OSX に JDK 9 Early Access Releases をインストール

cask と jenv をお使いの方を前提としております。

$ brew tap caskroom/versions
$ brew cask install java9-beta
$ jenv add /Library/Java/JavaVirtualMachines/jdk-9.jdk/Contents/Home
$ jenv shell oracle64-9-ea
$ java -version
java version "9-ea"
Java(TM) SE Runtime Environment (build 9-ea+95-2015-12-02-041743.javare.3968.nc)
Java HotSpot(TM) 64-Bit Server VM (build 9-ea+95-2015-12-02-041743.javare.3968.nc, mixed mode)
$ jshell
|  Welcome to JShell -- Version 9-ea
|  Type /help for help

-> "Hello JDK 9!"
|  Expression value is: "Hello JDK 9!"
|    assigned to temporary variable $1 of type String