Mockito Tutorial for Beginner in 5 Steps


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Mockito is the most popular mocking framework in Java.

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Git Repository

  • https://github.com/in28minutes/getting-started-in-5-steps/tree/master/mockito-in-5-steps

Pre-requisites

  • Java & Eclipse - https://www.youtube.com/playlist?list=PLBBog2r6uMCSmMVTW_QmDLyASBvovyAO3
  • JUnit - https://courses.in28minutes.com/p/junit-tutorial-for-beginners

IDE Configuration

Easier Static Imports

  • Window > Preferences > Java > Editor > Content Assist > Favorites
    • org.junit.Assert
    • org.mockito.BDDMockito
    • org.mockito.Mockito
    • org.hamcrest.Matchers
    • org.hamcrest.CoreMatchers

Reference

  • Visit Mockito Official Documentation - http://site.mockito.org/mockito/docs/current/org/mockito/Mockito.html

Overview

  • Step 1 : Setting up an example using http://start.spring.io.
  • Step 2 : Using a Stubs - Disadvantages
  • Step 3 : Your first mock.
  • Step 4 : Using Mockito Annotations - @Mock, @InjectMocks, @RunWith(MockitoJUnitRunner.class)
  • Step 5 : Mocking List interface

Step 1 : Setting up an example using http://start.spring.io.

Creating a Spring Project with Spring Initializr is a cake walk.

Spring Initializr http://start.spring.io/ is great tool to bootstrap your Spring Boot projects.

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As shown in the image above, following steps have to be done

  • Launch Spring Initializr and choose the following
    • Choose com.in28minutes.mockito as Group
    • Choose mockito-demo as Artifact
    • Choose Dependency
      • Web
  • Click Generate Project.
  • Import the project into Eclipse.
  • If you want to understand all the files that are part of this project, you can go here.

Let’s also set up a simple business class with a dependency.

  • SomeBusinessImpl depends on DataService for the data
  • findTheGreatestFromAllData has some business logic to find the greatest
package com.in28minutes.mockito.mockitodemo;

public class SomeBusinessImpl {
	private DataService dataService;

	public SomeBusinessImpl(DataService dataService) {
		super();
		this.dataService = dataService;
	}

	int findTheGreatestFromAllData() {
		int[] data = dataService.retrieveAllData();
		int greatest = Integer.MIN_VALUE;

		for (int value : data) {
			if (value > greatest) {
				greatest = value;
			}
		}
		return greatest;
	}
}
package com.in28minutes.mockito.mockitodemo;

public interface DataService {
	int[] retrieveAllData();
}

Step 2 : Using a Stubs - Disadvantages

Let’s use a stub to write a unit test for SomeBusinessImpl.

package com.in28minutes.mockito.mockitodemo;

import static org.junit.Assert.assertEquals;

import org.junit.Test;

public class SomeBusinessStubTest {
	@Test
	public void testFindTheGreatestFromAllData() {
		SomeBusinessImpl businessImpl = new SomeBusinessImpl(new DataServiceStub());
		int result = businessImpl.findTheGreatestFromAllData();
		assertEquals(24, result);

	}

}

class DataServiceStub implements DataService {
	@Override
	public int[] retrieveAllData() {
		return new int[] { 24, 6, 15 };
	}
}

Problems with Stub

  • How do I get DataServiceStub to return different data for different scenaris?
  • Everytime DataService interface is updated with new methods, the DataServiceStub implementations should be updated.

