collection**
If the collection implements `Cloneable`, you can clone it before passing it to the method. This way, any modifications made after the method call will not affect the cloned collection.java
// Example cloning
Collection originalCollection = new ArrayList();
Collection clonedCollection = new ArrayList(originalCollection);
// Pass clonedCollection to the method
5. Use Mockito Spy**
In some cases, using a spy instead of a mock can help. A spy wraps an existing object, allowing you to verify interactions while still using the real object's behavior. However, this does not directly solve the mutable collection issue unless you're verifying interactions on the spy itself.java
// Example usage of spy
List spyList = spy(new ArrayList());
// Use spyList in your test
verify(spyList).add(any());
Conclusion
Handling mutable collections with Mockito requires careful consideration of how the collection's state changes over time. By using custom answers, cloning collections, or refactoring to immutable collections, you can effectively verify method calls involving mutable collections.Citations:
[1] https://groups.google.com/g/mockito/c/_A4BpsEAY9s
[2] https://stackoverflow.com/questions/7798738/how-to-mock-a-method-which-relies-on-updating-mutable-objects
[3] https://mockk.io
[4] https://stackoverflow.com/questions/37931676/how-to-turn-a-mutable-collection-into-an-immutable-one/58415030
[5] https://kotlinlang.org/api/core/kotlin-stdlib/kotlin.collections/-mutable-collection/
[6] https://github.com/mockito/mockito/issues/967
[7] https://www.studytonight.com/java-examples/spy-in-mockito
[8] https://github.com/Eedanna/mockito/issues/126