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Mastering JUnit Test Cases with Argument Captors: A Step-by-Step Tutorial

April 3, 2023 / Last updated : April 3, 2023 Drona JUnit

Mastering JUnit Test Cases with Argument Captors: A Step-by-Step Tutorial

If you’re working with JUnit tests, you’ve likely encountered situations where you need to test a method that has complex input parameters. In these cases, you may find it helpful to use an Argument Captor to capture the values of the input parameters for further analysis. In this tutorial, we’ll explore the basics of JUnit Argument Captors and how to use them effectively.

What is a JUnit Argument Captor?

A JUnit Argument Captor is a tool that allows you to capture the arguments passed to a mocked object during a test. It’s particularly useful when you need to test a method that has complex input parameters, such as an object or a list. By using an Argument Captor, you can capture the input parameters passed to the method and verify that they meet your expectations.

Example

Let’s consider the following example: we have a UserService class that has a createUser method, which takes a User object as a parameter and returns a User object with an assigned ID. Our goal is to test this method using JUnit and an Argument Captor.



public class UserService {
    public User createUser(User user) {
        // some logic here to create user and assign ID
        return user;
    }
}

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public class UserService {

public User createUser(User user) {

// some logic here to create user and assign ID

return user;

}

Our test class will look something like this:



import org.junit.Before;
import org.junit.Test;
import org.mockito.ArgumentCaptor;
import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.mockito.MockitoAnnotations;
import static org.junit.Assert.assertEquals;
import static org.mockito.Mockito.verify;

public class UserServiceTest {

    @Mock
    private UserRepository userRepository;

    @InjectMocks
    private UserService userService;

    @Before
    public void setup() {
        MockitoAnnotations.initMocks(this);
    }

    @Test
    public void createUserTest() {
        ArgumentCaptor<User> userCaptor = ArgumentCaptor.forClass(User.class);

        User user = new User("John", "Doe", "johndoe@example.com");

        userService.createUser(user);

        verify(userRepository).save(userCaptor.capture());

        assertEquals(userCaptor.getValue().getFirstName(), user.getFirstName());
        assertEquals(userCaptor.getValue().getLastName(), user.getLastName());
        assertEquals(userCaptor.getValue().getEmail(), user.getEmail());
    }
}

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import org.junit.Before;

import org.junit.Test;

import org.mockito.ArgumentCaptor;

import org.mockito.InjectMocks;

import org.mockito.Mock;

import org.mockito.MockitoAnnotations;

import static org.junit.Assert.assertEquals;

import static org.mockito.Mockito.verify;

public class UserServiceTest {

@Mock

private UserRepository userRepository;

@InjectMocks

private UserService userService;

@Before

public void setup() {

MockitoAnnotations.initMocks(this);

}

@Test

public void createUserTest() {

ArgumentCaptor<User> userCaptor = ArgumentCaptor.forClass(User.class);

User user = new User("John", "Doe", "johndoe@example.com");

userService.createUser(user);

verify(userRepository).save(userCaptor.capture());

assertEquals(userCaptor.getValue().getFirstName(), user.getFirstName());

assertEquals(userCaptor.getValue().getLastName(), user.getLastName());

assertEquals(userCaptor.getValue().getEmail(), user.getEmail());

}

Let’s break down what’s happening in the test method:

We create an ArgumentCaptor object for the User class.
We create a new User object with some sample data.
We call the createUser method on the userService object, passing in the user object as a parameter.
We use the verify method from Mockito to verify that the userRepository.save method was called with the captured user object.
We use the assertEquals method to compare the properties of the captured User object to those of the original user object.

In this example, we’re using the Argument Captor to capture the User object that is passed to the userRepository.save method. We then compare the properties of this captured object to those of the original object to ensure that the method was called correctly.

Conclusion

JUnit Argument Captors are a powerful tool for testing methods with complex input parameters. By capturing the input parameters passed to a method, you can verify that the method is behaving correctly and meeting your expectations. By following the steps outlined in this tutorial, you should now have a solid understanding of how to use JUnit Argument Captors in your own test cases.