Step 3 : Your first mock.

package com.in28minutes.mockito.mockitodemo;

import static org.junit.Assert.assertEquals;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.when;

import org.junit.Test;

public class SomeBusinessMockTest {

	@Test
	public void testFindTheGreatestFromAllData() {
		DataService dataServiceMock = mock(DataService.class);
		when(dataServiceMock.retrieveAllData()).thenReturn(new int[] { 24, 15, 3 });
		SomeBusinessImpl businessImpl = new SomeBusinessImpl(dataServiceMock);
		int result = businessImpl.findTheGreatestFromAllData();
		assertEquals(24, result);
	}

	@Test
	public void testFindTheGreatestFromAllData_ForOneValue() {
		DataService dataServiceMock = mock(DataService.class);
		when(dataServiceMock.retrieveAllData()).thenReturn(new int[] { 15 });
		SomeBusinessImpl businessImpl = new SomeBusinessImpl(dataServiceMock);
		int result = businessImpl.findTheGreatestFromAllData();
		assertEquals(15, result);
	}

}

Notes

  • DataService dataServiceMock = mock(DataService.class) - We are using the mock method to create a mock.
  • when(dataServiceMock.retrieveAllData()).thenReturn(new int[] { 24, 15, 3 }) - stubbing the mock to return specific data

Step 4 : Using Mockito Annotations - @Mock, @InjectMocks, @RunWith(MockitoJUnitRunner.class)

package com.in28minutes.mockito.mockitodemo;

import static org.junit.Assert.assertEquals;
import static org.mockito.Mockito.when;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.mockito.junit.MockitoJUnitRunner;

@RunWith(MockitoJUnitRunner.class)
public class SomeBusinessMockAnnotationsTest {

	@Mock
	DataService dataServiceMock;

	@InjectMocks
	SomeBusinessImpl businessImpl;

	@Test
	public void testFindTheGreatestFromAllData() {
		when(dataServiceMock.retrieveAllData()).thenReturn(new int[] { 24, 15, 3 });
		assertEquals(24, businessImpl.findTheGreatestFromAllData());
	}

	@Test
	public void testFindTheGreatestFromAllData_ForOneValue() {
		when(dataServiceMock.retrieveAllData()).thenReturn(new int[] { 15 });
		assertEquals(15, businessImpl.findTheGreatestFromAllData());
	}

	@Test
	public void testFindTheGreatestFromAllData_NoValues() {
		when(dataServiceMock.retrieveAllData()).thenReturn(new int[] {});
		assertEquals(Integer.MIN_VALUE, businessImpl.findTheGreatestFromAllData());
	}
}

Notes

  • @Mock DataService dataServiceMock; - Create a mock for DataService.
  • @InjectMocks SomeBusinessImpl businessImpl; - Inject the mocks as dependencies into businessImpl.
  • @RunWith(MockitoJUnitRunner.class) - The JUnit Runner which causes all the initialization magic with @Mock and @InjectMocks to happen before the tests are run.

Step 5 : Mocking List interface

Mocking a method. Mock returns the same value on multiple calls.

@Test
public void testSize() {
	List listMock = mock(List.class);
	when(listMock.size()).thenReturn(10);
	assertEquals(10, listMock.size());
	assertEquals(10, listMock.size());
}

Setting the mock to return 10 on first call and 20 on the second call.

@Test
public void testSize_multipleReturns() {
	List listMock = mock(List.class);
	when(listMock.size()).thenReturn(10).thenReturn(20);
	assertEquals(10, listMock.size());
	assertEquals(20, listMock.size());
	assertEquals(20, listMock.size());
}

Customizing the mock based on a specific parameter value.

@Test
public void testGet_SpecificParameter() {
	List listMock = mock(List.class);
	when(listMock.get(0)).thenReturn("SomeString");
	assertEquals("SomeString", listMock.get(0));
	assertEquals(null, listMock.get(1));
}

Using a generic argument - Mockito.anyInt()

@Test
public void testGet_GenericParameter() {
	List listMock = mock(List.class);
	when(listMock.get(Mockito.anyInt())).thenReturn("SomeString");
	assertEquals("SomeString", listMock.get(0));
	assertEquals("SomeString", listMock.get(1));
}

