How to Write Test Cases in Java Application using Mockito and Junit?

Last Updated : 09 Mar, 2023

Mockito is an open-source testing framework used for unit testing of Java applications. It plays a vital role in developing testable applications. Mockito is used to mock interfaces so that a dummy functionality can be added to a mock interface that can be used in Unit Testing. Unit Testing is a type of software testing in which individual components of the software are tested. The major objective of using the Mockito framework is to simplify the development of a test by mocking external dependencies and using them in the test code. And as a result, Mockito provides a simpler test code that is easier to understand, more readable, and modifiable. Mockito can also be used with other testing frameworks like JUnit and TestNG.

JUnit framework is a Java framework that is also used for testing. Now, JUnit is used as a standard when there is a need to perform testing in Java. So in this article, we will be discussing test cases in a java application using Mockito and Junit.

Step by Step Implementation

Step 1: Create a Maven project in your favorite Java IDE (IHere we are using IntelliJ IDEA)

Step 2: When you have successfully created a maven project you have to add some dependencies in your pom.xml file. We have to add the following dependency in your pom.xml file.

Dependency for Mockito is as follows:

XML

<dependency>
    <groupId>org.mockito</groupId>
    <artifactId>mockito-all</artifactId>
    <version>2.0.2-beta</version>
</dependency>

Dependency for Junit is as follows:

XML

<dependency>
    <groupId>junit</groupId>
    <artifactId>junit</artifactId>
    <version>4.8.2</version>
</dependency>

Implementation: Below is the complete code for the pom.xml file

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>org.example</groupId>
    <artifactId>mockito-demo</artifactId>
    <version>1.0-SNAPSHOT</version>

    <dependencies>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.8.2</version>
        </dependency>
        <dependency>
            <groupId>org.mockito</groupId>
            <artifactId>mockito-all</artifactId>
            <version>2.0.2-beta</version>
        </dependency>
    </dependencies>

    <properties>
        <maven.compiler.source>11</maven.compiler.source>
        <maven.compiler.target>11</maven.compiler.target>
    </properties>

</project>

Step 3: Now you have to create one interface named as TodoService and one class named as TodoServiceImpl.

Java

// Java Program to Illustrate TodoService File

// Importing List class
import java.util.List;

// Interface
public interface TodoService {

    // Creating a simple method retrieveTodos()
    public List<String> retrieveTodos(String user);
}

Java

// Java Program to Illustrate TodoServiceImpl File

// Importing required classes
import java.util.ArrayList;
import java.util.List;

// Main class
public class TodoServiceImpl {

    // Creating a reference of
    // TodoService interface
    private TodoService todoService;

    // Constructor
    public TodoServiceImpl(TodoService todoService)
    {

        // This keyword refers to same instance itself
        this.todoService = todoService;
    }

    // Method
    // Filtering the string
    public List<String>
    retrieveTodosRelatedToJava(String user)
    {

        List<String> filteredTodos
            = new ArrayList<String>();
        List<String> todos
            = todoService.retrieveTodos(user);

        for (String todo : todos) {

            // Filtering the string that contains "Java"
            // keyword
            if (todo.contains("Java")) {
                filteredTodos.add(todo);
            }
        }
        return filteredTodos;
    }
}

Step 4: Now we are going to perform unit testing for the retrieveTodosRelatedToJava() method that is present inside the TodoServiceImpl.java file. To create the test class follow these steps. At first Right-click inside the TodoServiceImpl.java file.

Then click on the Generate button.

Then click on the Test button.

A pop-up window will be shown like this. Here you can modify your test class name. Also, check the setUp and the method that you want to perform unit testing.

Example:

Java

// Java Program to Illustrate TodoServiceImplMockTest File

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

// Importing required classes
import java.util.Arrays;
import java.util.List;
import org.junit.Before;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.Mock;
import org.mockito.runners.MockitoJUnitRunner;

// MockitoJUnitRunner gives automatic validation
// of framework usage, as well as an automatic initMocks()
@RunWith(MockitoJUnitRunner.class)

// Main class
public class TodoServiceImplMockTest {

    TodoServiceImpl todoBusiness;

    // The Mockito.mock() method allows us to
    // create a mock object of a class or an interface
    @Mock TodoService todoServiceMock;

    // Methods annotated with the @Before
    // annotation are run before each test
    @Before public void setUp()
    {
        todoBusiness = new TodoServiceImpl(todoServiceMock);
    }

    // @Test
    // Tells the JUnit that the public void method
    // in which it is used can run as a test case
    @Test
    public void testRetrieveTodosRelatedToSpring_usingMock()
    {
        List<String> todos
            = Arrays.asList("Learn Spring", "Learn Java",
                            "Learn Spring Boot");
        when(todoServiceMock.retrieveTodos("User"))
            .thenReturn(todos);

        List<String> filteredTodos
            = todoBusiness.retrieveTodosRelatedToJava(
                "User");
        assertEquals(1, filteredTodos.size());
    }

    @Test
    public void
    testRetrieveTodosRelatedToSpring_withEmptyList_usingMock()
    {
        List<String> todos = Arrays.asList();
        when(todoServiceMock.retrieveTodos("Dummy"))
            .thenReturn(todos);

        List<String> filteredTodos
            = todoBusiness.retrieveTodosRelatedToJava(
                "Dummy");
        assertEquals(0, filteredTodos.size());
    }
}

Now you have successfully written two test cases for your java application. Now to run the test cases click on the green button as shown in the below image. And you can see your test cases have been passed.