Java Home

Many Java applications need to know the location of a $JAVA_HOME directory. The $JAVA_HOME on Mac OS X should be found using the /usr/libexec/java_home command line tool on Mac OS X 10.5 or later. On older Mac OS X versions where the tool does not exist, use the fixed path "/Library/Java/Home". The /usr/libexec/java_home tool dynamically finds the top Java version specified in Java Preferences for the current user. This path allows access to the bin subdirectory where command line tools such as java, javac, etc. exist as on other platforms. The tool /usr/libexec/java_home allows you to specify a particular CPU architecture and Java platform version when locating a $JAVA_HOME.

Another advantage of dynamically finding this path, as opposed to hardcoding the fixed endpoint, is that it is updated when a new version of Java is downloaded via Software Update or installed with a newer version of Mac OS X. For this reason, it is important that developers do not install files in the JDKs inside of /System, since the changes will be lost with subsequent updates by newer versions of Java.

To obtain the path to the currently executing $JAVA_HOME, use the java.home System property.

Extension Libraries

Java software on other platforms often makes use of the $JAVA_HOME/lib/ext directory within a JDK installation to store support class or jar files. While Java for Mac OS X also contains a lib/ext directory, developers should not directly modify it for the same reasons mentioned above. The /Library/Java/Extensions directory can be used for additional jar files or JNI libraries that need to be placed on the system classpath. For more controlled access, the ~/Library/Java/Extensions directory can be used for user-level installation of support libraries. Items placed in either of these directories do not need to be named in an application's classpath and will be available to all applications run under the respective scope (system-level or user-level, depending which directory is used).

User Preferences

On Mac OS X, the Java runtime provides the java.util.prefs API which is backed by the standard Mac OS X Preferences API and directories. Simply using this pure Java API reads and stores your application's preferences in ~/Library/Preferences in a Mac OS X property list file. For applications that may already have their own preferences format, these preferences should be stored in the ~/Library/Preferences directory as well. This directory can be reached from Java code by creating a file with the path of System.getProperty("user.home") + "/Library/Preferences/" + "com.example.your.Application". An application that should have global preferences across all users could instead reside in /Library/Preferences, however this directory is not writable by non-admin users.

User Data

Some applications store user's data in database files or save-less file schemes which are not conventional documents the user can move around in the file system. On Mac OS X, these files should go into a directory with your application's name inside of the ~/Library directory. Your app's directory in the top-level of ~/Library should only be used for irreplaceable user data, and not preferences (like window positions or recently-used document lists). You can create a path to this directory by appending System.getProperty("user.home") + "/Library/" + "Your App Name".

Caches

Applications that use temporary files to cache downloaded resources or complex calculations should store them in the secure Mac OS X temp directory. Use the java.io.tmpdir System property concatenated with a unique identifier for your application to create a place in the secure temp directory for your application. This location is periodically purged of files that have not been modified for several days, but is present on the startup disk.

If your application uses its own purging strategy, it may be appropriate to use the ~/Library/Caches directory in the user's home directory. Keep in mind that this location may reside on a network mounted file system or in an encrypted FileVault image. You can create a directory in this location by appending System.getProperty("user.home") + "/Library/Caches/" + "com.example.your.Application".

Using the /tmp directory is not recommended.