Experimenting with jdk8

Debian linux

If you are on any variant of Debian linux (Mint / Ubuntu etc), there is the convenience of being able to set alternative java using update-alternatives:-

su -c "update-alternatives --config java"

or

sudo update-alternatives --config java

Output

There are 3 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                            Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java   1071      auto mode
  1            /opt/jdk1.7.0_51/bin/java                        100       manual mode
* 2            /opt/jdk1.8.0/bin/java                           100       manual mode
  3            /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java   1071      manual mode

Press enter to keep the current choice[*], or type selection number: 

ArchLinux

A bit drastic but you can use this script to change the symbolic links

#!/bin/bash
# This script iterates all symlinks in the BIN_DIR directory that have the same JDK path as 'java'
# and replaces every symlink with a new one that has the JDK path set from the first script argument.
#
# Useful if you want to switch symlinks from OpenJDK to any other JDK.
# E.g. ./replace-java-symlinks.sh /opt/java6
#
# author: Zdenek Obst, zdenek.obst-at-gmail.com

BIN_DIR=/usr/bin

if [[ $EUID -ne 0 ]]; then
  echo "You must be root to run this script"
  exit
fi

if [[ -z "$1" ]]; then
  echo "Pass the new JDK home as the first argument, e.g. /opt/java6"
  exit
fi

ACTUAl_JDK_PATH=`ls -l $BIN_DIR/java | awk '{print $11}' | sed 's:\/jre\/bin\/java::g'`
echo "=> Actual JDK path used by symlinks is: $ACTUAl_JDK_PATH"

NEW_JDK_PATH=`echo ${1%/}` # remove last slash if needed
echo "=> New JDK path for symlinks will be: $NEW_JDK_PATH"

read -p "Are suggested JDK paths correct? (y/n) " yn
if [ "$yn" != "y" ]; then
  echo "Cancelled on user's request"
  exit
fi

echo "----------------------------------------------"

FILES=`ls -l $BIN_DIR | grep $ACTUAl_JDK_PATH`

ls -l $BIN_DIR | grep $ACTUAl_JDK_PATH | while read file;
do
  FILENAME=`echo $file | awk '{print $9}'`
  SYMLINK=`echo $file | awk '{print $11}'`
  
  ACT=`echo $ACTUAl_JDK_PATH | sed 's:\/:\\\/:g'` # escape characters for SED
  NEW=`echo $NEW_JDK_PATH | sed 's:\/:\\\/:g'` # escape characters for SED
  
  NEWLINK=`echo $SYMLINK | sed "s/$ACT/$NEW/g"`
  
  if [ -f $NEWLINK ]; then
    echo "-> Replacing symlink: $BIN_DIR/$FILENAME -> $NEWLINK"
    rm $BIN_DIR/$FILENAME
    ln -s  $NEWLINK $BIN_DIR/$FILENAME    
  else
    echo "!! File $NEWLINK does not exist, skipping symlink re-creation"
  fi
  
done

Other linux

Simple but crude, ensure you have a user bin folder (and that it is on your PATH) create a bash script as follows,

#!/usr/bin/env bash
export JAVA_HOME="/opt/jdk1.8.0"
${JAVA_HOME}/bin/java $@

and make it executable chmod +x bin/java, but depends on convention that home/bin trumps /usr/bin

Windows

some nice person will edit this? Setting JAVA_HOME might do it.

MacOSX

some nice person with a rollneck sweater will edit this? Setting JAVA_HOME might do it.

Tools/Configuration

How do I check which version of the JVM JRuby is running on?

jruby -rjava -e "puts java.lang.System.get_property('java.version')"

prints out 1.8.0 with above setup.

How do I optimise performance (make use of jvm flags)

1. Edit your .jrubyrc in $HOME (for global configuration, NB: jruby-complete may also pick up this configuration up, especially if you kick-off ruby-processing with an installed jruby), apparently you can also have a .jrubyrc in a local folder and that takes precedence.

2. Edit data/jruby_args.txt in your local folder -XX:+TieredCompilation on its own seems to work best.