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Complete Code Example

/pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>

	<groupId>com.in28minutes.mockito</groupId>
	<artifactId>mockito-demo</artifactId>
	<version>0.0.1-SNAPSHOT</version>
	<packaging>jar</packaging>

	<name>mockito-demo</name>
	<description>Demo project for Spring Boot</description>

	<parent>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-parent</artifactId>
		<version>2.3.1.RELEASE</version>
		<relativePath/> <!-- lookup parent from repository -->
	</parent>

	<properties>
		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
		<project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
		<java.version>1.8</java.version>
<maven-jar-plugin.version>3.1.1</maven-jar-plugin.version>
	</properties>

	<dependencies>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter</artifactId>
		</dependency>

		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-test</artifactId>
			<scope>test</scope>
		</dependency>
	</dependencies>

	<build>
		<plugins>
			<plugin>
				<groupId>org.springframework.boot</groupId>
				<artifactId>spring-boot-maven-plugin</artifactId>
			</plugin>
		</plugins>
	</build>

	<repositories>
		<repository>
			<id>spring-snapshots</id>
			<name>Spring Snapshots</name>
			<url>https://repo.spring.io/snapshot</url>
			<snapshots>
				<enabled>true</enabled>
			</snapshots>
		</repository>
		<repository>
			<id>spring-milestones</id>
			<name>Spring Milestones</name>
			<url>https://repo.spring.io/milestone</url>
			<snapshots>
				<enabled>false</enabled>
			</snapshots>
		</repository>
	</repositories>

	<pluginRepositories>
		<pluginRepository>
			<id>spring-snapshots</id>
			<name>Spring Snapshots</name>
			<url>https://repo.spring.io/snapshot</url>
			<snapshots>
				<enabled>true</enabled>
			</snapshots>
		</pluginRepository>
		<pluginRepository>
			<id>spring-milestones</id>
			<name>Spring Milestones</name>
			<url>https://repo.spring.io/milestone</url>
			<snapshots>
				<enabled>false</enabled>
			</snapshots>
		</pluginRepository>
	</pluginRepositories>


</project>

/src/main/java/com/in28minutes/mockito/mockitodemo/DataService.java

package com.in28minutes.mockito.mockitodemo;

public interface DataService {
	int[] retrieveAllData();
}

/src/main/java/com/in28minutes/mockito/mockitodemo/MockitoDemoApplication.java

package com.in28minutes.mockito.mockitodemo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class MockitoDemoApplication {

	public static void main(String[] args) {
		SpringApplication.run(MockitoDemoApplication.class, args);
	}
}

/src/main/java/com/in28minutes/mockito/mockitodemo/SomeBusinessImpl.java

package com.in28minutes.mockito.mockitodemo;

public class SomeBusinessImpl {
	private DataService dataService;

	public SomeBusinessImpl(DataService dataService) {
		super();
		this.dataService = dataService;
	}

	int findTheGreatestFromAllData() {
		int[] data = dataService.retrieveAllData();
		int greatest = Integer.MIN_VALUE;

		for (int value : data) {
			if (value > greatest) {
				greatest = value;
			}
		}
		return greatest;
	}
}

/src/main/resources/application.properties


/src/test/java/com/in28minutes/mockito/mockitodemo/ListTest.java

package com.in28minutes.mockito.mockitodemo;

import static org.junit.Assert.assertEquals;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.when;

import java.util.List;

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

public class ListTest {

	@Test
	public void testSize() {
		List listMock = mock(List.class);
		when(listMock.size()).thenReturn(10);
		assertEquals(10, listMock.size());
		assertEquals(10, listMock.size());
	}

	@Test
	public void testSize_multipleReturns() {
		List listMock = mock(List.class);
		when(listMock.size()).thenReturn(10).thenReturn(20);
		assertEquals(10, listMock.size());
		assertEquals(20, listMock.size());
		assertEquals(20, listMock.size());
	}

