JDK 13 RFR of JDK-8221264: Refactor SourceVersion#latestSupported

[Joe Darcy]

Hello,

Responding to an rfe filed by Liam, please review a refactoring of the 
SourceVersion.latestSupported method:

     JDK-8221264: Refactor SourceVersion#latestSupported
     http://cr.openjdk.java.net/~darcy/8221264.0/

Patch below.

With this version, when it comes time to rev a release for 14, the line

+            return valueOf("RELEASE_" + Math.min(13, intVersion));

will need to be changed to

+            return valueOf("RELEASE_" + Math.min(14, intVersion));

In general the JSR 269 packages, javax.lang.model and 
javax.annotation.processing, are constrained by bootstrapping issue to 
be runnable on JDK (N-1); in other words, they cannot only run on JDK N. 
It may or may not be possible to compile the code to run on JDK (N-2) or 
earlier. For example, since JDK 9, the JSR 269 API has used default 
methods so the API certainly cannot be compiled and run earlier than JDK 8.

The Runtime.Version type was introduced in JDK 9; its feature() method 
in 10.

The proposed implementation is simple, even if it is not completely 
accurate in a current lower bound for running the API; I don't think 
being more accurate on this lower bound is necessary.

Thanks,

-Joe

--- 
old/src/java.compiler/share/classes/javax/lang/model/SourceVersion.java 
2019-03-22 10:09:33.870668998 -0700
+++ 
new/src/java.compiler/share/classes/javax/lang/model/SourceVersion.java 
2019-03-22 10:09:33.526668998 -0700
@@ -209,30 +209,13 @@
      private static final SourceVersion latestSupported = 
getLatestSupported();

      private static SourceVersion getLatestSupported() {
-        try {
-            String specVersion = 
System.getProperty("java.specification.version");
+        int intVersion = Runtime.version().feature();

-            switch (specVersion) {
-                case "13":
-                    return RELEASE_13;
-                case "12":
-                    return RELEASE_12;
-                case "11":
-                    return RELEASE_11;
-                case "10":
-                    return RELEASE_10;
-                case "9":
-                    return RELEASE_9;
-                case "1.8":
-                    return RELEASE_8;
-                case "1.7":
-                    return RELEASE_7;
-                case "1.6":
-                    return RELEASE_6;
-            }
-        } catch (SecurityException se) {}
-
-        return RELEASE_5;
+        if (intVersion >= 10) {
+            return valueOf("RELEASE_" + Math.min(13, intVersion));
+        } else {
+            return RELEASE_9;
+        }
      }

      /**