Introduction
This page is based on experience of setting up Java 8 on Ubuntu 14.04. Much of it may be applicable to other Java versions and distros.
Architecture
First we check which architecture we are running on.
Open a terminal and type the command :
arch
The output could be i686 or i386 for a 32 bits system or x86_64 for a 64 bits system.
Environment
This tutorial assume that you have at least one java installed on your system. It has not been tested on an Ubuntu without any installed java. However generalities should apply.
user@machine$ java -version java version "1.8.0_91" Java(TM) SE Runtime Environment (build 1.8.0_91-b14) Java HotSpot(TM) Server VM (build 25.91-b14, mixed mode)
The previous command shows us that we have a running java on our system. On Ubuntu 14.04 the Java commands are under the /usr/bin directory.
user@machine$ which java /usr/bin/java
The previous command shows us that we are running a java command from /usr/bin.
However in order to manage multiple version of Java the update-alternatives mechanism has been introduced. It allows multiple "providers" to be installed on the same machine, and configures which is the default.
This mechanism consists of two directories, /etc/alternatives who contains the symlinks and /var/lib/dpkg/alternatives who contains update-alternatives state information.
Let's see where is the java command:
user@machine$ ls -al /usr/bin/java lrwxrwxrwx 1 root root 22 Mar 12 2013 /usr/bin/java -> /etc/alternatives/java
As you can see the java command from /usr/bin is a symlink and will be resolved against the java command contained in /etc/alternatives.
user@machine$ file /etc/alternatives/java /etc/alternatives/java: symbolic link to `/usr/lib/jvm/ramdisk-java-8-oracle/jre/bin/java'
The previous command shows us where lies the java command and hopefully the directory /usr/lib/jvm/ramdisk-java-8-oracle/jre/bin contains the real java command.
user@machine$ ls -al /usr/lib/jvm/ramdisk-java-8-oracle/jre/bin/java -rwxr-xr-x 1 root root 5730 May 4 10:54 /usr/lib/jvm/ramdisk-java-8-oracle/jre/bin/java
Now take a look in the file /var/lib/dpkg/alternatives/java:
manual /usr/bin/java java.1.gz /usr/share/man/man1/java.1.gz /usr/lib/jvm/java-8-openjdk-i386/jre/bin/java 1069 /usr/lib/jvm/java-8-openjdk-i386/jre/man/man1/java.1.gz /usr/lib/jvm/java-8-oracle/jre/bin/java 1070 /usr/lib/jvm/java-8-oracle/man/man1/java.1.gz /usr/lib/jvm/ramdisk-java-8-oracle/jre/bin/java 1073
That file teaches us that there are several java command installed on this machine.
Java Layout
Either the Open JDK or the Oracle JDK install their environment below /usr/lib/jvm:
user@machine:/usr/lib/jvm$ ls -al total 100 drwxr-xr-x 5 root root 4096 Jul 11 12:07 . drwxr-xr-x 205 root root 53248 Jul 7 09:33 .. lrwxrwxrwx 1 root root 34 Jul 7 15:33 default-java -> /usr/lib/jvm/ramdisk-java-8-oracle lrwxrwxrwx 1 root root 41 Jul 7 15:34 .default-java.jinfo -> /usr/lib/jvm/.ramdisk-java-8-oracle.jinfo lrwxrwxrwx 1 root root 19 Apr 22 19:41 java-1.8.0-openjdk-i386 -> java-8-openjdk-i386 -rw-r--r-- 1 root root 3083 Jul 11 11:49 .java-1.8.0-openjdk-i386.jinfo drwxr-xr-x 7 root root 4096 Apr 25 12:34 java-8-openjdk-i386 lrwxrwxrwx 1 root root 30 Apr 25 12:39 .java-8-openjdk-i386.jinfo -> .java-1.8.0-openjdk-i386.jinfo drwxr-xr-x 8 root root 4096 May 4 10:55 java-8-oracle -rw-r--r-- 1 root root 3135 Jul 11 11:48 .java-8-oracle.jinfo drwxr-xr-x 2 root root 4096 Apr 25 12:42 openjdk-8 lrwxrwxrwx 1 root root 20 Jul 7 11:46 ramdisk-java-8-oracle -> /media/java-8-oracle -rw-r--r-- 1 root root 3563 Jul 11 11:47 .ramdisk-java-8-oracle.jinfo
Beyond the various directories, files and links in the /usr/lib/jvm directory several remarks need to be done.
First of all we use the debian layout by using the default-java symlink. Tomcat or Eclipse are tools who use the default-java symlink.
We see int the previous output that the default-java point to a ramdisk-java-8-oracle who is a local symlink who points to another directory.
Second we see .jinfo files, one per JDK installed. in our example three versions of the JDK are installed and we see three respective .jinfo files.
