Unit Test What is Mocking? and Why?

In simple, mocking is creating objects that mimic the behavior of real objects. Refer to the following case study :

Tested :

1. Java 1.8
2. JUnit 4.12
3. Mockito 2.0.73-beta

1. Java Examples

A simple Author and Book example.

1.1 BookService to return a list of books by author name.

package com.mkyong.examples.mock;
import java.util.List;
public interface BookService {
    List<Book> findBookByAuthor(String author);

package com.mkyong.examples.mock;
import java.util.List;
public class BookServiceImpl implements BookService {
    private BookDao bookDao;
    public BookDao getBookDao() {
        return bookDao;
    public void setBookDao(BookDao bookDao) {
        this.bookDao = bookDao;
    @Override
    public List<Book> findBookByAuthor(String name) {
        return bookDao.findBookByAuthor(name);

package com.mkyong.examples.mock;
import java.util.List;
public interface BookDao {
    List<Book> findBookByAuthor(String author);

package com.mkyong.examples.mock;
import java.util.List;
public class BookDaoImpl implements BookDao {
    @Override
    public List<Book> findBookByAuthor(String name) {
		// init database
        // Connect to DB for data
        // return data

1.2 A book validator.

package com.mkyong.examples.mock;
public interface BookValidatorService {
    boolean isValid(Book book);

package com.mkyong.examples.mock;
public class FakeBookValidatorService implements BookValidatorService {
    @Override
    public boolean isValid(Book book) {
        if (book == null)
            return false;
        if ("bot".equals(book.getName())) {
            return false;
        } else {
            return true;

1.3 Reviews the AuthorServiceImpl, it has dependencies on BookService (depends on BookDao) and BookValidatorService, it makes the unit test a bit hard to write.

package com.mkyong.examples.mock;
public interface AuthorService {
    int getTotalBooks(String author);

package com.mkyong.examples.mock;
import java.util.List;
import java.util.stream.Collectors;
public class AuthorServiceImpl implements AuthorService {
    private BookService bookService;
    private BookValidatorService bookValidatorService;
    public BookValidatorService getBookValidatorService() {
        return bookValidatorService;
    public void setBookValidatorService(BookValidatorService bookValidatorService) {
        this.bookValidatorService = bookValidatorService;
    public BookService getBookService() {
        return bookService;
    public void setBookService(BookService bookService) {
        this.bookService = bookService;
	//How to test this method ???
    @Override
    public int getTotalBooks(String author) {
        List<Book> books = bookService.findBookByAuthor(author);
        //filters some bot writers
        List<Book> filtered = books.stream().filter(
                x -> bookValidatorService.isValid(x))
                .collect(Collectors.toList());
        //other business logic
        return filtered.size();

2. Unit Test

Create a unit test for AuthorServiceImpl.getTotalBooks()

2.1 The AuthorServiceImpl has two dependencies, you need to make sure both are configured properly.

package com.mkyong.mock;
import com.mkyong.examples.mock.AuthorServiceImpl;
import com.mkyong.examples.mock.BookDaoImpl;
import com.mkyong.examples.mock.BookServiceImpl;
import com.mkyong.examples.mock.FakeBookValidatorService;
import org.junit.Test;
import static org.hamcrest.CoreMatchers.is;
import static org.junit.Assert.assertThat;
public class AuthorServiceTest {
    @Test
    public void test_total_book_by_mock() {
		//1. Setup
        AuthorServiceImpl obj = new AuthorServiceImpl();
        BookServiceImpl bookService = new BookServiceImpl();
        bookService.setBookDao(new BookDaoImpl()); //Where Dao connect to?
        obj.setBookService(bookService);
        obj.setBookValidatorService(new FakeBookValidatorService());
		//2. Test method
        int qty = obj.getTotalBooks("mkyong");
		//3. Verify result
        assertThat(qty, is(2));

To pass above unit test, you need to setup a database in DAO layer, else bookService will return nothing.

