JUnit 5 has finally arrived! It introduces a completely new extension model that allows to customize almost every aspect of test execution. For example, it lets you define custom conditions to decide whether a test should be executed or skipped. Test lifecycle callbacks allow to encapsulate common setup/teardown code in an extension. An extension can pass values or inject dependencies to a test by post-processing test instances, or resolving test method parameters. Moreover, you can even write an extension that specifies how a test method becomes a template for multiple tests and how to invoke those, e.g. multiple times with different parameters or a different setup. In this talk, we will go on an example-driven tour of the new extension API using real-world testing scenarios. We will learn about the utilities an extension can use, e.g. how it should store state, so you will be ready to write your own extensions after this session. The talk will be interesting for experienced Java developers with prior knowledge of JUnit (at least JUnit 4).

1. 5
JUNIT 5 EXTENSIONS
MARC PHILIPP

2. 5
MARC PHILIPP
So ware Engineer at
JUnit Maintainer since 2012
Twi er:
Web:
@marcphilipp
marcphilipp.de

3. 5
JUNIT 5 IS HERE!
5.0
September 10, 2017
5.1
February 18, 2018
5.2
April 29, 2018

4. 5
✋
SHOW OF HANDS

5. 5
JUNIT5
P L AT F O R M
Another Test
P A R T Y
T H I R D
J U P I T E RV I N TA G E

6. 5
JUNIT JUPITER
P L AT F O R M
JUPITER

7. 5
JUNIT JUPITER API
programming model for test authors
extension model for extension authors

8. 5
PROGRAMMING MODEL
import org.junit.jupiter.api.*;
import static org.junit.jupiter.api.Assertions.*;
class SimpleTest {
@Test
@DisplayName("1 + 1 = 2")
void onePlusOneEqualsTwo() {
assertEquals(2, 1 + 1);
}
}

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EXTENSION MODEL
import org.junit.jupiter.api.extension.*;
class MyCustomExtension
implements BeforeEachCallback, AfterEachCallback {
@Override
public void beforeEach(ExtensionContext context) {
// setup
}
@Override
public void afterEach(ExtensionContext context) {
// teardown
}
}

10. 5
EXTENSION REGISTRATION
Declara ve: @ExtendWith on classes or methods
Programma c: @RegisterExtension on fields
Global: via ServiceLoader
You can register as many extensions as you need
simultaneously

11. 5
PROGRAMMING MODEL MEETS
EXTENSION MODEL
import org.junit.jupiter.api.*;
import org.junit.jupiter.api.extension.*;
import static org.junit.jupiter.api.Assertions.*;
@ExtendWith(MyCustomExtension.class)
class SimpleTest {
@RegisterExtension Extension myExtension = new AnotherExtension(42);
@Test
@ExtendWith({FooExtension.class, BarExtension.class})
void onePlusOneEqualsTwo() {
assertEquals(2, 1 + 1);
}
}

12. 5
CONDITIONAL TEST
EXECUTION

13. 5
DEMO
h ps://github.com/marcphilipp/junit5‑extensions‑demo

14. 5
EXTENSION CONTEXT
Represents the current node in the test tree, e.g. test
method or class
Provides access to meta informa on about such a node,
e.g. display name, method, class

15. 5
EXTENSION CONTEXT
root : ExtensionContext
displayName = "JUnit Jupiter"
parent
class1 : ExtensionContext
displayName = "TestClass1"
parent
test1 : ExtensionContext
displayName = "test1()"
parent
test2 : ExtensionContext
displayName = "test2()"
parent
class2 : ExtensionContext
displayName = "TestClass2"
parent
test3 : ExtensionContext
displayName = "test3()"

16. 5
LESSONS LEARNED
Using custom logic to determine whether a test
class/method should be skipped
Registering an extension globally
Deac va ng a condi on without changing the code

17. 5
REUSABLE TEST
SETUP & TEARDOWN

18. 5
DEMO
h ps://github.com/marcphilipp/junit5‑extensions‑demo

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WHY THE STORE ABSTRACTION?
An extension needs to save and retrieve data, e.g. to clean
up in the end
Extensions are instan ated once and called for mul ple
tests
Using instance variables would be error‑prone
extensionContext
.getStore(Namespace.create(...))
.getOrComputeIfAbsent("key", key -> ...)

