## Wednesday, 29 January 2014

### Linux Alternatives and Oracle Java

If, like me, you prefer to run the Oracle version of Java on your Linux machine as the default JDK, you will often find that the Linux distro will have other ideas.  Fedora for example has a number of Java based applications as part of the distribution which will include a dependency on the OpenJDK.  When the distro installs OpenJDK is will generally be setup as the default for executing the various Java binaries (e.g. 'java', 'javac').  However, the team at Redhat built a system called alternatives which maintains a set of symbolic links that allows the user to switch between multiple implementations of a package the supports the same functionality.  I've managed to understand enough about the alternatives package that I can now easily switch between the Oracle JDK and the OpenJDK.

Alternatives has the notion of master and slave configurations.  The way that the OpenJDK sets up the alternatives is that it uses 2 masters 'java' for the runtime (JRE) and 'javac' for the development tools (JDK).  To set the Oracle JDK in a similar way you need a script like this:

Once you have run that script then you can configure the JRE and JDK of your choice:

$sudo alternatives --config java Which will provide a prompt allowing you to choose your desired JRE There is 2 program that provides 'java'. Selection Command ----------------------------------------------- * 1 /usr/lib/jvm/java-1.7.0-openjdk-1.7.0.60... + 2 /usr/java/latest/jre/bin/java Enter to keep the current selection[+], or type selection number: And run the same thing for the JDK:$ sudo alternatives --config javac

Jesus !

It's quiet easy to switch between jdk in gentoo:

$java-config-2 -L The following VMs are available for generation-2: 1) GCJ 4.7.2 [gcj-jdk] 2) IcedTea JDK 6.1.12.4 [icedtea-6] 3) IcedTea JDK 7.2.3.9 [icedtea-7] *) Oracle JDK 1.7.0.45 [oracle-jdk-bin-1.7] 5) Sun JDK 1.6.0.45 [sun-jdk-1.6]$ java -version
java version "1.7.0_45"
Java(TM) SE Runtime Environment (build 1.7.0_45-b18)
Java HotSpot(TM) Server VM (build 24.45-b08, mixed mode)

$java-config-2 -s 1 Now using gcj-jdk as your user JVM$ java -version
java version "1.5.0"
gij (GNU libgcj) version 4.7.2

Harish G. Naik said...

On my Ubuntu based distro, I often do this and it works:

for binary in $(ls /usr/lib/java/jdk1.7.0_51/bin/); do sudo update-alternatives --install /usr/bin/$binary $binary /usr/lib/java/jdk1.7.0_51/bin/$binary 0; done

Peter Veentjer said...

I just add my different Java environments in my .bashrc and select which one I want to use.

I always remove the installed one because they are installed at impractical locations.

JAVA_HOME=/java/jdk/jdk1.7.0_51/
#JAVA_HOME=/java/jdk/jdk1.8.0/
PATH=$JAVA_HOME/bin:$PATH

randy said...