2.3 Some disadvantages to perform tests like above :

1. This unit test is slow, because you need to start a database in order to get data from DAO.
2. This unit test is not isolated, it always depends on external resources like database.
3. This unit test can’t ensures the test condition is always the same, the data in the database may vary in time.
4. It’s too much work to test a simple method, cause developers skipping the test.

2.4 Solution
The solution is obvious, you need a modified version of the BookServiceImpl class – which will always return the same data for testing, a mock object!

3. Unit Test – Mock Object

3.1 Create a new MockBookServiceImpl class and always return the same collection of books for author “mkyong”.

package com.mkyong.mock;
import com.mkyong.examples.mock.Book;
import com.mkyong.examples.mock.BookService;
import java.util.ArrayList;
import java.util.List;
//I am a mock object!
public class MockBookServiceImpl implements BookService {
    @Override
    public List<Book> findBookByAuthor(String author) {
        List<Book> books = new ArrayList<>();
        if ("mkyong".equals(author)) {
            books.add(new Book("mkyong in action"));
            books.add(new Book("abc in action"));
            books.add(new Book("bot"));
        return books;
    //implements other methods...

3.2 Update the unit test again.

package com.mkyong.mock;
import com.mkyong.examples.mock.AuthorServiceImpl;
import com.mkyong.examples.mock.FakeBookValidatorService;
import org.junit.Test;
import static org.hamcrest.CoreMatchers.is;
import static org.junit.Assert.assertThat;
public class AuthorServiceTest {
    @Test
    public void test_total_book_by_mock() {
		//1. Setup
        AuthorServiceImpl obj = new AuthorServiceImpl();
        /*BookServiceImpl bookService = new BookServiceImpl();
        bookService.setBookDao(new BookDaoImpl());
        obj.setBookService(bookService);*/
        obj.setBookService(new MockBookServiceImpl());
        obj.setBookValidatorService(new FakeBookValidatorService());
		//2. Test method
        int qty = obj.getTotalBooks("mkyong");
		//3. Verify result
        assertThat(qty, is(2));

The above unit test is much better, fast, isolated (no more database) and the test condition (data) is always same.

3.3 But, there are some disadvantages to create mock object manually like above :

1. At the end, you may create many mock objects (classes), just for the unit test purpose.
2. If the interface contains many methods, you need to override each of them.
3. It’s still too much work, and messy!

3.4 Solution
Try Mockito, a simple and powerful mocking framework.

4. Unit Test – Mockito

4.1 Update the unit test again, this time, create the mock object via Mockito framework.

	<dependency>
            <groupId>org.mockito</groupId>
            <artifactId>mockito-core</artifactId>
            <version>2.0.73-beta</version>
	</dependency>

package com.mkyong.mock;
import com.mkyong.examples.mock.AuthorServiceImpl;
import com.mkyong.examples.mock.Book;
import com.mkyong.examples.mock.BookServiceImpl;
import com.mkyong.examples.mock.FakeBookValidatorService;
import org.junit.Test;
import java.util.Arrays;
import java.util.List;
import static org.hamcrest.CoreMatchers.is;
import static org.junit.Assert.assertThat;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.when;
public class AuthorServiceTest {
	    @Test
	    public void test_total_book_by_mockito() {
			//1. Setup
	        List<Book> books = Arrays.asList(
	                new Book("mkyong in action"),
	                new Book("abc in action"),
	                new Book("bot"));
	        BookServiceImpl mockito = mock(BookServiceImpl.class);
	        //if the author is "mkyong", then return a 'books' object.
	        when(mockito.findBookByAuthor("mkyong")).thenReturn(books);
	        AuthorServiceImpl obj = new AuthorServiceImpl();
	        obj.setBookService(mockito);
	        obj.setBookValidatorService(new FakeBookValidatorService());
			//2. Test method
	        int qty = obj.getTotalBooks("mkyong");
			//3. Verify result
	        assertThat(qty, is(2));

Done. Your feedback is appreciated

References

1. Mockito Official Site
2. DZone – Mockito
3. Wikipedia – Mock object
4. Unit tests with Mockito