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Behavior-Driven Development testing for Java using lambdas. Inspired by RSpec and Machine.Specifications.

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Why create another testing framework for Java, and why use lambdas?

JavaSpec attempts to be:

  • Concise: Simple behavior should be simple to test and describe in a small amount of space.
  • Searchable: Finding call sites in Java code is easy. Finding where a test calls your code should be just as easy.
  • Transparent: You shouldn't have to keep any caveats in mind when writing test code.

There are many testing libraries out there with some of these characteristics, but expresiveness does not need to come at the cost of adding complexity. For example you can write your tests in Ruby or Groovy (as the author once considered), but now you're adding more components between your test and production code, adding new dependencies, and losing out on searchability.

Lambdas are the weapon of choice for turning simple expressions into one-liners. A test with one assertion can be 1 line instead of several for tagging and creating whole, new test method.


JavaSpec is located in the Maven Central Repository, under the following coordinates:


It depends upon JUnit 4 and Java 8+.

Getting started

There's no magic in how JavaSpec works. This guide describes JavaSpec in terms of its similarities to popular libraries instead of pretending like these are radical, never-before-seen ideas.

It runs on JUnit

In JUnit, you create a test class and put @Test methods in it. JavaSpec is similar:

  • Make a test class.
  • Tag it with @RunWith(JavaSpecRunner.class).
  • Include 1 or more It fields in the class and assign a no-arg lambda to it. Put whatever code you would normally run in the @Test method in this lambda.
  • Run your tests anywhere you run JUnit. Maven (surefire plugin) and Eclipse Luna work.

A simple "Hello World" test looks like this:

class GreeterTest {
  It says_hello = () -> assertEquals("Hello World!", new Greeter().sayHello());

As with JUnit, you get 1 instance of your test class per test. Each It is its own test.

Finally, note that the It field is named says_hello instead of the conventional saysHello. This is done so that JavaSpec can convert that verb phrase into a human readable form by replacing underscores with spaces. When you run this test, JUnit will report results for says hello.

It's like Machine.Specifications

Machine.Specifications and JavaSpec represent the different steps of a test the same way:

  • An Establish lambda runs the Arrange part of your test. This runs first, when present.
  • A Because lambda runs the Act part of your test. This runs next, when present.
  • An It lambda does the Assert part of your test.
  • A Cleanup lambda - when present - always runs, even if a prior step failed.
  • If any step throws an exception, the test fails.

You can think of Establish and Because as what a @Before method would do in JUnit. These lambdas run before each It lambda in the same class (and also before It fields in inner classes). Cleanup is like @After in JUnit, running after each It in the same test class.

Unlike MSpec, your lambdas execute in an instance of the class in which they are declared. Non-static helper methods in your test class are fair game to be called from any step.

A JavaSpec test fixture looks like this:

class GreeterWithFixtureTest {
  private final PrintStreamSpy printStreamSpy = new PrintStreamSpy();
  private Widget subject;
  private String returned;

  Establish that = () -> subject = new Widget(printStreamSpy);
  Because of = () -> returned =;
  Cleanup close_streams = () -> {
    if(subject != null)

  It returns_bar = () -> assertEquals("bar", returned);
  It prints_baz = () -> assertEquals("baz", printStreamSpy.getWhatWasPrinted());

It's like RSpec

RSpec lets you organize hierarchies of tests and fixtures with describe and context, and each level in the tree can have its own before and after methods to work the test fixture. JavaSpec provides nested contexts by nesting context classes (inner, non-static classes) in the top-level test class.

Each class can have as many It lambdas as you want, plus up to 1 of each type of fixture lambda (Establish, Because, and Cleanup) to build up the test fixture. As with RSpec, setup runs outside-in and cleanup runs inside-out. If you happen to have an Because in an outer class and an Establish in an inner class and wonder which one runs first, the outer class lambdas run first (i.e. Because runs first).

An example of using nested contexts:

class WidgetTest {
  private Widget subject;
  Establish that = () -> subject = new Widget();

  class foo {
    private String returned;
    Because of = () -> returned = subject.sayHello();
    It says_hello = () -> assertEquals("Hello World!", returned);
  class bar {
    class given_a_multiple_of_3 {
      It prints_fizz = () -> assertEquals("fizz",;

    class given_a_multiple_of_5 {
      It prints_buzz = () -> assertEquals("buzz",;

In short:

  • Only tag the outer-most class with @RunWith(JavaSpecRunner.class). Don't tag any inner classes with this.
  • Make as many contexts as you like by making nested, non-static classes.
  • Add up to 1 each of Establish, Because and Cleanup to each context class.
  • Make as many tests as you want in each context class with It lambdas.

If you have any other questions

Hopefully JavaSpec works like you think it does.

For times when it doesn't, start by looking at the tests on JavaSpecRunner and related classes.

If that still doesn't do the trick, feel free to post an issue or submit a pull request with any suggested modifications.

Future work

Work is underway to make a new 2.0 release with an entirely different syntax that looks more like Mocha or Jasmine than like .NET's Machine.Specifications.

Release history

  • 1.0.1: Fixed Issue 5, catching some errors in initializing test classes.
  • 1.0: Full release. Renamed artifact to info.javaspec::javaspec-runner.
  • 0.5: Fixed an issue where specs with the same field / context class name were showing up as still running in IntelliJ. Also renamed JUnit test display names to human-readable names, replacing snake case underscores with spaces.
  • 0.4.2: Fixed Issue 2, so that only one instance of a context class is created for each test.
  • 0.4.1: Fixed Issue 1, dealing with being able to instantiate non-public context classes.
  • 0.4.0: Initial release
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