Faster and simpler Java 9 classpath implementation

  - Take advantage of the `/packages` index provided by the
    jrt file system to avoid (expensive) Files.exist for
    non-existent entries across the full list of modules.
  - Extends ClassPath directly which leads to a simpler
    implemnentation that using the base class.
  - Add a unit test that shows we can read classes and packages
    from the Java standard library.

Fixes scala/scala-dev#306

With this change bootstrap time under Java 9 was comparable to
Java 8. Before, it was about 40% slower.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/compiler/scala/tools/util/PathResolver.scala b/src/compiler/scala/tools/util/PathResolver.scala
index 188cabbc8d..f845656980 100644
--- a/src/compiler/scala/tools/util/PathResolver.scala
+++ b/src/compiler/scala/tools/util/PathResolver.scala
@@ -234,7 +234,7 @@ final class PathResolver(settings: Settings) {
 
     // Assemble the elements!
     def basis = List[Traversable[ClassPath]](
-      JImageDirectoryLookup.apply(),                // 0. The Java 9 classpath (backed by the jrt:/ virtual system)
+      JrtClassPath.apply(),                         // 0. The Java 9 classpath (backed by the jrt:/ virtual system, if available)
       classesInPath(javaBootClassPath),             // 1. The Java bootstrap class path.
       contentsOfDirsInPath(javaExtDirs),            // 2. The Java extension class path.
       classesInExpandedPath(javaUserClassPath),     // 3. The Java application class path.