10 Examples Of Mockito + JUnit in Java for Unit Testing

Hello guys, if you are writing unit test in Java then you know how difficult it is to write especially if you are testing a class which is dependent upon other class like HttpClient and you cannot connect to actual server. At those time, a mocking library like Mockito comes to rescue. Given the increased focus on unit testing and code coverage, I have find myself using Mockito more and more along with JUnit in last a couple of years but I haven't written many articles on Mockito yet but that is chaging now. In this article, I am going to share 10 essential Mockito examples which I belive every Java programmer should know. But, before we get to the 10 best examples that will teach you everything there is to know about Mockito in Java, let me tell you a little bit more about what it really is.

Mockito is basically an open-source framework that allows you to easily create test doubles or mocks. A test double is actually just a generic term for a particular case where you replace the production object for testing.

Mockito allows you to work with a variety of test doubles like stubs, spies, and mocks. Stubs are actually just some objects with a set of predefined return values. Spies are also very similar to stubs. But they can also record the stats related to how they are executed. 

Mocks are also objects with predefined return values to method executions. It also has recorded expectations of the executions.

10 Examples Of Mockito In Java

In this list, we have compiled 10 examples of Mockito in Java. Keep reading to find out more.

 

1. Setting Up Mockito In Maven

To add Mockito, you need to add the latest Mockito version by Maven. See the following example

pom.xml

<dependency>

    <groupId>org.mockito</groupId>

    <artifactId>mockito-core</artifactId>

    <version>4.6.1</version>

    <scope>test</scope>

</dependency>

build.gradle

testCompile group: 'org.mockito', name: 'mockito-core', version: '4.6.1'

You can also process Mockito annotations with the help of JUnit 5. But you will need to use the MockExtension. See the following example:

@ExtendWith(MockitoExtension.class)

public class ApplicationTest {

   //code

}

In case you are using the legacy JUnit 4,

@RunWith(MockitoJUnitRunner.class)

public class ApplicationTest {

//code

}

public class ApplicationTest {

@Rule public MockitoRule rule = MockitoJUnit.rule().strictness(Strictness.STRICT_STUBS);

//code

}

2. Annotations Is Mockito

You can annotate an object using the @Mock annotation. This will also make the code more readable and neat. You can specify the mock object name which will be useful in case there are errors or something. Check out the following example:

package com.journaldev.mockito.mock;

import java.util.List;

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

import static org.mockito.Mockito.*;

import org.junit.jupiter.api.BeforeEach;

import org.junit.jupiter.api.Test;

import org.mockito.Mock;

import org.mockito.MockitoAnnotations;

public class MockitoMockAnnotationExample {

@Mock

List<String> mockList;

@BeforeEach

public void setup() {

//if we don't call below, we will get NullPointerException

MockitoAnnotations.initMocks(this);

}

@SuppressWarnings("unchecked")

@Test

public void test() {

when(mockList.get(0)).thenReturn("JournalDev");

assertEquals("JournalDev", mockList.get(0));

}

}

3. Creating A Mock In Mockito

You can use the Mockito class mock() method for creating a mock object of a certain class or even a interface. It is also one of the easiest ways to create a mock.

package com.journaldev.mockito.mock;

import java.util.List;

import static org.mockito.Mockito.*;

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

import org.junit.jupiter.api.Test;

public class MockitoMockMethodExample {

@SuppressWarnings("unchecked")

@Test

public void test() {

// using Mockito.mock() method

List<String> mockList = mock(List.class);

when(mockList.size()).thenReturn(5);

assertTrue(mockList.size()==5);

}

}

In this example, you can see that we are using JUnit 5 for writing test cases along with Mockito to mock the objects.

4. Using Mockito spy() For Partial Mocking

If you are looking to mock only some specific behaviours, you can call the real methods for behaviours that are unstubbed. You can then create a spy object using the Mockito spy() method. 

package com.journaldev.mockito.mock;

import static org.junit.jupiter.api.Assertions.assertEquals;

import static org.mockito.Mockito.spy;

import static org.mockito.Mockito.when;

import java.util.ArrayList;

import java.util.List;

import org.junit.jupiter.api.Test;

public class MockitoSpyMethodExample {

@Test

public void test() {

List<String> list = new ArrayList<>();

List<String> spyOnList = spy(list);

when(spyOnList.size()).thenReturn(10);

assertEquals(10, spyOnList.size());

//calling real methods since below methods are not stubbed

spyOnList.add("Pankaj");

spyOnList.add("Meghna");

assertEquals("Pankaj", spyOnList.get(0));

assertEquals("Meghna", spyOnList.get(1));

}

}

5. The @InjectMocks Annotation

You can see the following example to understand how you can inject a mocked object into another object. 

package com.journaldev.mockito.mock;

import java.util.List;

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

import static org.mockito.Mockito.*;

import org.junit.jupiter.api.BeforeEach;

import org.junit.jupiter.api.Test;

import org.mockito.InjectMocks;

import org.mockito.Mock;

import org.mockito.MockitoAnnotations;

public class MockitoInjectMockAnnotationExample {

@Mock

List<String> mockList;

//@InjectMock creates an instance of the class and 

//injects the mocks that are marked with the annotations @Mock into it.

