I've been developing using Java on the Mac for a few years now. Historically (pre-2006 as much as I remember) Apple was pretty slow on getting JDKs working (remember the PPC Blackdown project), but as long as I've had by MacBookPro (Tiger+) developing has been pretty straightforward. There are some quirks about running various JDKs, but nothing too frustrating.

This changed with my Snow Leopard install. I upgraded to Snow Leopard a few weeks ago and didn't notice any problems until Friday when I tried doing a build to a Java 1.5 target.

My $JAVA_HOME = /System/Library/Frameworks/JavaVM.framework/Versions/1.5.0/Home
(which has worked fine for a few years)

But when I ran an ant task with build.compiler=javac1.5 (or target=1.5 in the <javac> task), I got classes compiled with java 1.6 (I could tell because when I ran "javap -verbose classname" I got major version=50).

Scratching my head for a bit I saw this message from ant "[javac] This version of java does not support the classic compiler; upgrading to modern" which made me think that something was wrong with my jdk.

When I ran "java -version" I got this output:
java version "1.6.0_15"
Java(TM) SE Runtime Environment (build 1.6.0_15-b03-219)
Java HotSpot(TM) 64-Bit Server VM (build 14.1-b02-90, mixed mode)

This kind of made sense because the java in my path was "/usr/bin/java" which symlinked to "/System/Library/Frameworks/JavaVM.framework/Versions/Current/Commands/java".

So I added $JAVA_HOME/bin to my path, like so: "export PATH=$JAVA_HOME/bin:$PATH". Now when I run "which java" I get "/System/Library/Frameworks/JavaVM.framework/Versions/1.5.0/Home/bin/java". But still when I run "java -version" I get the 1.6 output.

So after some googling, I finally come across this google groups article describing how Snow Leopard only includes jdk1.6.

This was confusing as if I look in /System/Library/Frameworks/JavaVM.framework/Versions I see:

but when I look more closely, 1.5 and 1.5.0 both symlink to CurrentJDK. Now there is a download that lets you have jdk1.5, but I found this very counter-intuitive that Apple would do this.

So I thought I would post an article with all the things I googled for an didn't see any matches to perhaps shave some minutes off the next java developer who is getting odd behavior with Snow Leopard. I am sure am glad I got this upgrade for free.

Update [2009.09.28 2251EDT]
: I discovered that if I tried to use some version name other than 1.5.0 I started getting this error:
"Shared archive: uninstalled generation
Bus error"
This was caused because even though I copied a 1.5 jdk to /System/Library/Frameworks/JavaVM.framework/Versions/1.5.0-leopard, when I tried to run java it was loading classes from /System/Library/Frameworks/JavaVM.framework/Versions/1.5.0/Classes/classes.jar (which pointed to the CurrentJDK). This cased the error above.

I fixed this by symlinking 1.5.0 -> 1.5.0-leopard.