when().thenThrow() for exceptions in JUnit

Introduction

We use when().thenThrow() to tell a test to expect an error from a specific action. This helps us check if our code handles problems correctly.

When you want to test how your code reacts to a failure from a service or method.

When you want to simulate an error like a network failure or invalid input in a test.

When you want to make sure your program throws the right exception in bad situations.

Syntax

JUnit

when(mockedObject.methodCall()).thenThrow(new ExceptionType("message"));

when() sets up the method call you want to fake.

thenThrow() tells the test to throw the exception when that method is called.

Examples

This makes getData() throw a RuntimeException with a message.

JUnit

when(service.getData()).thenThrow(new RuntimeException("Error occurred"));

This makes findById(1) throw an IllegalArgumentException.

JUnit

when(repository.findById(1)).thenThrow(new IllegalArgumentException("Invalid ID"));

Sample Program

This test creates a fake UserRepository that throws an exception when findById(1) is called. The test checks that getUser(1) throws the same exception with the correct message.

JUnit

import static org.mockito.Mockito.*;
import static org.junit.jupiter.api.Assertions.*;

import org.junit.jupiter.api.Test;

class UserService {
    UserRepository repo;
    UserService(UserRepository repo) {
        this.repo = repo;
    }
    User getUser(int id) {
        return repo.findById(id);
    }
}

interface UserRepository {
    User findById(int id);
}

class User {}

public class UserServiceTest {
    @Test
    void testGetUserThrowsException() {
        UserRepository mockRepo = mock(UserRepository.class);
        when(mockRepo.findById(1)).thenThrow(new RuntimeException("User not found"));

        UserService service = new UserService(mockRepo);

        RuntimeException thrown = assertThrows(RuntimeException.class, () -> {
            service.getUser(1);
        });

        assertEquals("User not found", thrown.getMessage());
    }
}

Important Notes

Always mock only the methods you need to test exceptions for.

Use assertThrows to check the exception type and message.

Summary

when().thenThrow() helps simulate errors in tests.

It is useful to check how your code handles exceptions.

Use it with assertThrows to verify the exception is thrown correctly.

Practice

1. What is the main purpose of using when().thenThrow() in JUnit tests?

easy

A. To verify that a method returns a specific value

B. To skip the execution of a test method

C. To create a new instance of a class

D. To simulate an exception being thrown by a mocked method

Solution

1. Step 1: Understand the role of when().thenThrow()

   This syntax is used in mocking frameworks like Mockito to simulate exceptions when a mocked method is called.

2. Step 2: Identify the purpose in testing

   It helps test how your code handles exceptions by forcing the mocked method to throw an exception.

3. Final Answer:

   To simulate an exception being thrown by a mocked method -> Option D

4. Quick Check:

   Exception simulation [OK]

Hint: Use when().thenThrow() to simulate errors in mocks [OK]

Common Mistakes:

• Confusing exception simulation with return value mocking
• Thinking it creates real exceptions instead of simulated ones
• Using it to skip tests instead of throwing exceptions

2. Which of the following is the correct syntax to make a mocked method throw an IllegalArgumentException using Mockito?

easy

A. when(mock.method()).throwException(IllegalArgumentException)

B. when(mock.method()).thenThrow(new IllegalArgumentException())

C. mock.method().thenThrow(IllegalArgumentException)

D. when(mock.method()).throws(IllegalArgumentException)

Solution

1. Step 1: Recall Mockito syntax for throwing exceptions

   The correct syntax uses when(mock.method()).thenThrow(new ExceptionType()).

2. Step 2: Match the options with correct syntax

   when(mock.method()).thenThrow(new IllegalArgumentException()) matches the correct syntax exactly, creating a new exception instance inside thenThrow().

