doReturn and doThrow in JUnit - Test Execution Trace

Test Overview

This test demonstrates how to use doReturn to mock a method's return value and doThrow to simulate an exception in JUnit with Mockito. It verifies correct behavior when the mocked method returns a value and when it throws an exception.

Test Code - JUnit with Mockito

JUnit

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

import org.junit.jupiter.api.Test;
import org.mockito.Mockito;

class Service {
    String fetchData() {
        return "real data";
    }
}

public class DoReturnDoThrowTest {

    @Test
    void testDoReturnAndDoThrow() {
        Service mockService = Mockito.mock(Service.class);

        // Use doReturn to mock fetchData to return "mocked data"
        doReturn("mocked data").when(mockService).fetchData();
        String result = mockService.fetchData();
        assertEquals("mocked data", result);

        // Use doThrow to mock fetchData to throw RuntimeException
        doThrow(new RuntimeException("Error occurred")).when(mockService).fetchData();
        RuntimeException thrown = assertThrows(RuntimeException.class, () -> {
            mockService.fetchData();
        });
        assertEquals("Error occurred", thrown.getMessage());
    }
}

Execution Trace - 7 Steps

Step	Action	System State	Assertion	Result
1	Test starts	JUnit test runner initialized	-	PASS
2	Mockito creates mock object of Service class	mockService is a mock instance of Service	-	PASS
3	doReturn("mocked data") set up for mockService.fetchData()	fetchData() will return "mocked data" when called	-	PASS
4	Call mockService.fetchData()	fetchData() returns "mocked data"	assertEquals("mocked data", result)	PASS
5	doThrow(RuntimeException) set up for mockService.fetchData()	fetchData() will throw RuntimeException when called	-	PASS
6	Call mockService.fetchData() expecting exception	fetchData() throws RuntimeException with message "Error occurred"	assertThrows(RuntimeException.class, ...), assertEquals("Error occurred", thrown.getMessage())	PASS
7	Test ends	All assertions passed	-	PASS

Failure Scenario

Failing Condition: If doReturn or doThrow is not set up correctly, fetchData() returns null or does not throw exception

Execution Trace Quiz - 3 Questions

What does doReturn("mocked data").when(mockService).fetchData() do?

A It makes fetchData() return "mocked data" when called

B It throws an exception when fetchData() is called

C It calls the real fetchData() method

D It disables fetchData() method

Key Result

Use doReturn() to safely mock methods especially on spy objects to avoid calling real methods, and use doThrow() to simulate exceptions for robust negative testing.