Mockito.when() is a fundamental method used in Mockito, a popular mocking framework for Java unit tests. It’s primarily utilized for stubbing, which means pre-defining the behavior of a mock object for specific method calls. Here’s an overview of its usage and importance in unit testing.
Syntax and Components
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Mockito.when(mockObject.methodCall(parameters)).thenReturn(returnValue); |
Components:
- mockObject: The mock instance of a class created using Mockito.
- methodCall(parameters): The method of the mock object for which behavior is defined.
- returnValue: The value that the method will return when called with specified parameters.
Example Usage
Consider a service MyService with a method getData(). To create a mock of MyService that returns specific data when getData() is called, you would use:
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// Creating a mock object of MyService MyService mockService = Mockito.mock(MyService.class); // Defining the behavior of mockService.getData() Mockito.when(mockService.getData()).thenReturn("Mock Data"); |
Advanced Usage
- Argument Matchers: Use matchers like
any(),eq(), etc., for a range of argument values. - Throwing Exceptions: Make the mock throw an exception using
.thenThrow(). - Multiple Calls: Define different behaviors for subsequent calls to the same method.
Important Notes
- Use
Mockito.when()only with mock objects, not with real instances. - It’s part of the given-when-then pattern in testing, used in the ‘Arrange’ or ‘Given’ phase.
- For void methods, use
Mockito.doReturn()orMockito.doThrow()instead.
Understanding Mockito.when() is essential for Java developers writing unit tests, as it helps create predictable and isolated test scenarios.
