same Java Classes compiled with maven and mill give different runtime result

[fmantz]

Hello,

lately I was playing around with Mill. My plan was to adapt the build process a bit. My idea: I wanted to limit the size of the output jar by filtering the class files needed by some classes only. Therefore, I inspected the bytecode of my class files with Apache Commons BCEL, a library I had never used before, but it seemed like a good fit to me.

Strangely, my first steps using BCEL were behaving differently when I compiled it with Mill or IntelliJ. Then I tried Maven, and Maven behaved the same way as IntelliJ. I tried it using different JDK versions (Temurin) and also on my Mac... same result. I'm not so sure if Mill has really something to do with that; it's not a compiler, but I find it strange anyway:

Here is my example setup using Java only:
https://github.com/fmantz/compare_mill_maven

Has anyone an idea, whats the problem?

[lefou]

Mill is using Zinc to compile Java code, by default. It should produce the same bytecode as if you would invoke the JDK compiler directly, but maybe, you found a bug?

As you didn't provide a description of the actual problem, I just assume you mean the failing test case TransitiveHullTest.getHull finding the unexpected class G. Since I'm not familiar with BCEL, I can't classify that issue.

[fmantz]

I do not think that this is needed, thanks. It may be an option to opt it in for Java modules to have full byte compatibility but I guess this is required in rare cases only. In this case someone could simply override compile as you proposed.

[lefou]

You may still want to open an issue to zinc (or sbt), since this may be a bug or some not anticipated side-effect. Raising awareness is probably the best way to fix this issue in the long term.

[fmantz]

Yes, I will address the issue somewhere. If I de-compile the class files with Intellij the result looks okay. It could be that the byte codes are equivalent but BCEL is only aware of a "standard representation".

[fmantz]

Update: I found the problem ... maven seems to set by default a compiler flag "-g" which mill+sbt does not. I added the compiler flag manually in my build.sbt and the problem is solved (same result after fixing my build.mill file).

https://docs.oracle.com/en/java/javacard/3.1/guide/setting-java-compiler-options.html