Write Unit Tests Faster with Instancio

When working on tests, we often need to create test data, which is one of the most tedious aspects of programming. Recently, I started using the Instancio library, which simplifies the process of creating objects with dummy data for unit tests.

In this article, let’s explore the power of the Instancio library (https://www.instancio.org/) and learn how to use it effectively in our tests.

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Installation

If you are using Maven and JUnit 5, simply add the following to your pom.xml file:

<dependency>
    <groupId>org.instancio</groupId>
    <artifactId>instancio-junit</artifactId>
    <version>5.2.1</version>
    <scope>test</scope>
</dependency>

In all examples we are going to use the following class:

@Data
public class Person {
  private String firstName;
  private String middleName;
  private String lastName;

  private LocalDate birthDate;

  private String addressLine1;
  private String addressLine2;
  private String city;
  private String state;
  private String zipCode;
}

Mocking Objects

Let’s explore how to test a PersonService class that validates whether the fields of a Person object are filled or not, first without using Instancio and then with Instancio.

// Without Instancio

@Test
void testPersonFields(){
  // Mock Person Object
  Person person = new Person();
  person.setFirstName("RandomString1");
  person.setMiddleName("RandomString2");
  person.setLastName("RandomString3");
  person.setBirthDate(LocalDate.of(1985, 2, 3));
  person.setAddressLine1("RandomString4");
  ... // And here we add all person fields

  boolean isOk = personService.isValidPerson(person);

  Assertions.assertTrue(isOk);
}

// With Instancio

@Test
void testPersonFields(){
  // Mock Person Object
  Person person = Instancio.create(Person.class);

  boolean isOk = personService.isValidPerson(person);

  Assertions.assertTrue(isOk);
}

Instancio can automatically identify all the fields of a Person object and generate random values for them. For example: Person(firstName=PGWNCHLPX, middleName=MIFNHR, lastName=IMVAD, birthDate=2022-02-02, addressLine1=CRVR, addressLine2=NZXDTPZ, city=APWN, state=CHRJZ, zipCode=QDW)

Instancio also allows us to customize specific fields in the following ways:

• Ignoring fields: All fields of the Person object will have random values, except those explicitly set to be ignored.
• Setting default values for fields: Specified fields will have default values, while all other fields will retain random values.
• Customizing random value generation rules: Specified fields will use custom rules to generate their random values.

@Test
void testPersonIgnoringMiddleName() {
  Person person = Instancio.of(Person.class)
                           .ignore(Select.field(Person::getMiddleName))
                           .create();

  boolean isOk = personService.isValidPerson(person);

  Assertions.assertTrue(isOk);
}

@Test
void testPersonSettingBirthDate() {
 Person person =
      Instancio.of(Person.class)
          .set(Select.field(Person::getBirthDate), LocalDate.of(1985, 2, 3))
          .create();

  boolean isOk = personService.isValidPerson(person);

  Assertions.assertTrue(isOk);
}

@Test
void testPersonGeneratingZipCode() {
  Person person =
      Instancio.of(Person.class)
          .generate(
              Select.field(Person::getZipCode),
              generators -> generators.string().digits().length(5))
          .create();

  boolean isOk = personService.isValidPerson(person);

  Assertions.assertTrue(isOk);
}

Creating Lists

Instancio can also generate lists with randomized elements, as shown below:

• The first example generates a list of Person objects using the default configuration, where the number of elements varies.
• The second example generates a list with exactly 10 elements.
• The third example generates a list of 10 elements, setting ‘London’ as the city for all of them.

@Test
void testPersonList() {
  List<Person> personList = Instancio.createList(Person.class);

  Assertions.assertTrue(personList.size() > 0);
}

@Test
void testPersonListWithSizeEqualsTo10() {
  List<Person> personList = Instancio.ofList(Person.class).size(10).create();

  Assertions.assertEquals(10, personList.size());
}

 @Test
void testPersonListWithSizeEqualsTo10AndDefaultCity() {
  List<Person> personList =
      Instancio.ofList(Person.class)
          .size(10)
          .set(Select.field(Person::getCity), "London")
          .create();

  Assertions.assertEquals(10, personList.size());
}

Creating Maps

Creating maps is as straightforward as creating lists. Here are a few examples:

• Using the default configuration to generate a map with an integer as the key and Person as the value.
• Generating a similar map but with a custom generator defined for the zip code.

@Test
void testPersonMap() {
  Map<Integer, Person> personMap = 
      Instancio.createMap(Integer.class, Person.class);

  Assertions.assertFalse(personMap.isEmpty());
}

@Test
void testPersonMapWithCustomGenerator() {
  Map<Integer, Person> personMap =
      Instancio.ofMap(Integer.class, Person.class)
          .generate(
              Select.field(Person::getZipCode),
              generators -> generators.string().digits().length(5))
          .create();

  Assertions.assertEquals(5, personMap.size());
}

Creating Streams

If you need to test your code with an infinite stream, you can use Instancio:

@Test
void testPersonStream() {
  Stream<Person> personStream = Instancio.stream(Person.class);

  Assertions.assertEquals(100, personStream.limit(100).count());
}