Fix Gradle Toolchain configuration

See gh-25787

gradle/toolchains.gradle

@@ -28,13 +28,11 @@
  *
  * @author Brian Clozel
  */
-def mainToolchain = 'mainToolchain'
-def testToolchain = 'testToolchain'
 
 plugins.withType(JavaPlugin) {
 	// Configure the Java Toolchain if the 'mainToolchain' property is defined
-	if (project.hasProperty(mainToolchain)) {
-		def mainLanguageVersion = JavaLanguageVersion.of(project.property(mainToolchain).toString())
+	if (project.hasProperty('mainToolchain') && project.mainToolchain) {
+		def mainLanguageVersion = JavaLanguageVersion.of(project.mainToolchain.toString())
 		java {
 			toolchain {
 				languageVersion = mainLanguageVersion
@@ -48,8 +46,8 @@ plugins.withType(JavaPlugin) {
 		}
 	}
 	// Configure a specific Java Toolchain for compiling and running tests if the 'testToolchain' property is defined
-	if (project.hasProperty(testToolchain)) {
-		def testLanguageVersion = JavaLanguageVersion.of(project.property(testToolchain).toString());
+	if (project.hasProperty('testToolchain') && project.testToolchain) {
+		def testLanguageVersion = JavaLanguageVersion.of(project.testToolchain.toString());
 		tasks.withType(JavaCompile).matching { it.name.contains("Test") }.configureEach {
 			javaCompiler = javaToolchains.compilerFor {
 				languageVersion = testLanguageVersion
@@ -65,7 +63,7 @@ plugins.withType(JavaPlugin) {
 
 plugins.withType(GroovyPlugin) {
 	// Fallback to JDK8
-	if (!project.hasProperty(mainToolchain)) {
+	if (!project.hasProperty('mainToolchain')) {
 		compileGroovy {
 			sourceCompatibility = JavaVersion.VERSION_1_8
 		}
@@ -74,8 +72,8 @@ plugins.withType(GroovyPlugin) {
 
 // Configure the Kotlin compiler if the 'mainToolchain' property is defined
 pluginManager.withPlugin("kotlin") {
-	if (project.hasProperty(mainToolchain)) {
-		def mainLanguageVersion = JavaLanguageVersion.of(project.property(mainToolchain).toString());
+	if (project.hasProperty('mainToolchain') && project.mainToolchain) {
+		def mainLanguageVersion = JavaLanguageVersion.of(project.mainToolchain.toString());
 		def compiler = javaToolchains.compilerFor {
 			languageVersion = mainLanguageVersion
 		}
@@ -96,15 +94,21 @@ pluginManager.withPlugin("kotlin") {
 		}
 	}
 	else {
+		// Fallback to JDK8
 		compileKotlin {
 			kotlinOptions {
 				jvmTarget = '1.8'
 			}
 		}
+		compileTestKotlin {
+			kotlinOptions {
+				jvmTarget = '1.8'
+			}
+		}
 	}
 	
-	if (project.hasProperty(testToolchain)) {
-		def testLanguageVersion = JavaLanguageVersion.of(project.property(testToolchain).toString());
+	if (project.hasProperty('testToolchain')  && project.testToolchain) {
+		def testLanguageVersion = JavaLanguageVersion.of(project.testToolchain.toString());
 		def compiler = javaToolchains.compilerFor {
 			languageVersion = testLanguageVersion
 		}
@@ -117,11 +121,4 @@ pluginManager.withPlugin("kotlin") {
 			}
 		}
 	}
-	else {
-		compileTestKotlin {
-			kotlinOptions {
-				jvmTarget = '1.8'
-			}
-		}
-	}
 }