3. Final Answer:

   when(mock.method()).thenThrow(new IllegalArgumentException()) -> Option B

4. Quick Check:

   Correct Mockito exception syntax [OK]

Hint: Use thenThrow(new Exception()) with when() for exceptions [OK]

Common Mistakes:

• Using throwException or throws which are not Mockito methods
• Not creating a new exception instance
• Calling thenThrow directly on mock.method()

3. Given the following code snippet, what will be the test result?

MyService mockService = mock(MyService.class);
when(mockService.getData()).thenThrow(new RuntimeException("Error"));

assertThrows(RuntimeException.class, () -> mockService.getData());

medium

A. Test passes because RuntimeException is thrown as expected

B. Test fails because no exception is thrown

C. Test fails due to syntax error in when().thenThrow()

D. Test passes but exception type is incorrect

Solution

1. Step 1: Analyze the mocking behavior

   The mocked method getData() is set to throw a RuntimeException when called.

2. Step 2: Understand the assertion

   assertThrows expects a RuntimeException from calling mockService.getData().

3. Final Answer:

   Test passes because RuntimeException is thrown as expected -> Option A

4. Quick Check:

   Exception thrown matches assertThrows [OK]

Hint: assertThrows expects the exception thrown by thenThrow() [OK]

Common Mistakes:

• Assuming no exception is thrown from mocked method
• Confusing exception types in assertThrows
• Thinking thenThrow causes syntax errors

4. Identify the error in the following test code using Mockito:

when(mockService.getData()).thenThrow(RuntimeException.class);
assertThrows(RuntimeException.class, () -> mockService.getData());

medium

A. Missing try-catch block around mockService.getData()

B. assertThrows should not be used with exceptions

C. Incorrect use of thenThrow with exception class instead of instance

D. No error; code is correct

Solution

1. Step 1: Check thenThrow argument type

   thenThrow requires an exception instance, not a class type.

2. Step 2: Identify correct usage

   The code should use thenThrow(new RuntimeException()) instead of passing the class.

3. Final Answer:

   Incorrect use of thenThrow with exception class instead of instance -> Option C

4. Quick Check:

   thenThrow needs exception instance [OK]

Hint: Use new Exception() inside thenThrow(), not exception class [OK]

Common Mistakes:

• Passing exception class instead of instance to thenThrow
• Expecting assertThrows to catch exceptions without mocking
• Adding unnecessary try-catch in test

5. You want to test that your service handles a NullPointerException thrown by a dependency's void call() method. Which of the following code snippets correctly mocks the dependency and verifies the exception handling using JUnit and Mockito?

hard

A. doThrow(new NullPointerException()).when(dependency).call();
assertThrows(NullPointerException.class, () -> service.callDependency());

B. when(dependency.call()).thenThrow(NullPointerException.class);
assertThrows(Exception.class, () -> service.callDependency());

C. when(dependency.call()).thenReturn(null);
assertThrows(NullPointerException.class, () -> service.callDependency());

D. when(dependency.call()).thenThrow(new NullPointerException());
assertThrows(NullPointerException.class, () -> service.callDependency());

Solution

1. Step 1: Understand mocking exceptions with void methods

   If dependency.call() is a void method, doThrow().when() is the correct way to mock exceptions.

2. Step 2: Verify correct assertion

   assertThrows(NullPointerException.class, () -> service.callDependency()) correctly checks that the service handles the exception.

3. Step 3: Analyze options

   when(dependency.call()).thenThrow(new NullPointerException()); assertThrows(NullPointerException.class, () -> service.callDependency()); uses when().thenThrow() which works only for non-void methods. when(dependency.call()).thenThrow(NullPointerException.class); assertThrows(Exception.class, () -> service.callDependency()); passes class instead of instance and wrong exception type in assertion. when(dependency.call()).thenReturn(null); assertThrows(NullPointerException.class, () -> service.callDependency()); returns null instead of throwing exception.

4. Final Answer:

   doThrow(new NullPointerException()).when(dependency).call(); assertThrows(NullPointerException.class, () -> service.callDependency()); -> Option A

5. Quick Check:

   Use doThrow for void methods + assertThrows [OK]

Hint: Use doThrow() for void methods, then assertThrows() to check [OK]

Common Mistakes:

• Using thenThrow() on void methods
• Passing exception class instead of instance
• Asserting wrong exception type
• Returning null instead of throwing exception