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brunoborges merged 2 commits into from
Feb 20, 2026
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Add JUnit 6 with JSpecify null safety pattern #65

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Copilot Feb 20, 2026
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Add JUnit 6 with JSpecify null safety pattern to tooling category
Copilot Feb 20, 2026
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2 changes: 1 addition & 1 deletion content/language/default-interface-methods.json
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Expand Up @@ -34,7 +34,7 @@
"state": "available",
"description": "Available since JDK 8 (March 2014)."
},
"prev": "tooling/aot-class-preloading",
"prev": "tooling/junit6-with-jspecify",
"next": "language/markdown-javadoc-comments",
"related": [
"language/private-interface-methods",
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2 changes: 1 addition & 1 deletion content/tooling/aot-class-preloading.json
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Expand Up @@ -35,7 +35,7 @@
"description": "Available as a standard feature in JDK 25 LTS (JEPs 514/515, Sept 2025)."
},
"prev": "tooling/built-in-http-server",
"next": "language/default-interface-methods",
"next": "tooling/junit6-with-jspecify",
"related": [
"tooling/jshell-prototyping",
"tooling/single-file-execution",
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58 changes: 58 additions & 0 deletions content/tooling/junit6-with-jspecify.json
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@@ -0,0 +1,58 @@
{
"id": 111,
"slug": "junit6-with-jspecify",
"title": "JUnit 6 with JSpecify null safety",
"category": "tooling",
"difficulty": "intermediate",
"jdkVersion": "17",
"oldLabel": "JUnit 5",
"modernLabel": "JUnit 6",
"oldApproach": "Unannotated API",
"modernApproach": "@NullMarked API",
"oldCode": "import org.junit.jupiter.api.Test;\nimport static org.junit.jupiter.api.Assertions.*;\n\nclass UserServiceTest {\n\n // JUnit 5: no null contracts on the API\n // Can assertEquals() accept null? Check source...\n // Does fail(String) allow null message? Unknown.\n\n @Test\n void findUser_found() {\n // Is result nullable? API doesn't say\n User result = service.findById(\"u1\");\n assertNotNull(result);\n assertEquals(\"Alice\", result.name());\n }\n\n @Test\n void findUser_notFound() {\n // Hope this returns null, not throws...\n assertNull(service.findById(\"missing\"));\n }\n}",
"modernCode": "import org.junit.jupiter.api.Test;\nimport org.jspecify.annotations.NullMarked;\nimport org.jspecify.annotations.Nullable;\nimport static org.junit.jupiter.api.Assertions.*;\n\n@NullMarked // all refs non-null unless @Nullable\nclass UserServiceTest {\n\n // JUnit 6 API is @NullMarked:\n // assertNull(@Nullable Object actual)\n // assertEquals(@Nullable Object, @Nullable Object)\n // fail(@Nullable String message)\n\n @Test\n void findUser_found() {\n // IDE warns: findById returns @Nullable User\n @Nullable User result = service.findById(\"u1\");\n assertNotNull(result); // narrows type to non-null\n assertEquals(\"Alice\", result.name()); // safe\n }\n\n @Test\n void findUser_notFound() {\n @Nullable User result = service.findById(\"missing\");\n assertNull(result); // IDE confirms null expectation\n }\n}",
"summary": "JUnit 6 adopts JSpecify @NullMarked, making null contracts explicit across its assertion API.",
"explanation": "JUnit 5 shipped without standardized nullability annotations, leaving developers to guess whether assertion parameters or return values could be null. JUnit 6 adopts JSpecify across its entire module: the @NullMarked annotation makes all unannotated types non-null by default, and @Nullable marks the exceptions. The Assertions class explicitly annotates parameters such as assertNull(@Nullable Object actual) and fail(@Nullable String message), so IDEs and static analyzers like NullAway and Error Prone can catch null misuse at compile time instead of at runtime.",
"whyModernWins": [
{
"icon": "📜",
"title": "Explicit contracts",
"desc": "@NullMarked on the JUnit 6 module documents null semantics directly in the API — no source-reading required."
},
{
"icon": "🛡️",
"title": "Compile-time safety",
"desc": "IDEs and analyzers warn when null is passed where non-null is expected, catching bugs before tests run."
},
{
"icon": "🌐",
"title": "Ecosystem standard",
"desc": "JSpecify is adopted by Spring, Guava, and others — consistent null semantics across your whole stack."
}
],
"support": {
"state": "available",
"description": "Available since JUnit 6.0 (October 2025, requires Java 17+)"
},
"prev": "tooling/aot-class-preloading",
"next": "language/default-interface-methods",
"related": [
"enterprise/spring-null-safety-jspecify",
"errors/helpful-npe",
"errors/require-nonnull-else"
],
"docs": [
{
"title": "JUnit 6 Assertions API",
"href": "https://docs.junit.org/current/api/org.junit.jupiter.api/org/junit/jupiter/api/Assertions.html"
},
{
"title": "JSpecify Nullness User Guide",
"href": "https://jspecify.dev/docs/user-guide/"
},
{
"title": "Upgrading to JUnit 6.0",
"href": "https://github.com/junit-team/junit-framework/wiki/Upgrading-to-JUnit-6.0/Core-Principles"
}
]
}