Let Fixture Monkey generates fully-customizable, randomly populated instance. Focus on the properties of the class that really matter in your test.
Let Fixture Monkey generates fully-customizable, randomly populated instance. Focus on the properties of the class that really matter in your test.
Designed by SeongIn Hong
Fixture Monkey is a Java & Kotlin library designed to generate controllable arbitrary test objects. Its most distinctive feature is the ability to freely access and configure any nested fields through path-based expressions.
It focuses on simplifying test writing by facilitating the generation of necessary test fixtures. Make your JVM tests more concise and safe with Fixture Monkey.
Fixture Monkey ships a Claude Code plugin that teaches the agent to write tests the way the library intends: enumerate the cases a method can produce, keep the ones worth testing, and pin only the properties that force the expected outcome — leaving everything else random, so the test stays out of unrelated diffs.
Install it with one line — paste it into Claude Code, or run it in a terminal:
claude plugin marketplace add naver/fixture-monkey --sparse .claude-plugin claude-plugins && claude plugin install fixture-monkey@fixture-monkeyThen just ask for tests — write tests for OrderService.calculate — and the skill activates on its own. /fixture-monkey:write-fixture invokes it explicitly.
The plugin only teaches the agent. Add the library itself to your build from Quick Start below.
Notes
--sparse limits the checkout to the plugin directories instead of cloning the whole library.naver/fixture-monkey shorthand clones over SSH by default. If Git SSH is not set up, prefix the command with CLAUDE_CODE_PLUGIN_PREFER_HTTPS=1.Using a different agent? The rules are published as plain documentation, so any tool can read them. Point your Cursor rule, AGENTS.md, or Copilot instructions file at:
https://naver.github.io/fixture-monkey/docs/agent-guide/writing-tests
https://naver.github.io/fixture-monkey/docs/agent-guide/api-referenceSee the AI Agents guide for the full procedure and the condensed Java/Kotlin API reference.
Add Fixture Monkey to your project:
// Java
testImplementation("com.navercorp.fixturemonkey:fixture-monkey-starter:1.2.3")
// Kotlin
testImplementation("com.navercorp.fixturemonkey:fixture-monkey-starter-kotlin:1.2.3")
com.navercorp.fixturemonkey
fixture-monkey-starter
1.2.3
test
com.navercorp.fixturemonkey
fixture-monkey-starter-kotlin
1.2.3
test
Create your first test object:
// Java
FixtureMonkey fixtureMonkey = FixtureMonkey.create();
Product product = fixtureMonkey.giveMeOne(Product.class);
// Kotlin
val fixtureMonkey = FixtureMonkey.create()
val product = fixtureMonkey.giveMeOne()…@Test
fun sampleOrder() {
val sut = FixtureMonkey.builder()
.plugin(KotlinPlugin())
.build()
val actual = sut.giveMeBuilder()
.setExp(Order::orderNo, "1")
.setExp(Order::productName, "Line Sally")
.minSizeExp(Order::items, 1)
.sample()
then(actual.orderNo).isEqualTo("1")
then(actual.productName).isEqualTo("Line Sally")
then(actual.items).hasSizeGreaterThanOrEqualTo(1)
}Note: Add "lombok.anyConstructor.addConstructorProperties=true" in lombok.config when using Lombok with Fixture Monkey
// Before: Manual object creation
Product product = new Product();
product.setId(1L);
product.setName("Test Product");
// ... many more setters
// After: With Fixture Monkey
Product product = fixtureMonkey.giveMeOne(Product.class);Stop writing boilerplate code for test object creation. Generate any test object with a single line of code.
// Set all product names to "Special Product" with a single expression
ArbitraryBuilder orderBuilder = fixtureMonkey.giveMeBuilder(Order.class)
.set("items[*].product.name", "Special Product");Bid farewell to endless getter/setter chains. Path expressions let you configure any nested field with a single line.
// Define once, reuse everywhere
ArbitraryBuilder productBuilder = fixtureMonkey.giveMeBuilder(Product.class)
.set("category", "Book")
.set("price", 1000);
// Reuse in different tests
Product product1 = productBuilder.sample();
Product product2 = productBuilder.size("reviews", 3).sample();Eliminate test code duplication by defining specifications once and reusing them across your test suite.
// Handles inheritance, circular references, and complex structures
Foo foo = FixtureMonkey.create().giveMeOne(Foo.class); // even with circular references
Bar bar = FixtureMonkey.create().giveMeOne(Bar.class); // even with inheritanceFrom simple POJOs to complex object graphs, Fixture Monkey handles all object structures.
// Each sample generates unique test data
Product sample1 = fixtureMonkey.giveMeBuilder(Product.class).sample();
Product sample2 = fixtureMonkey.giveMeBuilder(Product.class).sample();
assertThat(sample1).isNotEqualTo(sample2);Move beyond static test data to discover edge cases that static data might miss.
@Test
void testOrderProcessing() {
// Given
Order order = fixtureMonkey.giveMeBuilder(Order.class)
.set("items[*].quantity", 2)
.set("items[*].product.price", 1000)
.sample();
// When
OrderResult result = new OrderProcessor().process(order);
// Then
assertThat(result.getTotalAmount()).isEqualTo(4000); // 2 items * 2 quantity * 1000 price
assertThat(result.getStatus()).isEqualTo(OrderStatus.COMPLETED);
}Originally developed at Naver, Fixture Monkey has proven its reliability in handling complex business requirements at scale, supporting over 10,000 tests for South Korea's leading mobile payment service.
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