[jira] [Commented] (MJAR-62) Build-Jdk in Manifest.mf does not reflect which compiler version actually compiled the classes in the jar

["Jorge Solórzano (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/MJAR-62?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17693678#comment-17693678 ] 

Jorge Solórzano commented on MJAR-62:
-------------------------------------

This issue as it stands out today is irrelevant, and it actually does the correct thing, let me explain:

The {{MANIFEST.MF}} does not reflect the version of the compiler that built the jar because you can compile a class using a higher JDK version that targets a lower Java bytecode, so assume that you build a JAR that compiles the classes with JDK 17 but targets (–release) Java 11, which version do you use as *Build-Jdk* option? you are using Java 17 so it makes sense to set the version to {+}17{+}, but at the same time, you are compiling to Java 11 so you could argue that it must use {+}11{+}, but that is not true, and even the bytecode produced could be different.

But even if we try to use the "same" version in the *Build-Jdk* property that compiles the classes, the most important point that this is irrelevant now, is that we now have Multi-Release JARs ("MRJAR"), that contain bytecode generated for different Java versions in a versioned area of the JAR, so, if you build a JAR which compiles the base classes as Java 8, and have versioned classes for Java 9, Java 11, and Java 17, which version do you set in the *Build-Jdk* property? If you use the lower one (Java 8) that is not correct since it contains classes with different bytecode, and if you use the higher one (Java 17) that could not be entirely true as you might probably build the JAR using JDK 19.

In the end, the *Build-Jdk* should be used more as a property to signal which JDK was used to build (or a better term "assemble") the JAR rather than to know the compiled bytecode contained, when I mean that it actually does the correct thing is because if the JDK used to assemble the JAR is the one that Maven uses, then that's the JDK to be used in the property.

NOTE: This has no relation with the issue MJAR-289 since Toolchains handling is a different topic and this issue does not duplicate that, and while I agree that toolchain support could be improved, the same premise should be done, if maven-jar-plugin is aware of the toolchain, and the toolchain of maven-compiler-plugin is different from the toolchain used in maven-jar-plugin, which version should be used in the property? of course the one that uses maven-jar-plugin, yet doesn't need any special treatment since it will use the "java.version" property which again is the correct thing to do.

> Build-Jdk in Manifest.mf does not reflect which compiler version actually compiled the classes in the jar
> ---------------------------------------------------------------------------------------------------------
>
>                 Key: MJAR-62
>                 URL: https://issues.apache.org/jira/browse/MJAR-62
>             Project: Maven JAR Plugin
>          Issue Type: Bug
>            Reporter: Stefan Magnus Landrø
>            Priority: Major
>         Attachments: example-app.zip
>
>
> Manifest.mf does not reflect the version of the compiler that built the jar, but defaults to the version that maven runs under:  Build-Jdk: ${java.version}.
> I believe this makes users uncertain of which compiler was actually used to build the classes.



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