How do I handle exceptions when mocking retry logic with Mockito

Understanding Mockito Exception Handling

Mockito provides two primary methods for throwing exceptions: `when().thenThrow()` and `doThrow().when()`. The former is used for non-void methods, while the latter is used for void methods.

Using `when().thenThrow()`

This method is used to throw an exception when a non-void method is called. Here's an example:

java
import static org.mockito.Mockito.*;
import org.junit.jupiter.api.Test;

class MyServiceTest {
    @Test
    void testThrowException() {
        MyClass mockClass = mock(MyClass.class);
        when(mockClass.someMethod()).thenThrow(new RuntimeException("An error occurred"));
        assertThrows(RuntimeException.class, () -> {
            mockClass.someMethod();
        });
    }
}

Using `doThrow().when()`

For void methods, you can't use `when().thenThrow()` because there's no return value to mock. Instead, use `doThrow()`:

java
import static org.mockito.Mockito.*;
import org.junit.jupiter.api.Test;

class MyServiceTest {
    @Test
    void testThrowExceptionInVoidMethod() {
        MyClass mockClass = mock(MyClass.class);
        doThrow(new IllegalArgumentException("Invalid argument")).when(mockClass).voidMethod();
        assertThrows(IllegalArgumentException.class, () -> {
            mockClass.voidMethod();
        });
    }
}

Mocking Retry Logic

To test retry logic, you need to simulate failures and successes in a controlled manner. Here’s how you can do it using Mockito:

Example of Retry Logic

Suppose you have a method that retries a certain operation if it fails with a specific exception:

java
public void someMethod(String x) {
    int attempts = 0;
    boolean retry;
    do {
        retry = false;
        try {
            someFunction(x);
        } catch (ExceptionOne e) {
            if (attempts  {
    mockObject.someMethod("test");
});

Using Spring Retry

If you're using Spring, you can leverage the Spring Retry library to handle retries more elegantly. This library allows you to annotate methods with retry logic, making it easier to implement and test retries.

Here’s an example of how you might use Spring Retry with Mockito to test a method that retries:

java
// Define a service interface with retry logic
@Retryable(maxAttempts = 3, backoff = @Backoff(delay = 2000))
String call() throws Exception;

// In your test, mock the service to fail and then succeed
@Bean
public RemoteCallService remoteCallService() throws Exception {
    RemoteCallService remoteService = mock(RemoteCallService.class);
    when(remoteService.call())
        .thenThrow(new RuntimeException("Remote Exception 1"))
        .thenThrow(new RuntimeException("Remote Exception 2"))
        .thenReturn("Completed");
    return remoteService;
}

// Test the retry logic
@Test
public void testRetry() throws Exception {
    String message = this.remoteCallService.call();
    verify(remoteCallService, times(3)).call();
    assertThat(message, is("Completed"));
}

This approach ensures that the method is called the expected number of times before succeeding.

Conclusion

Mocking retry logic with Mockito involves simulating failures and successes in a controlled manner. By using `when().thenThrow()` and `doThrow().when()`, you can effectively test how your code handles exceptions during retries. Additionally, leveraging libraries like Spring Retry can simplify the implementation and testing of retry logic.