When Does The JDK Compile Version Matter?

Question

I have two Java artifacts being built. One needs to be built in 1.6, because PowerMock isn't compatible w/ 1.7 and we are using it in a lot of unit tests. Refactoring PowerMock out right now isn't an option as it will take too much time.

However, I want to use this artifact in a Java application built in 1.7 and run the whole thing in 1.7. I think that it should be Ok since it is just building some class files, which I doubt changed much if any probably as far back as 1.2 or earlier. Anyway, I obviously have a fuzzy understanding of this and I am interested to get a Java experts deep dive explanation as to when this would matter, when it wouldn't, and why.

Thanks!

Answer 1

Java is usually backwards compatible between versions so anything compiled on an old version should run fine on a newer JVM. In fact a lot of common libraries are compiled in as old a version as possible (usually Java 5 now a days) unless they need a newer feature to allow more people who are still stuck on old JVMs.

Having said that, there are a few gotchas you need to worry about. One problem I had on some Java 6 to 7 conversion was TreeMap with an initial value of null http://hariharanselvarajan-java.blogspot.com/2013/02/treemap-in-java-6-and-java-7.html

EDIT Here is a link to Oracle discussing what isn't compatible between 6 and 7 although I would imagine this only affects things that are recompiled: http://www.oracle.com/technetwork/java/javase/compatibility-417013.html

Answer 2

The compiled code should be backwards compatible, so if you run it all on java7 it shouldn't matter than some was compiled using java6.

When you try the other way you get an invalid major/minor version number error.

I would assume that you can mix & match java 6 and 7 code too, just as you can (with caution) mix and match pre & post generics java.