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From Apache Wiki <wikidi...@apache.org>
Subject [Harmony Wiki] Update of "OSGi-Harmony Runtime" by lvjing
Date Tue, 18 Aug 2009 05:17:01 GMT
Dear Wiki user,

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The following page has been changed by lvjing:

New page:
Introduction To OSGi-ed JRE

1 : The main purpose

OSGi has become one of the best ways to govern the jar files in a number of
applications Such as the power it exhibits in eclipse .And as we all know
,the Harmony runtime is a modularized one and each module , with its
manifest file , is just like one bundle in the OSGi framework .So we are
think of governing the modules in Harmony runtime in a OSGi-ed way.

2 : Ways to meet the main purpose

      Naturally, there are two ways to meet this purpose:

A  Implementing the OSGi framework in VM in C/C++ to govern all the java

B  Using one of the OSGi-ed framework implemented in java to implemented a
half OSGi-ed JRE.

      Finally I picked the plan B with the well known OSGi framework :Felix
.And the reasons are as follows

First : Implementing a OSGi-ed framework from scratch in c/c++ in the vm is
difficult and time-consuming .

Second : Plan A suffers from the facts that implementations differs huge
from VM to VM .Plan B won’t have this kind of problem as long as the modules
comply with the OSGi specifications and we have a adapted java.exe , the
rest is nearly the same.

Third : Beside the framework ,each OSGi implementation has a lot of add-ons
.This will facilitate future development of new features and utilities .For
example : the could use the bundle fileinstall to install all the jar files
lies in a folder ,very helpful in certain circumstances.This could be done
rapidly if we choose plan B .

3 : Problems and fixes

The difficulties ,or the main tasks ,of this project are as follows

A     Find the minimum working environment of OSGi-framework and modify the
rest modules in Harmony ,mainly the manifest file ,to resolve the coupling

B     Modify the classloading mechanism and booting of the jre to be able
run normal java applications

And the fixes are

For A:

Harmony is a modularized runtime .So determine the minimum working
environment is no more than choose the right module to stay with the VM
.However ,although the coupling is little , it does exist . So a
modification of the module is necessary ,mainly the manifest file .And as I
want  to remain the original structure of Harmony as largely as possible
,more contents (classes ) will be included in the minimum environment other
than splitting the modules apart .The final jre include the following
component :












the ICU4J folder.

The remaining jars are governed by the OSGi framework to handle. One more
thing to mention is the two classes
,”java.beans.PropertyChangelinstener”,”java.beans.PropertyChangeEvent” has
to be moved out of beans.jar .This is because of the coupling of beans.jar
with other modules in the minimum runtime .Well the good thing is these two
classes havn’t imports any other classes in the modules governed by the OSGi
framework .I think we could include them in any modules lie in the minimum
runtime as long as the VM could find them .

For B:

I added one extra sphere between the VM and the main class to run
.org.apache.osgi.FelixStarter. , the following is done in this class

First: Loading the configurations and Start Felix ,the OSGi framework we use

Second: Initializing the class org.apache.osgi.OsgiClassLoader ,this class
will load the main class and subsequently all the classes used in the main

Third: Transfer the arguments to the “real “ main class ,invoke the main
method .

The sequence of class loading is as follows :

First : Check the class name to determine whether it should be loaded by the
parent class loader .This work is done by specifying the
“org.osgi.framework.bootdelegation” property to include all the package
names in the modules stays in the VM .

Second : load the classes in the bundle governed by the framework .I will
examine each bundle's export and import property to search for the class
name ;

Third : the classes in the application’s class path .

Finally : we could not find the class and a ClassNotFoundException will be
thrown .

Both the OSGi framework and the Harmony do not allow the define , imports or
exports of java.* classes and packages ,so a lot of examines has been
removed to facilitate our purpose .

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