Java 5 compatibility is hard

From the times when I read the bug thread of Forge (about a year ago or so), it seemed to me that a lot of users, especially those on Macs or at work, were still using Java 5. I have no idea how it is now, but I still try to code and compile Java 5 compatible for exactly that reason.

Usually, it works out one way or another, but not today. I wanted to try JGroups (a story for another time, after I actually succeeded in it), but even the releases that are advertised (if you can call it that) as Java 5 compatible seem to be compiled in the Java 6 binary format. Since only part of the featureset I'm trying to implement depends on JGroups, I wanted to at least create compatible class files, even if using these features will fail on a Java 5 runtime.

There are basically two inputs to the compiler: my own Java 5 compatible code, and the already compiled Java 6 compatible bytecode. My output of choice was Java 5 compatible bytecode. After all, this output does not contain anything not Java 5 compatible, so I thought I was good to go. Using a Java 5 compiler will of course fail, because it can't read the necessary classes, but Using a Java 6 compiler with compatibility settings should work...

There are a few features new in Java 5, like generics, annotations, enums and for-each loops. It sounds reasonable that compiling Java 5 source code to Java 1.4 byte code won't work. Java 6 also has a new byte code format. It supports a new verification mechanism, and while the compatibility notes say that the compiler outputs this format only "by default", there is no way (I have found, at least) to change it: The most obvious way of setting the compiler arguments: javac -source 1.6 -target 1.5 gives an errror that source release 1.6 requires target release 1.6 - which exactly contradicts what is said in the compatibility notes.

It turns out that the Java 6 compiler can take Java 6 bytecode input even if the source setting is 1.5. That there's an error for 1.6 + 1.5, even though the Java language hasn't changed, is kind of absurd in my opinion... well, better having an absurd solution than none at all...