Add JUnit 6 with JSpecify null safety pattern to tooling category

Co-authored-by: brunoborges <129743+brunoborges@users.noreply.github.com>

Author: Copilot

content/language/default-interface-methods.json

@@ -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",

content/tooling/aot-class-preloading.json

@@ -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",

content/tooling/junit6-with-jspecify.json

@@ -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"
+    }
+  ]
+}