Will code compiled using 1.5 and 1.6 run on a 1.4 JRE? We weren't sure which Java versions the 1.4 JRE supports.
We know that if the code in question implements 1.5 or 1.6 supported features then it definitely won't compile... and that there are some risks with "backwards compiling" but wasn't sure if the 1.4 JRE would refuse to even load the 1.5/1.6 compiled classes or not.
Update: I confirmed you get a java.lang.UnsupportedClassVersionError exception if you run an 1.6 class file on JRE 1.4.
I don't think it will.
Occasionally (for reasons too complicated to explain), I try to run code in a 1.5 JRE that I compiled in a 1.6 JDK. It typically throws a java.lang.UnsupportedClassVersionError exception.
Only if you compile with javac -target 1.4 switch.
Obviously you will not be able to use 1.5+ features, such as Generics, Executors, etc.
You can cross-compile. This document shows you how:
http://java.sun.com/javase/6/docs/technotes/tools/solaris/javac.html#crosscomp-example
You must specify the specific major version you're targeting (1.4, it sounds like).
Using this technique, your best bet is to always use the newest javac you can find! That way you have all the latest bug fixes and performance improvements, and it's perfectly safe.
EDIT: note that this does address the problem of library incompatibilities, which was discussed in several answers!
Yes and no. You can run code compiled under Java 6 on a 1.4 jvm if you set the "source" and "target" javac options to the version you are targeting (e.g. 1.4) when you compile them. This will work in cases where you have not used any of the additional classes or language features that were added since the target version.
Good luck.
If you just build with the defaults of javac 1.6, your class files will not work on old versions of Java.
The best way to compile for older Java is just to use the older JDK releases. But if you really want to try to compile for older Java from newer, here are some instructions:
You also might be interested in http://en.wikipedia.org/wiki/Java%5Fbackporting%5Ftools
Yes, you can produce class files that are compatible with 1.4 with the 1.6 compiler (javac) however, simply doing this is not necessarily going to produce code that will work. The problem is that that will still compile against the 1.6 version of the API.
At first glance you would not expect this to be a problem since the contracts should not change but it is - I had a problem in that a new constructor that takes IIRC an integer was added to BigDecimal (in 1.5) and so at compile time the call to that constructor was specified however at runtime that constructor did not exist and so a runtime exception. You're probably going to have issues like this when methods are overloaded and you're relying on auto variable conversion.
The Javac app is in fact independent of the version of Java that it belongs to - you can specify a different API to use against 1.6 javac and in order to obviate any runtime issues this should be done.