feat: Migrate all Java unit tests from JUnit 4 to JUnit Jupiter

[He-Pin]

Motivation

Migrate all Java unit tests from JUnit 4 patterns (@ClassRule, @Rule, @RunWith, ExternalResource, extends JUnitSuite) to JUnit 5 Jupiter patterns (@RegisterExtension, @ExtendWith). This improves project maintainability and aligns with the modern testing ecosystem.

Additionally, rename all JUnit5* naming to JUnitJupiter* in test files, since "JUnit Jupiter" is the API name — not tied to version 5. With jupiter-engine already at version 6.0.3, using JUnit5 in class names would be misleading.

This addresses reviewer feedback from @pjfanning:

"Why is this called Junit5 in this new code? v6 is out. Can't we call the new classes JUnitJupiter or something like that?"

Modification

New Files

• PekkoJUnitJupiterActorSystemResource.java — JUnit Jupiter extension replacing JUnit 4 Rule-based PekkoJUnitActorSystemResource. Implements BeforeAllCallback, AfterAllCallback, BeforeEachCallback, AfterEachCallback, ParameterResolver. Thread-safe via synchronized methods with a classLevel flag distinguishing static (class-level) from instance (per-test) lifecycle.
• StreamTestJupiter.java — Base class for stream tests, replacing StreamTest (JUnit 4 version). Delegates to PekkoJUnitJupiterActorSystemResource.
• ActorTestKitJUnitJupiterTest.java — Renamed from ActorTestKitJUnit5Test.

Migration Patterns Applied (206 files)

Old (JUnit 4)	New (JUnit Jupiter)
@ClassRule public static PekkoJUnitActorSystemResource	@RegisterExtension static PekkoJUnitJupiterActorSystemResource
@Rule public PekkoJUnitActorSystemResource	@RegisterExtension PekkoJUnitJupiterActorSystemResource
extends JUnitSuite	(removed — not needed with Jupiter)
@RunWith(JUnitRunner.class)	(removed)
@Test(expected = X.class)	assertThrows(X.class, () -> {...})
Assert.assertEquals(msg, expected, actual)	assertEquals(expected, actual, msg)
Assert.assertTrue(msg, condition)	assertTrue(condition, msg)
extends StreamTest	extends StreamTestJupiter
@ExtendWith(TestKitJUnit5Extension.class)	@ExtendWith(TestKitJUnitJupiterExtension.class)
@JUnit5TestKit	@JUnitJupiterTestKit
JUnit5TestKitBuilder	JUnitJupiterTestKitBuilder

Backward Compatibility

• Old JUnit5TestKitBuilder, TestKitJUnit5Extension, JUnit5TestKit annotation in src/main are preserved (not deleted or deprecated — deprecation is a separate concern per @pjfanning).
• Old PekkoJUnitActorSystemResource and StreamTest preserved for users who still use JUnit 4.
• JUnit5TestKitBuilderSpec.scala preserved as it tests the old builder.

Dependencies

• junit-jupiter-engine: 6.0.3
• junit-platform-launcher: 6.0.3
• sbt-jupiter-interface: 0.17.0
• junit-interface: 0.11 (for JUnit 4 vintage compatibility)

Result

• ✅ All 206 test files migrated
• ✅ Zero remaining JUnit 4 annotations/imports in test files
• ✅ Zero printStackTrace calls
• ✅ Binary compatibility maintained
• ✅ Local compilation passes (all 11+ modules)
• ✅ Local tests pass (actor-testkit-typed: 171, actor-tests: 63, stream-tests: 215, docs: 46, coordination: 1, persistence-typed: 3)
• ✅ Cross-reviewed by 16 AI agents across 5 models (GPT 5.4, Gemini 3 Pro, Claude Opus 4.6, GPT 5.1, PR Reviewer)

References

• PR review comment: feat: Migrate all Java unit tests from JUnit 4 to JUnit Jupiter #2724 (comment)
• JUnit Jupiter documentation: https://junit.org/junit5/docs/current/user-guide/
• JUnit 6.0.3 release: JUnit Jupiter engine is now at version 6.x, making JUnit5 naming obsolete

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This code is Apache licensed under the Apache License, Version 2.0.

[He-Pin]

The javafmt can't run on Java 25:( , in JDK 25 — com.sun.tools.javac.util.Log$DeferredDiagnosticHandler.getDiagnostics() is gone

[He-Pin]

We should mark JUnit 4 support as deprecated, and mark it since 2.0.0.

[pjfanning]

Why is this called Junit5 in this new code? v6 is out.
Can't we call the new classes JUnitJupiter or something like that?
Junit5 seems like a bad name now.

We already have https://github.com/apache/pekko/blob/main/actor-testkit-typed/src/main/scala/org/apache/pekko/actor/testkit/typed/javadsl/JUnitJupiterTestKitBuilder.scala

I'm thinking about the new classes in this PR

• StreamTest5.java‎ could be renamed StreamTestJupiter.java
• PekkoJUnit5ActorSystemResource.java‎ could be renamed PekkoJUnitJupiterActorSystemResource.java
• Everywhere in your new code where you use TestKitJUnit5Extension, you could use TestKitJUnitJupiterExtension instead (the latter class already exists)

[He-Pin]

I will rebase on your current style:)