Friday, June 01, 2007

Module system interoperation kernel

Interoperation is currently a hot topic on the JSR 277 mailing list -- see the "Relationship to JSR 291" thread in the archive.

One of the approaches to interoperation is a 'kernel'. What follows is an extract from the README of the prototype plus the related class diagrams.


The goal is to enable JSR 277 to interoperate in a first class way with JSR 291 (OSGi). JSR 277 modules must be able to use JSR 291 modules and vice versa without significant restrictions.

The framework can probably enable other types of module systems to interoperate, but that isn't the primary goal.


The code [of the prototype] is only a strawman. It is not integrated with Felix or any other OSGi implementation. Nor is it integrated with JSR 277.

However, it outlines the interfaces that are likely to be needed to enable such module systems to interoperate well and provides several JUnit 4.1 testcases which drive the framework to illustrate the framework's features.

The code is licensed under the Apache License v2.0. See LICENSE.txt.

Module systems and wiring

The lack of a standard method of building delegation networks between class loaders requires each module system to interoperate specifically with each other module system. The framework provides a standard way for module systems to create delegation paths, or 'wires', to modules in other module systems.

We need a module system registry or 'kernel' to keep track of the available ModuleSystems. Something like the following.

public interface Kernel {
boolean registerModuleSystem(String moduleSystem, ModuleSystem ms);
boolean deregisterModuleSystem(String moduleSystem);
ModuleSystem getModuleSystem(String moduleSystem);
ModuleSystem[] getModuleSystems();

Module systems need to co-operate to resolve references to modules in other module systems. So each module system has an associated wire factory.

public interface WireFactory extends ModuleSystemRelated {
Wire resolve(Dependency dep);
Wire create(ResolutionContext ctx, Dependency dep);
Wire createLocal(ResolutionContext ctx, Dependency dep);
void complete(ResolutionContext ctx);

The most basic kind of dependency consists of a module name and a module version range. More sophisticated dependencies, such as package dependencies, are conceivable but omitted since they introduce much extra complexity.

A wire is a delegation path to the classloader of another module.

That leaves resolution contexts...

Resolution context

A resolution context is needed so that module systems can co-operate to resolve cyclic dependencies without ending up in an infinite loop. A resolution context also provides somewhere to anchor the state for sharing partially resolve modules and for back-tracking.

Constraint checking and back-tracking

A module system can anchor state off a resolution context so that, when asked more than once to create a wire for a given dependency, it can return each possibility in turn. To avoid interference between similar dependencies, new Dependency instances must be created for each module. If two modules have what appear to be the same dependency, then there need to be two Dependency instances.


The kernel class hierarchy shows the kernel with which module systems are
registered. Module systems and resolution contexts are each related to a
particular kernel (of which there is usually one per VM).
The module system class hierarchy shows wire factories, dependencies, and
wires which are each related to a particular module system.

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