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What is it?

Instancio is a Java library that automatically creates and populates objects for your unit tests.

Instead of manually setting up test data:

Address address  = new Address();
address.setStreet("street");
address.setCity("city");
//...

Person person = new Person();
person.setFirstName("first-name");
person.setLastName("last-name");
person.setAge(22);
person.setGender(Gender.MALE);
person.setAddress(address);
//...

You can simply do the following:

Person person = Instancio.create(Person.class);

This one-liner returns a fully-populated person, including nested objects and collections.

The object is populated with random data that can be reproduced in case of test failure.

What else can Instancio do?

1. Create collections of objects:

List<Person> persons = Instancio.ofList(Person.class).size(10).create();

2. Create streams of objects:

Stream<Person> persons = Instancio.stream(Person.class).limit(5);

3. Create generic types:

Pair<List<Foo>, List<Bar>> pairOfLists = Instancio.create(new TypeToken<Pair<List<Foo>, List<Bar>>>() {});

4. Customise generated values:

Person person = Instancio.of(Person.class)
    .generate(field(Person::getDateOfBirth), gen -> gen.temporal().localDate().past())
    .generate(field(Phone::getAreaCode), gen -> gen.oneOf("604", "778"))
    .generate(field(Phone::getNumber), gen -> gen.text().pattern("#d#d#d-#d#d-#d#d"))
    .subtype(all(AbstractAddress.class), AddressImpl.class)
    .supply(all(LocalDateTime.class), () -> LocalDateTime.now())
    .onComplete(all(Person.class), (Person p) -> p.setName(p.getGender() == Gender.MALE ? "John" : "Jane"))
    .create();

5. Create reusable templates (Models) of objects:

Model<Person> simpsons = Instancio.of(Person.class)
    .set(field(Person::getLastName), "Simpson")
    .set(field(Address::getCity), "Springfield")
    .generate(field(Person::getAge), gen -> gen.ints().range(40, 50))
    .toModel();

Person homer = Instancio.of(simpsons)
    .set(field(Person::getFirstName), "Homer")
    .create();

Person marge = Instancio.of(simpsons)
    .set(field(Person::getFirstName), "Marge")
    .create();

6. Use data feeds for data defined in external sources (e.g. CSV or JSON):

List<Person> person = Instancio.ofList(Person.class)
    .size(10)
    .applyFeed(all(Person.class), feed -> feed.ofResource("persons.csv"))
    .create();

7. Define custom feeds for reuse across tests:

@Feed.Source(resource = "persons.csv")
interface PersonFeed extends Feed {

  @TemplateSpec("${firstName} ${lastName}")
  FeedSpec<String> fullName();

  @FunctionSpec(params = {"fullName", "dateOfBirth"}, provider = BioProvider.class)
  FeedSpec<String> bio();

  class BioProvider implements FunctionProvider {

    String describePerson(String fullName, LocalDate dateOfBirth) {
      int age = Period.between(dateOfBirth, LocalDate.now()).getYears();
      return String.format("%s is %d years old", fullName, age);
    }
  }
}

// Usage:
List<Person> person = Instancio.ofList(Person.class)
    .applyFeed(all(Person.class), feed -> feed.of(PersonFeed.class))
    .create();

8. Inject arguments into fields and parameters with JUnit 5:

@ExtendWith(InstancioExtension.class)
class ExampleTest {

    @Given
    private List<String> randomList;

    @Test
    void example1(@Given String randomSting, @Given(SomeCustomProvider.class) customString) {
        //...
    }

    @ValueSource(strings = {"foo", "bar"})
    @ParameterizedTest
    void example2(String fooOrBar, @Given long randomLong) {
        // ...
    }
}

9. Run parameterized tests against many samples of generated inputs:

@ExtendWith(InstancioExtension.class)
class ExampleTest {

    @InstancioSource(samples = 1000)
    @ParameterizedTest
    void example(UUID randomUuid, Foo randomFoo, @Given(BarProvider.class) Bar customBar) {
        // runs the test method 1000 times against generated inputs
    }
}

Main Features

• Fully reproducible data in case of test failures.
• Support for generics, record and sealed classes, Java 21 sequenced collections.
• Data generation based on JPA and Bean Validation annotations (Hibernate, jakarta, javax).
• Flexible configuration options and SPIs.
• Support for defining custom generators.
• InstancioExtension for Junit 5 @ExtendWith.
• Object population via fields and setters.

Getting Started

• User guide (instancio.org)

• Javadocs (javadoc.io)

• Quickstart (github.com) sample Maven project with usage examples

  git clone https://github.com/instancio/instancio-quickstart.git

Maven Coordinates

If you have JUnit 5 on the classpath, use the instancio-junit dependency.

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

To use Instancio with JUnit 4, TestNG, or standalone, use instancio-core:

<dependency>
    <groupId>org.instancio</groupId>
    <artifactId>instancio-core</artifactId>
    <version>LATEST</version>
    <scope>test</scope>
</dependency>

Feedback

Feedback and bug reports are greatly appreciated!

• Please submit an issue for bug reports and feature requests.
• For general feedback or questions, please create a post in the Discussions.

Please check the User Guide, previous issues and discussions before creating a new one.

Sponsors

If you like Instancio, please consider supporting its maintenance and development by becoming a sponsor.

A big thank you to the current project sponsors:

• @bit-dot
• @darthblonderss
• @mevlana14
• @tix320

Thanks to JetBrains and YourKit for supporting this project with their open source licenses.

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