Java 1.6 & Eclipse on Mac OS X

These are some notes on using Java 1.6 on Mac OS X with Eclipse.

Mac OS X lagged behind for some time on adopting Java 1.6, but it was finally released as Java for Mac OS X 10.5 Update 1. Unfortunately, this release has some limitations: it only runs in 64-bit mode and only on Mac OS X 10.5 (Leopard). This meant no Java 1.6 support on Mac OS X 10.4, no support for PowerPC systems, and no support on the early 32-bit Intel Macs released in 2006.

To make Java 1.6 the default Java execution environment, run the Java Preferences application (found in the Utilities subfolder of the Applications folder). There you can drag the Java versions into any order you wish for both applications and applets. So to default to using Java 1.6 for everything, you can just drag Java SE 6 to the top of both the application and applet lists. The Java Preferences application will change around symlinks to correspond to your new choices. Mac OS X 10.6 (Snow Leopard) appears to include only Java SE 6, but includes both 32 and 64-bit versions, so it should allow Java 1.6 to be run on the oldest Intel Macs (no PPC because Snow Leopard does not support PPC).

Some command line scripts expect the JAVA_HOME environment variable to be set to the directory that contains the Java distribution being used. Also, some scripts might require different versions of Java. To support this, Apple introduced a new utility called /usr/libexec/java_home. By default, java_home just returns the home value appropriate based on the selection in the Java Preferences utility, but command line arguments can request different versions (see the manual page linked earlier). The output of java_home is intended to be assigned to the JAVA_HOME environment variable in a shell initialization script (like .profile or .cshrc).

This brings us to Eclipse, the Java development environment. Eclipse can be used to develop for multiple Java versions. When creating a Java project, Eclipse prompts for the desired JRE version in the new project dialog box. To ensure that Eclipse tracks the language differences between versions, also select the desired version in Preferences->Java->Compiler->Compiler compliance level to match your project.

However, despite these changes, Eclipse itself does not necessarily run using the Java version being used for the project being developed. Initially, the latest version of Eclipse (3.5) was released only in 32-bit mode for both Carbon (an older and deprecated Mac OS API) and Cocoa (the modern Mac OS API). Since Leopard only supported Java 1.6 in 64-bit mode, this meant that Eclipse was always running under Java 1.5. With the release of Eclipse 3.5.1, there are now 64-bit Cocoa downloads available, and these will run under Java 1.6. For some users, this may not be important, but for those doing Eclipse tool development (such as the Hackystat Eclipse sensor) it is very helpful. I use JAXB in some of my Ant build scripts, and JAXB is built into Java 1.6 but not Java 1.5. When running Eclipse in Java 1.5, many of my Ant scripts report spurious errors about being unable to locate JAXBException, but these all vanish when running Eclipse under Java 1.6. I should note that at least some people disagree with this advice, and suggest using the 32-bit Cocoa version on Leopard instead of the 64-bit version. Everyone apparently agrees that on Snow Leopard you want the 64-bit Cocoa version of Eclipse (unless you are on a 32-bit Intel Mac).

So if you want to use Java 1.6 for your application and run Eclipse under Java 1.6 there are two options:

• Use Mac OS X 10.5 Leopard (with all software updates) on a Core 2 Duo Intel Mac, with Java Preferences set to use Java SE 6 as the default Java version, and use the 64-bit Cocoa version of Eclipse 3.5.1.
• Use Mac OS X 10.6 Snow Leopard on an Intel Mac, and use the 64-bit Cocoa version of Eclipse 3.5.1 [note, I have not specifically tested this configuration, but it should work. Confirmed, this works]