Compiling Java code for all target platforms at once

[erik.joelsson at oracle.com]

On 8/25/22 4:11 PM, some-java-user-99206970363698485155 at vodafonemail.de 
wrote:
> Hello,
> the JDK source code contains many Java classes which are specific to one platform / operating system, for example
> classes to interact with the file system or the AWT and Swing related classes are different for Windows and Linux.
>
> Is there a way to compile the Java source code for all platforms supported by the JDK at once? My goal is not to build
> a functional JDK, but to use the class files (or rather the compilation process) for static code analysis, specifically
> for analysis with CodeQL. I assume this might also be useful for other code analysis tools, and maybe also in general
> to detect Java compilation errors with the JDK source code.
>
> It appears the OpenJDK building instructions are always for building for a specific target platform and therefore
> skip compiling the Java code for other platforms. Additionally the `configure` command checks also check for tools
> needed to compile native code (e.g. Xcode or Visual Studio), even though for pure Java compilation they would not be
> needed (?). Therefore when you try to use multiple configurations (as described in the build instructions) it would
> require you to install all the tools for all platforms (if that is even possible on the same host operating system).
>
> Ideally I am looking for a solution which satisfies the following:
> - Compiles the Java source code for all supported platforms
> - Compiles all the Java source code in one build command execution
>    - Or at least compiles the Java source code with separate build command executions, but which can all be
>      run from the same host operating system (no need to use separate operating systems)
> - Does not require any of the tools for native compilation to be installed
>
> Though I am happy about any solution which gets as close as possible to this. Thanks already in advance!

This is not a usecase I have ever encountered before, and the build 
system does not have anything close to supporting it. I don't think you 
can achieve this without some serious effort.

Even if you were to include all the java source dirs, I'm pretty sure 
you couldn't compile them straight into the same output dir. Different 
versions of the same java class can appear in different platform 
directories. We also have a lot of generated java code where some may 
get generated differently depending on platform. For example in 
make/modules/java.base/gensrc/GensrcMisc.gmk, we use the native 
preprocessor to generate constants into a java class.

It's not possible to cross compile all platforms from any single OS, so 
you can't provide native toolchains for separate builds on the same host.

Depending on how close to correct you need the java class files to be 
produced, it could be possible to hack around in the build system to 
achieve something that is perhaps close enough, but as I said already, 
it won't be trivial. I doubt we would want to incorporate any such 
change into the build permanently without some very strong motivation.

/Erik