Support features of JDK 11+

[retronym]

In general, Scala now works just fine on JDK 11

For the current support status of each JDK version, see https://docs.scala-lang.org/overviews/jdk-compatibility/overview.html

JDK9 JEP list

• JEP 223: New Version-String Scheme
  • Update our version parsing in isJavaAtLeast (and elsewhere?) SI-9750 scala.util.Properties.isJavaAtLeast works with JDK9 scala#5276
• JEP 220: Modular Run-Time Images (includes SI-9103)
  • Adapt our classpath to use jrt:/ virtual filesystem. WIP retronym:review/5112
• Update Zinc to understand that jrt:// classpath entries should have a timestamp based on the JDK (maybe $JAVA_HOME/jmods/*.mod? Find out exactly what file backs jrt://)
• Adapt REPL :javap command (currently gets IllegalArgumentException in JavapClass$JavapTool$.apply(JavapClass.scala:337)
• JEP-261 Module System (Support JEP-261 Module System #529)
  • javac allows programmers to declare module requirements, exports inline in the sourcetree ("module-info.java" files). What should we do?
  • app class loader is no longer a URLClassloader. Lots of places enumerate the classpath for JARs to initialize settings for a compiler. Experiment, document.
  • Update classfile reader to parse module-info.class (perhaps using ASM)
  • create module readability / accessibilty graph (following transitive "public exports")
• JEP-238 Multi-Release JAR Files
  • See if this feature is used in practice
  • Update classpath implementation to be able to consume such JARs. (WIP)
• JEP 280: Indify String Concatenation
  • Under target:jvm-1.9, consider emitting Javac style string concat with indy.
    • Use StringConcatFactory for string concatenation on JDK 9+ scala#9556
  • Update to ASM 5.1 (?). As suggested by @varming Varming/5.1 scala-asm-legacy#11
• JEP-193 VarHandles introduce a number of new methods annotated with @PolymorphicSignature. Expand our compiler support to these methods.
  • Maybe: foster creation of a library of Java 8/9 shims that use invokedynamic to use some Java 9 features (e.g VarHandle) when available but fall back to Java 8 alternatives (e.g. Unsafe._), as prototyped in retronym/scala@a95f03e
• Infrastructure:
  • Make JDK9 EA builds available in CI: Make it possible to use early access builds of OpenJDK scala-jenkins-infra#174
• SBT
  • Review Zinc's use of sun.boot.classpath: https://github.com/sbt/zinc/blob/1.x/internal/zinc-compile-core/src/main/scala/sbt/internal/inc/CompilerArguments.scala#L83
• Make mixin fields non-final to avoid verify errors. Use explicit fences to preserve memory effects: Trait encoding vs the persnickety JVM 9 verifier: final fields only updatable from within <init> #408

[retronym]

To whet the collective appetite, here's my branch that adapts to JEP 220 up and running:

⚡ qscala
Welcome to Scala 2.12.0-20160425-151134-5e07d57 (Java HotSpot(TM) 64-Bit Server VM, Java 9-ea).
Type in expressions for evaluation. Or try :help.

scala> java.util.List.of(1, 2, 3)
res0: java.util.List[Int] = [1, 2, 3]

[varming]

With less than nine months to the release of JDK9 not being able to run Scala programs on the early access releases is a serious concern, so it is great to see that work has started.

[SethTisue]

@retronym did 2.12.1 check some more of these boxes?

[retronym]

I have submitted a PR to SBT to start the ball rolling for Java 9 support. Because SBT internally uses an old version of scalac to compile build definitions, I decided to create a tool to export a jrt:/modules/*/** to rt.jar to bridge JDK9 to the older compiler.

This approach also enables your project to be compiled with, say, Scala 2.11.8, which might help some more folks test out JDK9, without first needing to upgrade to the 2.12 series.

Warning: we are yet to implement the additional access checks required by JEP-261. Until then, you can get LinkageError-s at runtime for code that passes the Scala type checker. So while I encourage testing and experimentation, use this at your own risk in production.

[retronym]

Discussion on the Java Language Spec changes related to modules, thread started by Eclipse JDT seeking to implement the spec. http://mail.openjdk.java.net/pipermail/jigsaw-dev/2017-May/012433.html