How to determine JAVA_HOME on Debian/Ubuntu?

Question

On Ubuntu it is possible to have multiple JVMs at the same time. The default one is selected with update-alternatives. But this does not set the JAVA_HOME environment variable, due to a debian policy.

I am writing a launcher script (bash), which starts a java application. This java application needs the JAVA_HOME environment variable. So how to get the path of the JVM which is currently selected by update-alternatives?

Answer 1

For the JRE, something like this should do the trick:

JAVA_HOME=$(readlink -f /usr/bin/java | sed "s:bin/java::") 

Answer 2

danadam's solution can easily be adopted to retrieve the JDK (i.e. not JRE) path as required:

JAVA_HOME=$(readlink -f /usr/bin/javac | sed "s:/bin/javac::") 

• Looks for javac Java compiler (instead of java) included in JDK (but not JRE).
• Has no trailing / (stripped off by sed s:/bin... instead of s:bin...)

Answer 3

export JAVA_HOME=$(dirname $(dirname $(readlink -f /usr/bin/java)))

In .bashrc was handy for me.

Answer 4

So, you're saying that this command does nothing for you?

sudo update-alternatives --config java 

Answer 5

I installed java with

sudo apt-get install openjdk-7-jre-headless 

and then to find the location

ls -al /etc/alternatives/java 

Answer 6

As an extension of danadams answer:

First of all, install the 2nd Java JRE as the 3rd java option, with priority of "3":

sudo alternatives --install /usr/lib/jvm/jre jre /opt/IBM/java/jre/bin/java 3 

Then, you can list them:

update-alternatives --list java 

You can set the alternative by hand , using this:

sudo alternatives --config java /opt/IBM/java/jre/bin/java 

Then, your script can set it on the fly, like so:

sudo alternatives --set java /opt/IBM/java/jre/bin/java JAVA_HOME=$(readlink -f /usr/bin/java | sed "s:bin/java::") 

This better illustrates what the 'sed' command is doing. Although you still need to set the links for javaw and javac, etc, as well, each done separately.

Answer 7

A while ago I created a tutorial on the Ubuntu forum on how to install the latest JRE/JDK from the Java website. It also covers on how to enable it system-wide, by adding the JRE/JDK location to the PATH variable. If you like, you can also add JAVA_HOME to the script, mentioned at the end of the topic.

Check it out: http://ubuntuforums.org/showthread.php?t=1437100

Answer 8

If java is configured with update-alternatives or was added to your PATH variable manually, then no hardcoded "/usr/bin/java" is needed. I use this solution in my .bashrc:

export JAVA_HOME=$(readlink -m $(which java)/../..)