Run tests on all JDKs but only build on 18

.github/workflows/build.yml

@@ -9,22 +9,6 @@ jobs:
   jvm:
     runs-on: ubuntu-latest
 
-    strategy:
-      fail-fast: false
-      matrix:
-        java-version:
-          - 8
-          - 9
-          - 10
-          - 11
-          - 12
-          - 13
-          - 14
-          - 15
-          - 16
-          - 17
-          - 18
-
     steps:
       - name: Checkout
         uses: actions/checkout@v2
@@ -36,7 +20,7 @@ jobs:
         uses: actions/setup-java@v2
         with:
           distribution: 'zulu'
-          java-version: ${{ matrix.java-version }}
+          java-version: 18
 
       - name: Test
         run: ./gradlew build
@@ -55,7 +39,7 @@ jobs:
         uses: actions/setup-java@v2
         with:
           distribution: 'zulu'
-          java-version: 17
+          java-version: 18
 
       - name: Upload Artifacts
         run: ./gradlew publish

build.gradle.kts

@@ -99,4 +99,29 @@ subprojects {
       )
     }
   }
+
+  // Copied from https://github.com/square/retrofit/blob/master/retrofit/build.gradle#L28.
+  // Create a test task for each supported JDK.
+  for (majorVersion in 8..18) {
+    // Adoptium JDK 9 cannot extract on Linux or Mac OS.
+    if (majorVersion == 9) continue
+
+    val jdkTest = tasks.register<Test>("testJdk$majorVersion") {
+      val javaToolchains = project.extensions.getByType(JavaToolchainService::class)
+      javaLauncher.set(javaToolchains.launcherFor {
+        languageVersion.set(JavaLanguageVersion.of(majorVersion))
+      })
+
+      description = "Runs the test suite on JDK $majorVersion"
+      group = LifecycleBasePlugin.VERIFICATION_GROUP
+
+      // Copy inputs from normal Test task.
+      val testTask = tasks.getByName<Test>("test")
+      classpath = testTask.classpath
+      testClassesDirs = testTask.testClassesDirs
+    }
+    tasks.named("check").configure {
+      dependsOn(jdkTest)
+    }
+  }
 }