20. 5
STORE
Map ‑like interface for extensions to save and retrieve
data, e.g. store.put(key, value)
Accessed via a Namespace : Enables sharing data across
extensions, but makes it a deliberate decision (e.g.
Namespace.GLOBAL )
Reading from a Store follows the hierarchy upwards, if a
key is not found

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STORE
root : ExtensionContext
displayName = "JUnit Jupiter"
parent
class1 : ExtensionContext
displayName = "TestClass1"
parent
store
test1 : ExtensionContext
displayName = "test1()"
a : Store
+ get(Object, Class<T>): T, ...
parent
b : Store
+ get(Object, Class<T>): T, ...
parent
c : Store
+ get(Object, Class<T>): T, ...
Namespace
store
Namespace
store
Namespace

22. 5
AUTOMATIC CLEAN‑UP
CloseableResource instances in a Store are automa cally
closed when the scope of the corresponding
ExtensionContext ends.
class DockerClientResource implements CloseableResource {
private final DockerClient dockerClient;
DockerClientResource() {
var config = DefaultDockerClientConfig.createDefaultConfigBuilder
dockerClient = DockerClientBuilder.getInstance(config).build();
}
DockerClient get() {
return dockerClient;
}
@Override public void close() throws Throwable {
dockerClient.close();
}
}

23. 5
LESSONS LEARNED
Using the Store class for extension state
Using Lifecycle Callbacks to enable reuse of common
setup/teardown code
Implemen ng mul ple Extension interfaces in a single
extension

24. 5
RESOLVING TEST
PARAMETERS

25. 5
DEMO
h ps://github.com/marcphilipp/junit5‑extensions‑demo

26. 5
LESSONS LEARNED
How to resolve test method parameters in an
Extension ?
You can also inject parameters into test class constructors
and @BeforeEach / @AfterEach methods etc.

27. 5
PROVIDING
ARGUMENTS FOR
PARAMETERIZED
TESTS

28. 5
DEMO
h ps://github.com/marcphilipp/junit5‑extensions‑demo

29. 5
LESSONS LEARNED
Using Parameterized Tests
Wri ng a custom ArgumentsProvider that loads data
from a JSON file, …

30. 5
FROM TESTS TO TEST
TEMPLATES

31. 5
DEMO
h ps://github.com/marcphilipp/junit5‑extensions‑demo

32. 5
LESSONS LEARNED
Execu ng a test mul ple mes with different contexts
Implemen ng a TestInvocationContextProvider

33. 5
SUPPORT CLASSES
Package org.junit.platform.commons.support contains:
AnnotationSupport to scan for custom annota ons,
including meta‑annota ons
ReflectionSupport for classpath scanning, finding
methods, invoking them etc.

34. 5
SUMMARY

35. 5
EXTENSION POINTS
Lifecycle: BeforeAllCallback , BeforeEachCallback ✅,
BeforeTestExecutionCallback ,
TestExecutionExceptionHandler ,
AfterTestExecutionCallback , AfterEachCallback ✅,
AfterAllCallback
Other: ExecutionCondition ✅,
TestInstancePostProcessor , ParameterResolver ✅,
TestTemplateInvocationContextProvider ✅

36. 5
JUNIT JUPITER IS EXTENSIBLE
A lot of extension points to choose from
The JUnit team will add more in future releases
Combining mul ple extension points in one extension is
very powerful!

37. 5
THIRD‑PARTY EXTENSIONS
Spring, Mockito, Docker, Wiremock, JPA,
Selenium/WebDriver, DbUnit, Ka a, Jersey, GreenMail,
S3Mock, Citrus Framework, XWiki, …
h ps://github.com/junit‑team/junit5/wiki/Third‑party‑
Extensions

38. 5
GETTING STARTED
User Guide:
Sample projects for Gradle, Maven, and Ant:
Javadoc:
h p://junit.org/junit5/docs/current/user‑guide/
h ps://github.com/junit‑team/junit5‑samples
h p://junit.org/junit5/docs/current/api/

39. 5
WANTED: FEEDBACK!
StackOverflow:
Code & Issues:
Twi er:
h p://stackoverflow.com/ques ons/tagged/junit5
h ps://github.com/junit‑team/junit5/
h ps://twi er.com/juni eam

40. 5
EXAMPLE CODE
h ps://github.com/marcphilipp/junit5‑extensions‑demo

41. 5
QUESTIONS?
or @marcphilipp @juni eam

42. 5
THANKS!