Re: Separating out RMIClassLoader dependencies

Sun, 18 Oct 2009 16:40:54 -0700

Ignore this message, it was sent during drafting. The real message will follow. 

Peter Firmstone wrote:

Thanks Nic for your comments, pls see reply below.

Niclas Hedhman wrote:

A big chunk of your bundle analysis already exist in a widely used OSGi
tool, named BND. Most OSGi developers use it, since it is near impossible to 
create the metadata by hand, especially the "uses" constraint.

Definitely agree, quite useful for eliminating syntax errors.

But before that we spoke of figuring out how a particular implementation
wire up a random set of bundles. That is not predictable, nor does the OSGi spec provides an API to find out. Well, kind of... If you have the loaded 
class, you can ask from which bundle it came, but unfortunately the
StackElement doesn't give you the Class, only the class name, which isn't 
enough.
Bytecode analysis, can identify all method signatures, all fields, the level of access, public, package private, protected or private.
It can also identify the serialVersionUID for classes that implement Serializable.
In addition if a bundle is used, the Metadata in the bundle indicates packages to be exported, which defines the publicly accessible API.
This information can be stored in Static Mirror Objects, for each bundle, package and class.
It is also possible to support the Java Product Versioning Specification, the difference being that there would be one Classloader per package, instead of per bundle. Public and Protected Public Class API signatures and serialVersionUID's stored in class mirror objects, would determine API compatibility. Preference would be given to compatible Package Implementations that had greater version ID's.
Since the Static Bytecode Anaysis would allow identification of API signatures, most importantly a Packages Public API, Protected API and Class serialVersionUID's, it allows comparison of API between different package versions. - Package API.
Special consideration has to be given to Package API that is shared between multiple packages, lazy loading requires some thought too. I think all dependencies will have to be resolved prior to loading for all packages. Failed resolution would require a JVM restart, with state persistence.
It's possible to additionally support the Java Product Versioning Specification, where maximum sharing of classes is desired, eg globally visible classes, as this increases the number of classloaders and reduces the probability that a Package API conflict cannot be handled gracefully, considering the size and availability of OSGi bundle repository's, the likelyhood isn't great, but it may be useful where new software is undergoing rapid development and API revision, where shared classes need to be persisted.
Perhaps I should name the Mirror Interface, in Reflective Terms, StaticMirror - to indicate it doesn't change, with implementations such as ClassStaticMirror, PackageStaticMirror and BundleStaticMirror, Mirror objects produced by analysis could be stored in a hash that looks up packages by name.
I was also thinking that for dependency resolution, I could have a PackageDependencyMirror Object, which ClassLoaders could submit to a codebase service for resolution of the latest compatible packages. The required packages 





But I agree with you that with Jini tech and OSGi's well-defined
classloading semantics one can do really cool stuff. Including getting rid 
of some of the more troubling Jini aspects (codebase).

-- Niclas

On Oct 16, 2009 8:21 PM, "Peter Firmstone" <[email protected]> wrote:

Hmm, that sounds like opportunity.
A dedicated codebase service has all the time in the world to burn processor 
cycles on analysis, as its main task is simply serving up jar files over
networks.
Bytecode analysis, identifies class, package and possibly module API, which can be stored with the harvested metadata in mirror objects, one for each class, package and module. Optional Package metadata could be a potential 
source of information too.
Bundles, depend upon and export packages. Once the API is identified using 
bytecode analysis, fast comparison using mirror objects could identify
compatibility.

This could be utilised in two ways:

1. As a check of backward compatibility for modules / packages over
different release versions, for substituting later compatible bundle
versions, if desired.
2. When a dependency on a package only utilises a subset of that package, the actual API requirements may still be satisfied, even though different release versions of that package may not usually be interchangeable or fully 
compatible.
A module or bundle would exist within its own classloader, in the local jvm. The packages or modules upon which it depends could be made available from 
other classloaders.
Bundles could be uploaded to codebase services or a codebase service could retrieve the bundles from designated repository's, perform analysis, then 
make the bundles available in a location independent manner, to prevent
codebase loss and allow for redundant codebase services.
Then all one need do is to upload application bundles to the codebase server 
and register a service, the service and bytecode could be provided
independently, the codebase service can provide entire application bytecode 
built on Jini services and other third party libraries.

Have a look on springsource, there are many OSGi jar bundles available,
these are simply jar files with Metadata.
Someone's done so much hard work already, why not ride the wave? There are 
already support tools available to create application bundle manifests.
The reason I'm considering bundles, is it reduces the number of classloaders required, one per bundle as opposed to one per package. One cannot rely on 
standard java Package meta data to exist in jar files.

********This doesn't require an OSGi framework.*********

http://blog.springsource.com/2008/02/18/creating-osgi-bundles/

http://www.springsource.com/repository/app/bundle?query=A

http://www.springsource.com/repository/app/faq

>From the website:


    What is the SpringSource Enterprise Bundle Repository?
The SpringSource Enterprise Bundle Repository is a collection of open source libraries commonly used for developing enterprise Java applications with the Spring Framework. The repository contains jar files (bundles) and library definition (".libd") files. A library defines a collection of bundles that 
are often used together for some purpose (e.g. the "Spring Framework"
library). There are hundreds of bundles contained in the repository.

The repository meets the following criteria:

  * Every jar file in the repository is a valid OSGi bundle. Any jar
    you download from the repository can be deployed as-is into an
    OSGi Service Platform and the SpringSource dm Server. It can also
    be used as a regular jar file outside of OSGi.
  * Every bundle and library has full version information associated
    with it. The package export information for a bundle contains
    version information, and the package import information for a
    bundle contains full version range compatibility information.
  * The repository is transitively complete. The mandatory
    dependencies of any bundle are guaranteed to also be in the
    repository. Most of the optional dependencies of any bundle in the
    repository will also be present. The bundles listed in any library
    definition are guaranteed to be in the repository.
  * The repository is self-consistent. Before any artefact is uploaded
    to the repository, we verify that it can be installed, resolved,
    and started in an OSGi Service Platform (using the same profile as
    the SpringSource dm Server) alongside all of the other bundles in
    the repository.
  * The repository can be used from Ivy and Maven based builds.

Cheers,

Peter.
Niclas Hedhman wrote: > > On Fri, Oct 16, 2009 at 1:04 PM, Niclas Hedhman < 
[email protected]> wro...

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