Advice + proposals regarding automodule naming
rfscholte at apache.org
Mon Jan 16 09:37:08 UTC 2017
This is a message from Robert Scholte and Brian Fox. We both have been
talking about this topic several weeks with other Maven developers and
came to the conclusion that we should warn the jigsaw team with their
current approach regarding auto modules. We will share our experiences,
thoughts, conclusions and will suggest two proposals.
Traditionally, the Java ecosystem has been very mature in terms of naming
and namespacing. The reverse fqdn introduced into the java package was a
great choice to ensure classes don’t conflict. Popular build tools such as
Maven and nearly all those that followed built upon that this key concept
with the introduction of “GroupId” also using the fqdn as part of the name
to ensure the coordinates were properly namespaced.
We’ve seen some ecosystems diverge from this leading to new challenges
that ultimately had to be reversed. A great example can be seen in the “
tragic mistake from npm creators ”  which was to launch without a
namespace concept. Eventually, NPM started running out of useful names and
had to backtrack to introduce “scopes” which is really just a namespace
. The real problem here is that the major change in namespace was
backed in after several years of momentum without it. It’s taken a long
time for tooling and best practice to catch up to scopes and in the
interim, people have been left with a dual mode, some namespaced, some not
namespaced situation that has created chaos. 
The real issue at hand here as we consider behaviors in the jigsaw
automodule revolves around two well studied concepts.
The most important is the “Default effect”  which states that whatever
the default behavior is will become the most prominent best practice. A
default that uses a filename to generate a very short, un-namespaced
module id effectively sets the behavior to create generic names that will
eventually conflict...exactly what we’ve seen in npm.
Additionally, The switching costs introduced in overcoming a default
un-namespaced module id to one with a unique namespace is also significant
once you consider all the potential users. This is why API change is hard,
and changing the module id after the fact from the default is effectively
an API change.
The second principal at hand is the “Principle of least astonishment”. We
want to find a default that doesn’t violate what most users would consider
to be the most obvious. One could argue the current auto module algorithm
doesn’t violate this principle, but it’s important to consider alternate
suggestions in this light.
First, lets explore the potential downsides if the default effect takes
hold with the currently generated auto module id. In Apache Maven, the
artifact id is the part of the coordinate that generates the filename.
This means that com.somecompany:artifact:version will become
artifact-version.jar, which would result in automodule id “artifact”.
Armed with this understanding, that does an analysis of the Maven
ecosystem have to say about potential conflicts in the automodule id?
If we ignore the groupid and version of all the components in the Maven
Central repository, we end up with over 13,500 (7% of the total
group:artifact combinations) conflicts. This does not consider conflicts
across other repositories, or within customer portfolios yet it is pretty
telling. Conflicts will happen. In some cases, the number of conflicts on
the same common names is well above 100. The list of conflicts as of
October, 2016 can be seen here. 
At this point, hopefully we’ve made the case for at least establishing a
default module id that
1. Uses namespaces to minimizes id conflicts when possible
2. Leverages the default effect to create a de facto best practice
3. Follows the principle of least astonishment
We have two potential proposals that solve these goals.
Proposal 1: Leverage existing coordinates when available.
Maven is inarguably the most popular build system for Java components,
with Maven Central being the default and largest repository of Java
components in the world. By default, every jar built by Maven
automatically gets a simple properties file inserted into it with its
unique coordinates. Now, not every jar in Central was built with Maven,
however 94% of them were, as we can find the pom.properties file in
1,806,023 of the 1,913,561 central components . Talk about the default
effect in action!
It’s further important to recognize that given a jar with a pom.properties
declaring coordinates, it means that the project itself has chosen those
coordinates as their own name. In other words, this is how they refer to
themselves, even if other consumers may not be using Maven directly.
If automodule were able to peek inside a jar and generate the default id
using the groupid and artifactid present in the file, this would nearly
eliminate all instances of id conflict because a significant portion of
the Java ecosystem is in fact built with Maven. Additionally, the fact
that 1.8 million (and counting) modules would have namespace as the
default behavior means we’ve taken a huge step in setting the best
practice of picking module ids with a namepace. Additionally, since the
project itself has chosen these coordinates and uses them as their primary
distribution mechanism, this follows the principle of least astonishment
to consumers regardless of their chosen build system. Finally, since all
of the above are true, it’s unlikely the project would need to migrate to
a new module id when they adopt jigsaw natively, thus avoiding an API
switching cost for their users.
Proposal 2: Drop automodules
Right now Jigsaw tries to calculate a module name solely based on the name
of the jar file, which now already causes issues. Besides the fact that
the module name is not guaranteed unique compared with its Maven
coordinate, there are extra transformations which makes it even less
guaranteed that it is unique; e.g. dashes are replaced by dots (which are
both valid artifactId characters), in some cases the number and their
following characters are stripped off. For artifacts like
jboss-servlet-api_4.0_spec it makes sense, however we already see issues
here where commons-lang, commons-lang2 and commons-lang3 get the same
even though they have different artifactIds and contain different
packages. Choosing different artifactIds and packages was a very wise
decision because it made it possible that these jars could live next to
each other. Removing that separation by the authors is a very unwise
Another known example is the jsrNNN jars, which now all get jsr as the
Is it highly unlikely there is one single rule to capture all the use
cases and which always result in a module name we can work with.
For that reason the other proposal is to simply drop automodules. Don’t
try to come up with a name for unnamed jars. It might look like the
feature of automodules makes migrating easier because every dependency
will get a name so can complete your module-info for all requirements, but
we expect that once Jigsaw comes to speed the invalid module names are
actually blocking further development due to name collisions or forced
renaming by transitive modular jars.
The advantage of this proposal is that library builders are not forced to
keep the proposed module name in order to maintain backwards compatibility
with the default.. Instead library builders can pick a more suitable
module name. The modular system doesn’t allow the same package to be
exported by multiple jars (and automodules exports every package). Library
builders can fix this is their new jars, however if end users would
require both jars because they were specified as requirements in different
transitive jars, you cannot compile this project. There’s just no
dependency-excludes like Maven has, because “requires” in the module-info
really means requires. Dropping automodules will prevent these kind of
issues, because a package can only be exported by a named module.
Sure, this means that for end users they cannot refer to every jar in
their module-info. But at least if they add a “requires” to their
module-info, they can ensure that it’ll always refer to the intended
modular jar. With build tools like Maven the chance of missing artifacts
on the classpath has already been reduced a lot. In general builds have
become quite stable, so we don’t expect that developers will translate all
dependencies to the module-info file, especially if we warn them about the
possible consequences of depending on automodules. Only referring to named
modules and even a single “requires” is already a gain. There’s no reason
to try to speed this up and give the developer the false impression that
it’ll keep working when upgrading to real modular jars. Focus should be on
the target, not on the path how to reach it.
Dropping the automodules will prevent a lot of discussions about what is
the correct way to select a module name and will give the responsibility
for the name back to the place where it belongs: the developer.
 The fact that so much of the npm ecosystem is effectively
not-namespaced is has actually
created potential build time malware injection possibilities. If I know of
a package in use by a
company through log analysis, bug report analysis etc, I could potentially
go register the same
name in the default repo with a very high semver and know that it’s very
likely this would be
picked up over the intended internally developed module because there’s no
 http://openjdk.java.net/jeps/261 #Risk and assumptions
More information about the jpms-spec-experts