Possible to generate bytecode for older java version?

[broesdecat]

Hi,

I am trying to generate java code for ros messages for Kuka Sunrise.
The basic setup works fine, except that Kuka Sunrise runs on Java 6, and my catkin workspace on docker runs on Java 8. Hence, generated java code for ros messages does not work on the robot (unsupportedClassVersionError). Due to various other issues, I cannot downgrade my container to a lower Java version (Gradle having issues with openjdk, oracle java<1.8 installers being no longer available).
So I was wondering whether rosjava / genjava support cross-compiling from Java 8 to Java 6, as that would also resolve my problem.

Thx!
Broes

[jubeira]

Hi @broesdecat!
Yes, it is possible indeed. You will have to compile rosjava_core again, adding

sourceCompatibility = 1.6
targetCompatibility = 1.6

to the build.gradle file inside subprojects task.
I did that myself and uploaded the resulting artifacts to a personal mvn repo here: https://github.com/jubeira/rosjava_mvn_repo/commits/java_6_improvements.

If you compile the code on your own, you can verify if the bytecodes are OK by extracting a class from a Jar, and then running
javap -v extractedClass.class | grep major

Hope it helps!

Edit: the same applies for message artifacts. In that case, go to genjava/templates/genjava_project/build.gradle.in and add the compatibility lines there. That file is used as a template script to generate the messages, so if you indicate to build them with Java 6 compatible bytecodes, the resulting messages shall be compatible.

[broesdecat]

@jubeira Great answer, worked exactly as advertised and does the job :) Thx!

[jubeira]

glad it helped 😄
could you close this issue please?