Automatic module names

[Stephen Colebourne]

Back in October, I raised the issue of modules names generally and for
automatic modules specifically [1]. The short thread came to no
conclusion, but recent threads have again raised similar problems. The
problem is that automatic modules have magical name creation from a
filename, which is brittle and unlike anything else in Java.

I also recently looked at the Joda-Convert and Joda-Beans libraries,
to see if I could add module-info in preparation for Java 9. I quickly
backed away, again because of the same issue. Put simply, I am
unwilling to write a module-info file that refers to a dependency that
is not yet a module. And I have to advise all open source projects to
do the same. Given this, there can be no simple migration to the JPMS
for open source projects. Each open source project must wait for all
its dependencies to migrate to JPMS (by adding a module-info and
publishing to Maven Central).

The issue is clear. If I write this:

module org.joda.convert {
  requires guava;
}

where guava is an automatic module, I am locking in the name of the
guava dependency, something that I do not control. The name "guava" is
just a guess. The guava authors might choose "com.google.guava" or
something else entirely.

In a closed system of modules, ie. a private application, automatic
modules are fine, because the requires clause can be changed if it
turns out the guess was wrong. But once published as an open source
project to Maven Central or elsewhere, the guess cannot be fixed if it
is wrong (without releasing a new version of the library, which is not
an acceptable solution).

I also strongly believe that module names cannot be flat and
unstructured, such as "joda-convert" or "guava". They must have
structure, such as the domain name or a Maven-style group name
"org.joda.convert" or "org.joda:joda-convert". The potential for
clashes has been shown by the Maven team [2].

Some brainstormed possible changes:

- Remove the automatic module concept altogether

- Define a clear mapping from Maven Central co-ordinates to module
name that includes the group, artifact and classifier

- Provide a text file to JPMS that allows incorrect module names to be
mapped to the correct name

- Publicly advise against using automatic modules for open source projects

- Change rules of Maven Central to prevent modular jars being added
that depend on an automatic module

- Allow requires clauses to have aliases - requires org.guava.guava OR guava.

- Allow modules to have aliases - module org.guava.guava AKA guava


Given that applications can depend on libraries that haven't been
released in years, this has the potential to be a critical problem for
the ecosystem. My preference remains to define a clear mapping from
the widely adopted Maven Central naming strategy to JPMS modules.
Ideally, this would be a formal group concept in the JPMS, something
that I believe is sorely lacking.

Stephen

[1] http://mail.openjdk.java.net/pipermail/jigsaw-dev/2016-October/009631.html
[2] http://mail.openjdk.java.net/pipermail/jpms-spec-observers/2017-January/000707.html