I can second that by an example using Groovy. We have not yet taken
steps to make jigsaw modules, but we quite some time ago we have split
the code base in what would become eventually become modules in the
future. Because of history this did mean to split packages. So we have a
couple of jars (that want to be modules), with overlapping package names.
Examples of package conflicts with the "base module":
* The groovy-console has a conflict in groovy.ui
* groovy-ant has a conflict in groovy.util
* groovy-docgenerator in org.codehaus.groovy.tools
* groovy-nio in org.codehaus.groovy.runtime
* groovy-sql in org.codehaus.groovy.runtime
* groovy-swing in org.codehaus.groovy.runtime
* groovy-test in goovy.lang and groovy.util
* groovy-xml in org.codehaus.groovy.runtime and goovy.util
That's 7 "modules" with conflicts out of 19. One should remember here,
that the codebase is like 10+ years old, and something that is now a
"module" used to be fixed part of the language in the past. So some poor
naming decision have been made. The org.codehaus.groovy.runtime
conflicts could probably be solved, but the groovy.util ones are
difficult, since this is a package imported by default (like java.lang
in java), so moving classes to differing packages will break Groovy
scripts... very elementary here for example GroovyTestCase in
groovy-util, which is in the groovy-test "module"..
also... there is a automated meta class lookup system, that is based on
the package name with a prefix. So someone could provide a meta class
for java.util.ArrayList, while another does this for LinkedList. If they
are using modules, they cannot be loaded at the same time. Granted, I
don't like this mechanism, and I am looking for ways to deprecate it in
the near future, but it is another example of same-package conflicts.
bye Jochen
On 30.10.2015 12:28, Stephen Colebourne wrote:
On 29 October 2015 at 13:48, Alan Bateman <alan.bate...@oracle.com> wrote:
The restriction is that no two modules with the same package (irrespective
of whether it is exported or not, or who reads the modules) can be mapped to
the same class loader.
FWIW, I think that this could be a headache for module adoption.
Here are three cases that appear to be troubled by these restrictions:
- a large project that has taken an existing project (module) and
split it in two. In order to preserve backwards compatibility, the
author wants to retain the package names. But doing so is not
possible, because a single package now needs to be split across two
modules.
- extending a package scoped API from another project. While hardly
pleasant this is sometimes the only viable approach to get a task
done. As an example, the ImmutableList class from Google Guava has a
package scoped constructor. It is perfectly possible today for an
application to write a class that extends ImmutableList, so long as
that class is placed in the com.google.common.collect package. The
module system has the potential to block this extension point. (I
imagine that cglib and similar tools also generate code in other
peoples modules)
- where a stable interface is copied into two separate jar files to
avoid a dependency. An example of this was commons-beanutils where 4
interfaces were copied from commons-collections. While this has now
been changed, it was considered useful at the time, and given the
stability of the interfaces in question, caused no problems.
https://issues.apache.org/jira/browse/BEANUTILS-379
FWIW, I'm most concerned about the first one going forward. However,
the other two are cases I've seen used, and if used broadly in the
wild could slow adoption (if it is hard to make those jar files
modular, and they are widely depended on).
Stephen
