Proposal: #AvoidConcealedPackageConflicts, #StaticLayerConfiguration, & #MultipleModuleVersions

Mon Mar 6 17:16:25 UTC 2017

Issue summaries
---------------
 #AvoidConcealedPackageConflicts -- Provide a simple means to load two
 modules, without using reflection, when they contain non-exported
 packages of the same name. This could be done by loading them via
 different class loaders or, alternatively, via
 #StaticLayerConfiguration. [1]
 #StaticLayerConfiguration -- Layers can, at present, only be created
 dynamically, via reflection. If there were a way to specify them
 statically, i.e., at startup time, then they could be used to solve the
 kinds of version conflicts that are presently addressed via shading or
 shadowing. [2]
 #MultipleModuleVersions -- Allow multiple distinct modules of a given
 name to be loaded in a convenient fashion, without using reflection.
 This could be done by creating new layers automatically, or by relaxing
 the constraints on multiple versions within a layer, or by some other
 means (cf. #StaticLayerConfiguration, #AvoidConcealedPackageConflicts).
 Addressing this issue may entail reconsidering the multiple versions
 non-requirement. [3]
Proposal
--------
These overlapping issues do reflect actual, practical problems. There
are, however, already effective -- if somewhat crude -- solutions to
these problems via techniques such as shading (in Maven) and shadowing
(Gradle). More sophisticated solutions could be designed and implemented
in a future release.
The lack of immediate solutions to these problems should not block a
developer who wants to modularize an existing class-path application.
If such an application works properly on the class path today then it
likely does not have conflicting packages anyway, since conflicting
packages on the class path almost always lead to trouble.
[1] http://openjdk.java.net/projects/jigsaw/spec/issues/#AvoidConcealedPackageConflicts
[2] http://openjdk.java.net/projects/jigsaw/spec/issues/#StaticLayerConfiguration
[3] http://openjdk.java.net/projects/jigsaw/spec/issues/#MultipleModuleVersions