Multiple Java versions on OS X, and their paths

Java version management on OS X is a wee bit complicated. Here’s what I understand.

All different versions of Java are installed into the directory:
/System/Library/Frameworks/JavaVM.framework/Versions/

For example, the JDK home directory for 1.4.2 would be Versions/1.4.2/Home/.

There are three ways to access specific versions.

1. The hardcoded default. There is the hardcoded default version that comes with the current version of the OS. For OS X Leopard, this is Java 1.5. This version can always be accessed through the path /Library/Java/Home.
2. The Java Preferences application. recent versions of Java on OS X install an application “Java Preferences” into /Applications/Utilities. It allows you to select the preferred version by dragging it to the top of the list. There is one list for applets, and one list for applications and the command line. The terminal commands (e.g., java) use that version from the second list. The path of this version, in case you need it, can be obtained by running the command /usr/libexec/java_home.
3. Setting JAVA_HOME. By doing so one can override the choice that is made in the Java Preferences application. If the JAVA_HOME is set, the command line applications will use that version. However, /usr/libexec/java_home will still return the path of the version that was selected in Java Preferences.

“Magic” versions. In addition to the different Java versions, the Versions directory also contains several “magic” directories:

Versions/CurrentJDK/ is the hardcoded default version of the OS, so on Leopard it will always be an alias pointing to /Versions/1.5. Note that this is not affected by whatever version is selected in Java Preferences or via JAVA_HOME. Messing manually with the symlink to make it point to a different version is probably not a good idea. /Library/Java/Home points here.

Versions/Current/ is an alias that points to Versions/A/. This, in turn, is not a proper Java version like the other directories in /Versions/. It contains internal parts of the OS X Java machinery, e.g., in Versions/Current/Commands there are “fake” binaries such as java, javac and javadoc that internally use /usr/libexec/java_home (and JAVA_HOME, if set) to find the “real” binary. When you call Java commands from the command line, you actually invoke these “fake” binaries (via symlinks in /usr/bin). These are system internals, and poking around in there too much is probably not a good idea.

Finding the current version. So, what is the correct way to determine the location of the current Java version on OS X, say from a shell script?

1. If JAVA_HOME is set, use that.
2. Otherwise, invoke /usr/libexec/java_home to find the path.
3. If that fails, fall back to /Library/Java/Home.

Should you set JAVA_HOME? If you don’t need it, don’t set it at all. If you need it, setting it via

export JAVA_HOME=`/usr/libexec/java_home`

is probably not a bad idea, because this will reflect future changes to the selected version from the Java Preferences application.