RPKG Overview
Alternative design and implementation of the java package concept due
to a design flaw in the java class loader interface. In this case,
RPKG, sealed packages with individual manifest records are represented
with package information identities.
This package subsystem (RPKG) is able to represent packages in foreign
(child) class loaders by virtue of its initialization proceedure.
Problem statement
A design flaw in the implementation of the java package subsystem
prevents dereferencing a package from a foreign class loader.
The structure of the class loader runtime is a series of class loaders
that incrementally expand the scope of the class path. This class
loader "chain" is available using the class loader "parent" method.
The "parent" relationship is a singly linked list of class loaders,
from the "boot class path" class loader to the "application class
path" class loader, and then possibly including additional scopes.
Each class loader manages the loading of classes, and packages, from
its class path. A library managed by a parent is available from its
child, but the library managed by the child is not available to the
parent.
java/lang/ClassLoader
java/lang/Package
# /usr/lib/jvm/java-8-openjdk-amd64/src/java/lang/ClassLoader.java
1604 /**
1605 * Returns a <tt>Package</tt> that has been defined by this class loader
1606 * or any of its ancestors.
1607 *
1608 * @param name
1609 * The package name
1610 *
1611 * @return The <tt>Package</tt> corresponding to the given name, or
1612 * <tt>null</tt> if not found
1613 *
1614 * @since 1.2
1615 */
1616 protected Package getPackage(String name) {
1617 Package pkg;
1618 synchronized (packages) {
1619 pkg = packages.get(name);
1620 }
1621 if (pkg == null) {
1622 if (parent != null) {
1623 pkg = parent.getPackage(name);
1624 } else {
1625 pkg = Package.getSystemPackage(name);
1626 }
1627 if (pkg != null) {
1628 synchronized (packages) {
1629 Package pkg2 = packages.get(name);
1630 if (pkg2 == null) {
1631 packages.put(name, pkg);
1632 } else {
1633 pkg = pkg2;
1634 }
1635 }
1636 }
1637 }
1638 return pkg;
1639 }
A library using the java package object class is unable to dereference
packages found in a child class loader because access to the java
package subsystem is protected.
Problem solution
As a result, the design flaw needs to be repaired by replacing the
package concept.
RPKG initialization
Each leaf or tail of the class loader chain needs to be initialized.
Package.Init(Main.class);
This action creates and stores each sealed package listed in each
manifest in the available class path.
RPKG examination
The effect of initialization may be examined by listing the contents
of the package subsystem.
Package.List(System.out);
JAR manifest
Name: syntelos/rpkg/
Sealed: true
Implementation-Title: "syntelos.rpkg"
Implementation-Version: "1.0.3"
Implementation-Vendor: "John Pritchard, Syntelos"
Each package available to the RPKG package subsystem has a JAR
manifest entry that is sealed.
Update
2018/12 -- 1.0.1
[DONE] Export package list from user class loader.
2018/12 -- 1.0.2
[DONE] Export package init class loader.
[TODO] Review package init, which presumes jar files.
2018/12 -- 1.0.3
[DONE] Read archive manifest from manifest reference.