Default JAVA, $JAVA_HOME vs sudo update-alternatives --config java

Question

I am on Ubuntu and I have set the following in my ~/.bashrc file:

export JAVA_HOME=/opt/jdk1.8.0_91
export PATH=$JAVA_HOME/bin:$PATH

and then:

echo $JAVA_HOME
>/opt/jdk1.8.0_91

java -version
>java version "1.8.0_91"
>Java(TM) SE Runtime Environment (build 1.8.0_91-b14)
>Java HotSpot(TM) 64-Bit Server VM (build 25.91-b14, mixed mode)

at the first glance, the command sudo update-alternatives --config java was not showing my manually installed Java, so I installed it to the command with sudo update-alternatives --install /usr/bin/java java /opt/jdk1.8.0_91 1.

Now, the command sudo update-alternatives --config java drops down the list of all Java versions being installed like that:

0            /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java   1071      auto mode
1            /opt/jdk1.7.0_51/bin/java                        1         manual mode
* 2            /opt/jdk1.8.0_91                                 1         manual mode
3            /usr/lib/jvm/java-6-openjdk-amd64/jre/bin/java   1061      manual mode
4            /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java   1071      manual mode

But switching between these using the prompt of the sudo update-alternatives --config java does not affect $JAVA_HOME and then $java -version.

My question is, what does sudo update-alternatives --config java exactly do after switching to another alternative with respect to the settings in the $JAVA_HOME$ variable?

Answer 1

It only changes a symlink located (on most distro I guess) at /etc/alternatives/java. Absolutely NO change in the environment variable you set $JAVA_HOME is made.

First look at from where the command is found, you can do :

$which java
/usr/bin/java

The which command shows /usr/bin/java in my Debian distro. This file is a symlink which points to /etc/alternatives/java.

$ls -l /usr/bin | grep java
java -> /etc/alternatives/java

Then you follow the symlink :

$ls -l /etc/alternatives/java
/etc/alternatives/java -> /path/to/my/java/installation/1.x/bin/java

This shows that /etc/alternatives/java is another symlink. When you do an update-alternatives on java, you just change this symlink target to another one.

Then, why doesn't the executed version change when you do the update-alternatives command ? I guess it's because of the order the executables are found in $PATH. Since you added a directory to the PATH environment variable, there are now two possible java executables : one in /usr/bin and the other in /opt/jdk1.8.0_9, but only the first one found will be taken into account when you'll type java commands.

And because you set

PATH=$JAVA_HOME/bin:$PATH

The first one will be found in $JAVA_HOME/bin aka /opt/jdk1.8.0_91 . Because you made /opt/jdk1.8.0_9 appear before /usr/bin which is defined by default in the the PATH variable. You can check it by typing in a terminal

$echo $PATH
/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games:/path/to/my/java/installation/1.x/bin

You can see that my java/bin dir is located after the others defined in the PATH.

To correct this, you just have to concatenate $JAVA_HOME/bin after $PATH, like this :

PATH=$PATH:$JAVA_HOME/bin

This way you will be able to choose the default java executable from alternatives and the java exe found in $JAVA_HOME/bin will be discarded. But to be consistent, in most cases you should choose the same java exe as in $JAVA_HOME/bin.