doReturn and doThrow in JUnit - Build an Automation Script

Test service method behavior using doReturn and doThrow with Mockito

Preconditions (3)

Step 1: Mock the dependency method to return a specific value using doReturn

Step 2: Call the service method that uses the mocked dependency

Step 3: Verify the returned value matches the mocked value

Step 4: Mock the dependency method to throw an exception using doThrow

Step 5: Call the service method that uses the mocked dependency and expect an exception

Step 6: Verify the exception is thrown as expected

✅ Expected Result: The test passes when the service method returns the mocked value and throws the mocked exception as specified

Automation Requirements - JUnit 5 with Mockito

Assertions Needed:

assertEquals to verify returned value

assertThrows to verify exception thrown

Best Practices:

Use @ExtendWith(MockitoExtension.class) for Mockito support

Use doReturn() and doThrow() for mocking void or spy methods

Use descriptive method names for test clarity

Use explicit assertions for expected outcomes

Automated Solution

JUnit

import static org.mockito.Mockito.doReturn;
import static org.mockito.Mockito.doThrow;
import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.junit.jupiter.api.Assertions.assertThrows;

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.mockito.junit.jupiter.MockitoExtension;

@ExtendWith(MockitoExtension.class)
public class ServiceTest {

    @Mock
    Dependency dependency;

    @InjectMocks
    Service service;

    @Test
    void testDoReturn() {
        // Arrange
        doReturn("mocked response").when(dependency).getData();

        // Act
        String result = service.processData();

        // Assert
        assertEquals("mocked response", result);
    }

    @Test
    void testDoThrow() {
        // Arrange
        doThrow(new RuntimeException("mocked exception")).when(dependency).getData();

        // Act & Assert
        RuntimeException thrown = assertThrows(RuntimeException.class, () -> {
            service.processData();
        });
        assertEquals("mocked exception", thrown.getMessage());
    }
}

// Supporting classes
class Service {
    private final Dependency dependency;

    public Service(Dependency dependency) {
        this.dependency = dependency;
    }

    public String processData() {
        return dependency.getData();
    }
}

interface Dependency {
    String getData();
}

This test class uses JUnit 5 and Mockito to automate the manual test case steps.

Annotations: @ExtendWith(MockitoExtension.class) enables Mockito support. @Mock creates a mock for the dependency. @InjectMocks injects the mock into the service.

testDoReturn: Uses doReturn to mock dependency.getData() to return "mocked response". Then calls service.processData() and asserts the returned value matches.

testDoThrow: Uses doThrow to mock dependency.getData() to throw a RuntimeException. Then asserts that calling service.processData() throws the expected exception with the correct message.

This approach clearly separates setup, action, and verification steps, making the test easy to read and maintain.

Common Mistakes - 3 Pitfalls

Using when().thenReturn() with spy objects causing real method calls

Not using assertThrows to verify exceptions

Mocking methods without specifying behavior leading to null returns

Bonus Challenge

Now add data-driven testing with 3 different return values and 3 different exceptions for the mocked method

Show Hint

Practice

1. What is the main purpose of doReturn in JUnit mocking?

easy

A. To specify a value that a mocked method should return when called

B. To make a mocked method throw an exception

C. To verify how many times a method was called

D. To reset the state of a mock object

Solution

1. Step 1: Understand doReturn usage

   doReturn is used to tell a mock what value to return when a specific method is called.

2. Step 2: Differentiate from doThrow

   doThrow makes the method throw an exception, not return a value.

3. Final Answer:

   To specify a value that a mocked method should return when called -> Option A

4. Quick Check:

   doReturn sets return value [OK]

Hint: Remember: doReturn returns a value, doThrow throws exception [OK]

Common Mistakes:

• Confusing doReturn with doThrow
• Thinking doReturn verifies method calls
• Assuming doReturn resets mocks

2. Which of the following is the correct syntax to make a mock method throw an exception using doThrow?

easy

A. doThrow().when(mockObject).methodName(new IOException());

B. doThrow(IOException).when(mockObject).methodName();

C. doThrow(new IOException()).when(mockObject).methodName();

D. doThrow(new IOException).methodName().when(mockObject);

Solution

1. Step 1: Recall correct doThrow syntax

   The correct syntax is doThrow(new ExceptionType()).when(mock).method(); to specify the exception and method.

