Github user tzezula commented on the pull request:
https://github.com/apache/ant/pull/16#issuecomment-199305236
Yes. Unfortunately the javac `-modulesourcepath` option for the multi
module compilation is more complex than existing path arguments. It's described
in [JEP 261: Module System](http://openjdk.java.net/jeps/261). The important
part is:
> In large systems the source code for a particular module may be spread
across several different directories. In the JDK itself, e.g., the source files
for a module may be found in any one of the directories
src/<module>/share/classes, src/<module>/<os>/classes, or
build/gensrc/<module>, where <os> is the name of the target operating system.
To express this in a module source path while preserving module identities we
allow each element of such a path to use braces ({ and }) to enclose
commas-separated lists of alternatives and a single asterisk ( * ) to stand for
the module name. The module source path for the JDK can then be written as
>{src/*/{share,<os>}/classes,build/gensrc/*}
In simple words the `javac -modulesourcepath
{src/*/{share,<os>}/classes,build/gensrc/*} -d build` means that modules will
be searched in the `src` or `build/gensrc` folders. The java sources of the
modules located in the `src` are in folders `share/classes` or `<os>/classes`
(relative to module). The java sources of the modules located in `build/gensrc`
are directly in module folder. The path before the `*` in the path element is a
path to module and the path after the `*` is a path in the module to the actual
source root.
During the multi module compilation the javac creates module directories in
the `build` folder containing the module classes.
The Ant Javac task needs to interpret the sourcepath elements to find all
modules and their java files to do the up to date check.
For example in a project structure:
```
src/java.base/share/classes/
src/java.logging/share/classes/
build/java.base/
build/java.logging/
```
and javac options `-modulesourcepath src/*/{share,<os>}/classes -d build`
the modules are `java.base` and `java.logging`. The `java.base` module has
sources in the `src/java.base/share/classes/` and is compiled into the
`build/java.base/` folder. The `java.logging` module has sources in the
`src/java.logging/share/classes/` and is compiled into the
`build/java.logging/` folder. The Ant Javac task needs to do up to date check
of sources in `src/java.base/share/classes/`
(`src/java.logging/share/classes/`) with classes in `build/java.base/`
(`build/java.logging/`).
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