java - Towards the "true" definition of JAVA_HOME

Question

As a Java developer who switches between *NIX systems (OS X, Ubuntu, ...), although I can always "get" my JDK up and running, it seems that there is no clear definition of JAVA_HOME in many packages which require JAVA_HOME to be set :

For example

• MAVEN : "JAVA_HOME, refers to your JDK directory"
• HADOOP : "JAVA_HOME, which specifies the path to the Java 1.5.x installation"
• SUN : "JAVA_HOME is the directory that contains the JRE"

I thus have 2 questions regarding this matter (any insights would be welcome, also, but concretely, I have these two questions) :

1) Does mac os X java installation copy the target from the "JavaVM.frameworks" directory into usr/bin ?

2) What is the "definition" of JAVA_HOME ? Clearly, we cannot define JAVA_HOME as simply "the place where java is installed", because this definition is ambiguous, since java can exist both in a HOME location (i.e. in /System/Library/Fraemworks/.....) , or alternatively, it may also be directly in the /usr/bin directory ?

MY THOUGHTS

I believe that JAVA_HOME is ACTUALLY meant to refer to more than just a binary "java" program. JAVA_HOME probably is meant to refer to the location of a set of java related directories AND binaries... But still, I am not really clear what this comprises, and wether or not this definition which I am proposing is precise enough to be useful.

Answer 1

There is no "true" definition of JAVA_HOME. This variable is not used by the Java Runtime Environment, and has no specification as part of the Java platform.

It is merely a convention that is used by some applications that run on top of the Java platform. Since there's no standard, each application is free to make its own rules about the directory to which this variable should refer; you should read the application's documentation to find out what it needs.

That said, every application I've found that uses this variable will work if you set it to the top level directory of a JDK installation (not the JRE within the JDK, but the JDK itself). This directory should contain "bin" and "lib" subdirectories that contain the java executable and the runtime libraries, respectively.

Some applications will also work if you point it at a JRE, but if it needs development tools like javac, it will fail.

Usually, I specify JAVA_HOME on the command line when I run a tool than needs it, because I work with multiple versions of Java, like this:

JAVA_HOME=/usr/local/jdk1.6.0_29 ant