2. Step 2: Check each option

   doThrow(new IOException()).when(mockObject).methodName(); matches the correct syntax exactly. Others have misplaced parentheses or wrong order.

3. Final Answer:

   doThrow(new IOException()).when(mockObject).methodName(); -> Option C

4. Quick Check:

   Correct doThrow syntax = doThrow(new IOException()).when(mockObject).methodName(); [OK]

Hint: Use new Exception() inside doThrow, then when(mock).method() [OK]

Common Mistakes:

• Omitting 'new' before exception
• Placing parentheses incorrectly
• Calling method before when()

3. Given the following code snippet, what will be the output when mockService.getData() is called?

MyService mockService = mock(MyService.class);
doReturn("Hello").when(mockService).getData();
doThrow(new RuntimeException()).when(mockService).getData();
System.out.println(mockService.getData());

medium

A. Hello

B. RuntimeException is thrown

C. null

D. Compilation error

Solution

1. Step 1: Analyze the order of stubbing

   First doReturn("Hello") is set, then doThrow(RuntimeException) overrides it for the same method.

2. Step 2: Determine which stub applies on method call

   The last stub doThrow takes effect, so calling getData() throws RuntimeException.

3. Final Answer:

   RuntimeException is thrown -> Option B

4. Quick Check:

   Last stub wins, so exception thrown [OK]

Hint: Last doReturn or doThrow for method overrides previous [OK]

Common Mistakes:

• Assuming first stub applies
• Expecting returned string instead of exception
• Thinking code won't compile

4. Identify the error in the following code snippet using doReturn and doThrow:

doReturn("Success").when(mockService).process();
doThrow(new IllegalArgumentException()).when(mockService).process();
doReturn("Done").when(mockService).process();
String result = mockService.process();

medium

A. The code will throw IllegalArgumentException at runtime

B. Cannot use doReturn and doThrow on the same method

C. doThrow must be called after doReturn to work

D. The last doReturn overrides previous stubs, so no error

Solution

1. Step 1: Understand stub overriding order

   Stubs are applied in order; the last stub for a method is effective.

2. Step 2: Check the last stub

   The last stub is doReturn("Done"), so calling process() returns "Done" without exception.

3. Final Answer:

   The last doReturn overrides previous stubs, so no error -> Option D

4. Quick Check:

   Last stub wins, no exception thrown [OK]

Hint: Last stub for method call is effective, no conflict error [OK]

Common Mistakes:

• Thinking doThrow always throws exception regardless of order
• Believing mixing doReturn and doThrow causes compile error
• Expecting exception thrown despite last stub

5. You want to test a method that calls externalService.fetch(). You want to simulate two calls: the first returns "Data1", the second throws an IOException. Which is the correct way to stub fetch() using doReturn and doThrow?

hard

A. doReturn("Data1").doThrow(new IOException()).when(externalService).fetch();

B. doReturn("Data1").when(externalService).fetch(); doThrow(new IOException()).when(externalService).fetch();

C. doReturn("Data1").when(externalService).fetch(); doReturn("Data2").when(externalService).fetch();

D. doThrow(new IOException()).doReturn("Data1").when(externalService).fetch();

Solution

1. Step 1: Understand chaining stubs for multiple calls

   Mockito allows chaining stubs with doReturn(...).doThrow(...).when(mock).method(); to simulate sequential calls.

2. Step 2: Analyze options

   doReturn("Data1").doThrow(new IOException()).when(externalService).fetch(); correctly chains doReturn then doThrow for first return and second throw. Separate doReturn("Data1").when(externalService).fetch(); doThrow(new IOException()).when(externalService).fetch(); overrides stubs, causing all calls to throw the exception. The other options use repeated returns or incorrect order.

3. Final Answer:

   doReturn("Data1").doThrow(new IOException()).when(externalService).fetch(); -> Option A

4. Quick Check:

   Chain doReturn then doThrow for sequential calls [OK]

Hint: Chain doReturn().doThrow() for sequential call behaviors [OK]

Common Mistakes:

• Overwriting stubs instead of chaining
• Wrong order of doThrow and doReturn
• Expecting multiple when() calls for sequence