Mocking Method Behavior with Custom Implementations using Mockito

Mockito is a widely-used open-source Java testing framework that allows developers to create mocks of objects and define their behavior for unit testing. By using Mockito, developers can isolate specific parts of their code and verify how these parts interact with other components. One powerful feature of Mockito is the ability to mock method behavior with custom implementations. This allows developers to simulate specific scenarios and test how their code handles them.

What is mocking method behavior?

Mocking method behavior refers to the process of creating a mock object that resembles a real object and defining specific behaviors for its methods. When a method of the mock object is called during testing, it will execute the defined behavior instead of the actual implementation. This enables developers to control the behavior of external dependencies or complex objects, making it easier to write effective unit tests.

The basics of mocking method behavior with Mockito

Before diving into custom implementations, let's first understand the basics of mocking method behavior using Mockito. Here's a brief example:

// Creating a mock object
MyClass myMock = Mockito.mock(MyClass.class);

// Defining method behavior
Mockito.when(myMock.myMethod()).thenReturn("Mocked behavior");

// Calling the mocked method
String result = myMock.myMethod();

// Verifying the behavior
assertEquals("Mocked behavior", result);

In the example above, we first create a mock object of the MyClass class using Mockito.mock(). Then, using Mockito.when(), we define the behavior of the myMethod() method as "Mocked behavior". When we call myMethod() on the mock object, it will return the mocked value instead of executing the actual implementation.

Finally, we can verify if the mocked method was called using Mockito.verify(). We can also assert that the returned result matches the expected behavior using an assertion library like JUnit.

Implementing custom behavior

While using thenReturn() is useful in many scenarios, there may be situations where we need more flexibility in defining custom behaviors. Mockito provides various methods to achieve this level of customization.

Performing actions with thenAnswer()

The thenAnswer() method allows us to define custom behavior by implementing the Answer interface. This gives us full control over the method's execution and its arguments. Here's an example:

Mockito.when(myMock.myMethod()).thenAnswer(new Answer<>() {
    public Object answer(InvocationOnMock invocation) {
        // Custom behavior implementation
        return "Custom behavior";

In the example above, we define the behavior of myMethod() as a custom implementation of the Answer interface. Inside the answer() method, we can implement any custom logic required and return the desired result.

Throwing exceptions with thenThrow()

Sometimes, we need to test how our code handles exceptions thrown by external dependencies or complex objects. Mockito allows us to throw exceptions when a specific method is called using the thenThrow() method. Here's an example:

Mockito.when(myMock.myMethod()).thenThrow(new RuntimeException("Exception message"));

In this example, we define that when myMethod() is called on the mock object, it should throw a RuntimeException with the given message. This allows us to simulate exceptional scenarios and verify if our code handles them correctly.

Chaining behaviors with thenReturn() and thenAnswer()

Mockito also enables us to chain multiple behaviors for the same method using both thenReturn() and thenAnswer(). This can be useful when we want to mock different behaviors for different invocations of the same method. Here's an example:

    .thenReturn("First behavior")
    .thenAnswer(new Answer<>() {
        public Object answer(InvocationOnMock invocation) {
            // Custom behavior implementation for subsequent invocations
            return "Subsequent behavior";

In this example, the first call to myMethod() will return "First behavior" while subsequent calls will execute the custom behavior defined in the Answer implementation.


Mocking method behavior with custom implementations using Mockito is a powerful technique for writing effective unit tests. By controlling the behavior of external dependencies or complex objects, developers can create specific testing scenarios and verify the behavior of their code. Mockito provides various methods like thenAnswer() and thenThrow() to help developers define custom behaviors flexibly. By incorporating these techniques into your unit testing practices, you can improve the reliability and quality of your code.

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