Support concise try-with-resources with java9

pmd-java/etc/grammar/Java.jjt

@@ -4,6 +4,7 @@
  * A single underscore "_" is an invalid identifier in java9.
  * Diamond operator for anonymous classes is only allowed with java9.
  * Add support for module-info.java.
+ * Allow more concise try-with-resources statements with java9.
  * Andreas Dangel 09/2017
  *====================================================================
  * Add support for new Java 8 annotation locations.
@@ -356,6 +357,11 @@ public class JavaParser {
       throwParseException("Cannot use module declaration when running in JDK inferior to 9 mode!");
     }
   }
+  private void checkForBadConciseTryWithResourcesUsage() {
+    if (jdkVersion < 9) {
+      throwParseException("Cannot use concise try-with-resources when running in JDK inferior to 9 mode!");
+    }
+  }
 
   // This is a semantic LOOKAHEAD to determine if we're dealing with an assert
   // Note that this can't be replaced with a syntactic lookahead
@@ -2247,11 +2253,9 @@ void Resources() :
 void Resource() :
 {}
 {
-	( "final" | Annotation() )*
-	Type()
-	VariableDeclaratorId()
-	"="
-	Expression()
+   LOOKAHEAD(2) ( ( "final" | Annotation() )* Type() VariableDeclaratorId() "=" Expression() )
+   |
+   Name() {checkForBadConciseTryWithResourcesUsage();}
 }
 
 void CatchStatement() :

pmd-java/src/test/java/net/sourceforge/pmd/lang/java/ast/JDKVersionTest.java

@@ -257,4 +257,14 @@ public class JDKVersionTest {
     public final void jdk9ModuleInfo() {
         parseJava9(loadSource("jdk9_module_info.java"));
     }
+
+    @Test(expected = ParseException.class)
+    public final void jdk9TryWithResourcesInJava8() {
+        parseJava18(loadSource("jdk9_try_with_resources.java"));
+    }
+
+    @Test
+    public final void jdk9TryWithResources() {
+        parseJava9(loadSource("jdk9_try_with_resources.java"));
+    }
 }

pmd-java/src/test/resources/net/sourceforge/pmd/lang/java/ast/jdkversiontests/jdk9_try_with_resources.java

@@ -0,0 +1,7 @@
+public class InputJava9TryWithResources {
+ public static void main() {
+  MyResource resource1 = new MyResource();
+  MyResource resource2 = new MyResource();
+  try (resource1; resource2) { }
+ }
+}