Configuring java using the alternatives system (on Fedora/Centos/Redhat)

On most red-hat based systems, alternative versions of the same software (from say different vendors) are managed using the "alternatives" system. Here is an example :

$ which java | xargs file
/usr/bin/java: symbolic link to `/etc/alternatives/java'

$ file /etc/alternatives/java

/etc/alternatives/java: symbolic link to `/usr/lib/jvm/jre-1.6.0-openjdk.x86_64/bin/java'

Here in this case we are looking at the java pointing to the one bundled by openjdk. Say if you have installed the Sun (or Oracle rather :|) JDK and if the path to that is /opt/java/bin , then you can add the sun version of java to the alternatives system using : 

$ alternatives --install /usr/bin/java java /opt/java/bin/java 100000 

The last number there is a priority, set it high so that any priority based overrides are ineffective. Now by doing : 

$ alternatives --config java 

you should be able to switch between the alternative versions of java !