	@Test
	public void testGet_SpecificParameter() {
		List listMock = mock(List.class);
		when(listMock.get(0)).thenReturn("SomeString");
		assertEquals("SomeString", listMock.get(0));
		assertEquals(null, listMock.get(1));
	}

	@Test
	public void testGet_GenericParameter() {
		List listMock = mock(List.class);
		when(listMock.get(Mockito.anyInt())).thenReturn("SomeString");
		assertEquals("SomeString", listMock.get(0));
		assertEquals("SomeString", listMock.get(1));
	}
}

/src/test/java/com/in28minutes/mockito/mockitodemo/MockitoDemoApplicationTests.java

package com.in28minutes.mockito.mockitodemo;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;

@RunWith(SpringRunner.class)
@SpringBootTest
public class MockitoDemoApplicationTests {

	@Test
	public void contextLoads() {
	}

}

/src/test/java/com/in28minutes/mockito/mockitodemo/SomeBusinessMockAnnotationsTest.java

package com.in28minutes.mockito.mockitodemo;

import static org.junit.Assert.assertEquals;
import static org.mockito.Mockito.when;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.mockito.junit.MockitoJUnitRunner;

@RunWith(MockitoJUnitRunner.class)
public class SomeBusinessMockAnnotationsTest {

	@Mock
	DataService dataServiceMock;

	@InjectMocks
	SomeBusinessImpl businessImpl;

	@Test
	public void testFindTheGreatestFromAllData() {
		when(dataServiceMock.retrieveAllData()).thenReturn(new int[] { 24, 15, 3 });
		assertEquals(24, businessImpl.findTheGreatestFromAllData());
	}

	@Test
	public void testFindTheGreatestFromAllData_ForOneValue() {
		when(dataServiceMock.retrieveAllData()).thenReturn(new int[] { 15 });
		assertEquals(15, businessImpl.findTheGreatestFromAllData());
	}

	@Test
	public void testFindTheGreatestFromAllData_NoValues() {
		when(dataServiceMock.retrieveAllData()).thenReturn(new int[] {});
		assertEquals(Integer.MIN_VALUE, businessImpl.findTheGreatestFromAllData());
	}
}

/src/test/java/com/in28minutes/mockito/mockitodemo/SomeBusinessMockTest.java

package com.in28minutes.mockito.mockitodemo;

import static org.junit.Assert.assertEquals;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.when;

import org.junit.Test;

public class SomeBusinessMockTest {

	@Test
	public void testFindTheGreatestFromAllData() {
		DataService dataServiceMock = mock(DataService.class);
		when(dataServiceMock.retrieveAllData()).thenReturn(new int[] { 24, 15, 3 });
		SomeBusinessImpl businessImpl = new SomeBusinessImpl(dataServiceMock);
		int result = businessImpl.findTheGreatestFromAllData();
		assertEquals(24, result);
	}

	@Test
	public void testFindTheGreatestFromAllData_ForOneValue() {
		DataService dataServiceMock = mock(DataService.class);
		when(dataServiceMock.retrieveAllData()).thenReturn(new int[] { 15 });
		SomeBusinessImpl businessImpl = new SomeBusinessImpl(dataServiceMock);
		int result = businessImpl.findTheGreatestFromAllData();
		assertEquals(15, result);
	}

}

/src/test/java/com/in28minutes/mockito/mockitodemo/SomeBusinessStubTest.java

package com.in28minutes.mockito.mockitodemo;

import static org.junit.Assert.assertEquals;

import org.junit.Test;

public class SomeBusinessStubTest {
	@Test
	public void testFindTheGreatestFromAllData() {
		SomeBusinessImpl businessImpl = new SomeBusinessImpl(new DataServiceStub());
		int result = businessImpl.findTheGreatestFromAllData();
		assertEquals(24, result);

	}

}

class DataServiceStub implements DataService {
	@Override
	public int[] retrieveAllData() {
		return new int[] { 24, 6, 15 };
	}
}

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