Let's see the content of the .java-8-oracle.jinfo:
name=java-8-oracle alias=java-8-oracle priority=1070 section=main jre ControlPanel /usr/lib/jvm/java-8-oracle/jre/bin/ControlPanel jre java /usr/lib/jvm/java-8-oracle/jre/bin/java jre javaws /usr/lib/jvm/java-8-oracle/jre/bin/javaws jre jcontrol /usr/lib/jvm/java-8-oracle/jre/bin/jcontrol jre jjs /usr/lib/jvm/java-8-oracle/jre/bin/jjs jre keytool /usr/lib/jvm/java-8-oracle/jre/bin/keytool jre orbd /usr/lib/jvm/java-8-oracle/jre/bin/orbd jre pack200 /usr/lib/jvm/java-8-oracle/jre/bin/pack200 jre policytool /usr/lib/jvm/java-8-oracle/jre/bin/policytool jre rmid /usr/lib/jvm/java-8-oracle/jre/bin/rmid jre rmiregistry /usr/lib/jvm/java-8-oracle/jre/bin/rmiregistry jre servertool /usr/lib/jvm/java-8-oracle/jre/bin/servertool jre tnameserv /usr/lib/jvm/java-8-oracle/jre/bin/tnameserv jre unpack200 /usr/lib/jvm/java-8-oracle/jre/bin/unpack200 jre jexec /usr/lib/jvm/java-8-oracle/jre/lib/jexec jdk appletviewer /usr/lib/jvm/java-8-oracle/bin/appletviewer jdk extcheck /usr/lib/jvm/java-8-oracle/bin/extcheck jdk idlj /usr/lib/jvm/java-8-oracle/bin/idlj jdk jar /usr/lib/jvm/java-8-oracle/bin/jar jdk jarsigner /usr/lib/jvm/java-8-oracle/bin/jarsigner jdk javac /usr/lib/jvm/java-8-oracle/bin/javac jdk javadoc /usr/lib/jvm/java-8-oracle/bin/javadoc jdk javafxpackager /usr/lib/jvm/java-8-oracle/bin/javafxpackager jdk javah /usr/lib/jvm/java-8-oracle/bin/javah jdk javap /usr/lib/jvm/java-8-oracle/bin/javap jdk javapackager /usr/lib/jvm/java-8-oracle/bin/javapackager jdk jcmd /usr/lib/jvm/java-8-oracle/bin/jcmd jdk jconsole /usr/lib/jvm/java-8-oracle/bin/jconsole jdk jdb /usr/lib/jvm/java-8-oracle/bin/jdb jdk jdeps /usr/lib/jvm/java-8-oracle/bin/jdeps jdk jhat /usr/lib/jvm/java-8-oracle/bin/jhat jdk jinfo /usr/lib/jvm/java-8-oracle/bin/jinfo jdk jmap /usr/lib/jvm/java-8-oracle/bin/jmap jdk jmc /usr/lib/jvm/java-8-oracle/bin/jmc jdk jps /usr/lib/jvm/java-8-oracle/bin/jps jdk jrunscript /usr/lib/jvm/java-8-oracle/bin/jrunscript jdk jsadebugd /usr/lib/jvm/java-8-oracle/bin/jsadebugd jdk jstack /usr/lib/jvm/java-8-oracle/bin/jstack jdk jstat /usr/lib/jvm/java-8-oracle/bin/jstat jdk jstatd /usr/lib/jvm/java-8-oracle/bin/jstatd jdk jvisualvm /usr/lib/jvm/java-8-oracle/bin/jvisualvm jdk native2ascii /usr/lib/jvm/java-8-oracle/bin/native2ascii jdk rmic /usr/lib/jvm/java-8-oracle/bin/rmic jdk schemagen /usr/lib/jvm/java-8-oracle/bin/schemagen jdk serialver /usr/lib/jvm/java-8-oracle/bin/serialver jdk wsgen /usr/lib/jvm/java-8-oracle/bin/wsgen jdk wsimport /usr/lib/jvm/java-8-oracle/bin/wsimport jdk xjc /usr/lib/jvm/java-8-oracle/bin/xjc plugin xulrunner-1.9-javaplugin.so /usr/lib/jvm/java-8-oracle/jre/lib/i386/libnpjp2.so plugin firefox-javaplugin.so /usr/lib/jvm/java-8-oracle/jre/lib/i386/libnpjp2.so plugin iceape-javaplugin.so /usr/lib/jvm/java-8-oracle/jre/lib/i386/libnpjp2.so plugin iceweasel-javaplugin.so /usr/lib/jvm/java-8-oracle/jre/lib/i386/libnpjp2.so plugin mozilla-javaplugin.so /usr/lib/jvm/java-8-oracle/jre/lib/i386/libnpjp2.so plugin midbrowser-javaplugin.so /usr/lib/jvm/java-8-oracle/jre/lib/i386/libnpjp2.so plugin xulrunner-javaplugin.so /usr/lib/jvm/java-8-oracle/jre/lib/i386/libnpjp2.so
As you can see this file contains the description of the each java tool, either jre, jdk or plugin is associated with an a particular command or library.
In case this file doesn't exist, create one with the previous content and adjust the path accordingly.
The priority value needs to be unique among the JVM installations.
The following command shows the installed JVM and their according priority number.
user@machine$ update-java-alternatives -l default-java 1073 /usr/lib/jvm/default-java java-1.8.0-openjdk-i386 1069 /usr/lib/jvm/java-1.8.0-openjdk-i386 java-8-openjdk-i386 1069 /usr/lib/jvm/java-8-openjdk-i386 java-8-oracle 1070 /usr/lib/jvm/java-8-oracle ramdisk-java-8-oracle 1073 /usr/lib/jvm/ramdisk-java-8-oracle
Choose a priority number who do not collapse an existing one.
Then setup default-java:
cd /usr/lib/jvm sudo ln -s ramdisk-java-8-oracle default-java sudo ln -s .ramdisk-java-8-oracle.jinfo .default-java.jinfo