[eclipse-archived/xtext-core#1835] better treatment of unsupported Java versions in Xtext

Signed-off-by: Christian Dietrich <christian.dietrich@itemis.de>

Author: cdietrich

xtext-gradle-builder/src/main/java/org/xtext/gradle/builder/XtextGradleBuilder.xtend

@@ -230,12 +230,16 @@ class XtextGradleBuilder implements IncrementalXtextBuilder {
 			attachToEmfObject(resourceSet)
 			language2GeneratorConfig.putAll(
 				gradleRequest.generatorConfigsByLanguage.mapValues [ gradleConfig |
+					val javaVersion = JavaVersion.fromQualifier(gradleConfig.javaSourceLevel.toString)
+					if (javaVersion === null) {
+						gradleRequest.logger.warn("Xtext does not support Java " + gradleConfig.javaSourceLevel.toString + ", falling back to " + JavaVersion.JAVA8.label)
+					}
 					new GeneratorConfig => [
 						generateSyntheticSuppressWarnings = gradleConfig.isGenerateSyntheticSuppressWarnings
 						generateGeneratedAnnotation = gradleConfig.isGenerateGeneratedAnnotation
-						includeDateInGeneratedAnnotation = gradleConfig.isIncludeDateInGeneratedAnnotation
+						includeDateInGeneratedAnnotation = 	gradleConfig.isIncludeDateInGeneratedAnnotation
 						generatedAnnotationComment = gradleConfig.generatedAnnotationComment
-						javaSourceVersion = JavaVersion.fromQualifier(gradleConfig.javaSourceLevel.toString)
+						javaSourceVersion = javaVersion ?: JavaVersion.JAVA8
 					]
 				]
 			)