As some of you might have seen in the SVN logs, I have started to work
on making Cocoon work under OSGi again [1].
This will, for the moment, not affect the trunk more than that I might
refactor or reorganize some stuff based on needs for the OSGi integration.
The main reasons for working on OSGi integration is that OSGi makes it
possible to push the modularization of Cocoon a bit further.
* Blocks can be dynamically installed, started, stopped and updated
while the rest of the webapp continue to run. Take a look at Peter
Kriens demo from last year to see how cool this is [2].
* Each block (bundle) has its own classloader and import and export Java
packages from other bundles based on OSGi specific info in the
metainf.mf. In this way the "classloader hell" is handled. Several
versions of the same jar can be used in the same application.
* Each block has its own Spring context and publish and use services
(beans) from other blocks based on a special Spring-OSGi configuration [3].
For background on OSGi [4], [5] and [6] might be good. And the
specification [7] is fairly readable for being a specification.
The work on getting there mainly conisists of: bundelizing the Cocoon
jars and all needed jars, make beans OSGi services and creating
infrastucture for building, configuring and running Cocoon in OSGi.
Bundelizing
===========
To make a jar working in OSGi it must be converted to a bundle. Which
means that its Manifest.mf must be extended with some OSGi specific,
info about what packages it exports and imports and some other things.
Felix contains an excelent Maven plugin that makes this work rather easy
[8].
Also the Felix, Spring-OSGi, Eclipse Equinox and OPS4J has already
bundelized many popular Java libraries.
Services
========
The Spring-OSGi project [9] creates OSGi-Spring integration. There is a
custom name space with constructions for publishing beans as OSGi
services and for using OSGi services as Spring bean references. The idea
is that each bundle (block) has its own Sprin application context, and
that it can publish beans and use beans from other bundles through
OSGi:s service architecture.
There is a Spring OSGi extender bundle that automatically detects
bundles with a Spring configurations in the META-INF/spring directory
and takes care about its services.
Infrastructure
==============
Although the Cocoon OSGi integration should work in any OSGi
implementation I use Felix during development.
I have set up a Maven assembly that creates a directory structure with
all needed jars and configuration files. So that one just can start the
Felix OSGi framework with parts of Cocoon inside (see the README [10]).
For logging I use OPS4J Pax Logging [11], which makes all major logging
frameworks available in an OSGi friendly way.
I use an HTTP service from Eclipse Equinox as the Felix one doesn't
support Servlet 2.4 only 2.3. An activator [14], [15] has the same role
as the DispatcherServlet in the ordinary context. But it is dynamic so
it will detect and use servlet services that comes and goes.
The servlet protocol is available as a "real" Java protocol and is
available with java.net.URL, using the OSGi URL service.
Status
======
This far some basic examples that use the servlet service fw works. See
the README [10] for how to build and test the stuff.
I'll continue with bundelizing the Cocoon core.
There are a number of open problems:
* How to handle configuration
* Spring-OSGi doesn't support prototype scope that well
* How handle the massive "import" of services that is needed for a sitemap?
We had some partial answers to the above question last time we worked on
Cocoon OSGi integration. Hopefully they can be adapted to the Spring
OSGi context.
An overview of earlier work can be found in [12] and in my presentation
[13].
/Daniel
[1] http://svn.apache.org/repos/asf/cocoon/whiteboard/osgi/
[2] http://www.aqute.biz/Blog/20070703
[3] http://www.springframework.org/osgi/specification
[4] http://www.aqute.biz/OSGi/Tutorial
[5] http://felix.apache.org/site/presentations.data/osgi-berlin-20070321.pdf
[6] http://www.osgi.org/osgi_technology/index.asp?section=2
[7] http://www2.osgi.org/Download/Release4V401
[8] http://felix.apache.org/site/maven-bundle-plugin-bnd.html
[9] http://www.springframework.org/osgi/
[10] http://svn.apache.org/repos/asf/cocoon/whiteboard/osgi/README.txt
[11] http://wiki.ops4j.org/confluence/display/ops4j/Pax+Logging
[12] http://marc.info/?l=xml-cocoon-dev&m=114237414521595&w=2
[13] http://www.cocoongt.org/archive/2005/Slides-and-recordings.html
[14]
http://svn.apache.org/repos/asf/cocoon/whiteboard/osgi/core/cocoon-servlet-service/cocoon-servlet-service-impl-osgi/src/main/java/org/apache/cocoon/servletservice/osgi/Activator.java
[15]
http://svn.apache.org/repos/asf/cocoon/whiteboard/osgi/core/cocoon-servlet-service/cocoon-servlet-service-impl-osgi/src/main/resources/META-INF/spring/servlet-listener.xml
