Multiple java versions on debian

Debian has a nice way to handle multiple java installations on a the same machine. Let’s say that for some reason you want to have sun-java 1.5 and also 1.6 installed on the server, we can easily configure the default one with the update-java-alternatives command (part of the java-common package). Here is how it can be used:

To see what versions of java we have installed on the system (from debian packages):

update-java-alternatives -l
java-1.5.0-sun 53 /usr/lib/jvm/java-1.5.0-sun
java-6-sun 63 /usr/lib/jvm/java-6-sun

We can see that the default version is 1.6 in my case (as it was the last installed):

java -version
java version "1.6.0_20"
Java(TM) SE Runtime Environment (build 1.6.0_20-b02)
Java HotSpot(TM) Client VM (build 16.3-b01, mixed mode, sharing)

We can change the default version with: update-java-alternatives –jre -s , like:

**update-java-alternatives** --jre -s java-1.5.0-sun

and now the default is 1.5:

java -version
java version "1.5.0_22"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_22-b03)
Java HotSpot(TM) Client VM (build 1.5.0_22-b03, mixed mode, sharing)

This is quite handy if you need to have multiple java versions installed, and need a quick way to change the default one (you can access any of them directly from their own path of course).