@InjectMocks

Fruits mockFruits;

@BeforeEach

public void setup() {

//if we don't call below, we will get NullPointerException

MockitoAnnotations.initMocks(this);

}

@SuppressWarnings("unchecked")

@Test

public void test() {

when(mockList.get(0)).thenReturn("Apple");

when(mockList.size()).thenReturn(1);

assertEquals("Apple", mockList.get(0));

assertEquals(1, mockList.size());

//mockFruits names is using mockList, below asserts confirm it

assertEquals("Apple", mockFruits.getNames().get(0));

assertEquals(1, mockFruits.getNames().size());

mockList.add(1, "Mango");

//below will print null because mockList.get(1) is not stubbed

System.out.println(mockList.get(1));

}

}

class Fruits{

private List<String> names;

public List<String> getNames() {

return names;

}

public void setNames(List<String> names) {

this.names = names;

}

}

6. The @Spy Annotation

As the name suggests, you can use the @Spy annotation for spying on an object. Check out the following example. 

package com.journaldev.mockito.mock;

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

import static org.mockito.Mockito.*;

import org.junit.jupiter.api.BeforeEach;

import org.junit.jupiter.api.Test;

import org.mockito.MockitoAnnotations;

import org.mockito.Spy;

public class MockitoSpyAnnotationExample {

@Spy

Utils mockUtils;

@BeforeEach

public void setup() {

MockitoAnnotations.initMocks(this);

}

@Test

public void test() {

when(mockUtils.process(1, 1)).thenReturn(5);

//mocked method

assertEquals(5, mockUtils.process(1, 1));

//real method called since it's not stubbed

assertEquals(20, mockUtils.process(19, 1));

}

}

class Utils{

public int process(int x, int y) {

System.out.println("Input Params = "+x+","+y);

return x+y;

}

}

7. ToDoBusinessMock.java Example

Check out the following piece of code to see how you can use ToDoBusinessMock.java:

import static org.junit.Assert.assertEquals;  

import static org.mockito.Mockito.mock;  

import static org.mockito.Mockito.when;  

  

import java.util.Arrays;  

import java.util.List;  

import org.junit.Test;  

  

public class ToDoBusinessMock {  

  

    @Test  

    public void testusing_Mocks() {  

          

        ToDoService doService = mock(ToDoService.class);  

           

        List<String> combinedlist = Arrays.asList(" Use Core Java ", " Use Spring Core ", " Use w3eHibernate ", " Use Spring MVC ");  

        when(doService.getTodos("dummy")).thenReturn(combinedlist);  

          

        ToDoBusiness business = new ToDoBusiness(doService);  

      

        List<String> alltd = business.getTodosforHibernate("dummy");   

          

        System.out.println(alltd);  

        assertEquals(1, alltd.size());  

    }  

 }  

8. Using TestList.java

This is very helpful for mocking a list class.

import static org.junit.Assert.*;  

import static org.mockito.Mockito.when;  

  

import java.util.List;  

import org.junit.Test;  

import org.mockito.Mock;  

  

public class TestList {  

  

    @Test  

    public void testList_ReturnsSingle_value() {  

  

        List mocklist = mock(List.class);  

                           when(mocklist.size()).thenReturn(1);  

  

        assertEquals(1, mocklist.size());  

        assertEquals(1, mocklist.size());  

          

                          System.out.println( mocklist.size());  

        System.out.println(mocklist);  

    }  

 }  

10. How To Get Multiple Return Values For A Test

You can also mock a list class with multiple return values. The list in the following example contains three items.

import static org.junit.Assert.*;  

import static org.mockito.Mockito.when;  

  

import java.util.List;  

import org.junit.Test;  

import org.mockito.Mock;  

  

public class TestList {   

      

      @Test   

      public void testList_Returns_MultipleValues() {  

        

      List mocklist = mock(List.class);  

      when(mocklist.size()).thenReturn(1).thenReturn(2).thenReturn(3);  

        

      assertEquals(1, mocklist.size());   

      assertEquals(2, mocklist.size());  

      assertEquals(3, mocklist.size());  

        

      System.out.println(mocklist.size());   

      System.out.println(mocklist);  

        

      }  

 }    

Conclusion

If you liked this list of 10 examples of Mockito in Java, feel free to share it with your friends and family.