$ brew install Caskroom/versions/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+102-2016-01-21-003129.javare.4316.nc)
Java HotSpot(TM) 64-Bit Server VM (build 9-ea+102-2016-01-21-003129.javare.4316.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