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From conflue...@apache.org
Subject [CONF] Apache Tuscany Docs 2.x > Classloading
Date Thu, 06 May 2010 08:09:01 GMT
Space: Apache Tuscany Docs 2.x (http://cwiki.apache.org/confluence/display/TUSCANYxDOCx2x)
Page: Classloading (http://cwiki.apache.org/confluence/display/TUSCANYxDOCx2x/Classloading)

Edited by Simon Laws:
Need to detail the various ways the Tuscany runtime loads classes and the issues involved.
Here are some notes to kick of this document

h1. Classloading Objectives

The runtime must work in both OSGi and non-OSGI environments. I.e. we can't rely on the OSGi
service registry for extensibility
The runtime must not be genberally environment specific. I.e. no buddy classloading

h1. General Patterns

h2. Extension loading - JSE

Tuscany finds extensions by looking for META-INF/services files on the classpath. 

h2. Extension loading - OSGi

It's a bit more complicated here. The extensibility-equinox bundle is given the entire OSGi
context at start up and from there is looks in all of the loaded bundles looking for META-INF/services
files. It caches them against the bundle in which they are found. 

The tuscany-extensibility-equinox bundle also has a dynamic import 

DynamicImport-Package: org.apache.tuscany.sca.extensibility.equinox,

Which allows it to generally load any classes in the runtime

h2. Split Packages - JSE

We don't take any special account of this in JSE (?)
We avoid split packages across the JARs we create as it messes OSGi up. 

h2. Split Packages - OSGI

We avoid split packages across the bundles we create
They may exist in third party bundles (or jars that we turn into bundles) so we need a way
round it

The Tuscany eclipse plugin is used to generate bundles manifest for jars which don't have
them. This is done automatically with all packages exported (?) and the resulting bundle it
in the distribution modules directory in the following form


The MANIFEST.MF is generated and will have a bundle classpath pointing to the jar (which doesn't
itself have a manifest

The runtime (node-launcher-equinox) has code to load these directories as bundles. 

There is a way of overriding these automatically generated bundles so that split packages
(or any other manifest problems) can be worked round. Generate the manifest manually and put
it in 


Update distribution/pom.xml to configure the Tuscany version of the maven bundle plugin to
apply this manifest

                                 <!-- artifactAggregations (below) is the right approach
to solving the split 
                                      package between axis-kernel and axis2-transport-http
however the Tuscany 
                                      runtime doesn't take any notice of it so using a fragment
at the moment -->

You'll note that there is an artifact aggregation element that doesn't work at the moment.
This should aggregate the two bundles together so that a split package isn't an issue. As
this doesn't work at the moment another way to achieve the same result is to make one package
a fragement of the other by configuring separate manifests manually.

NOTE!!!!! you also need to put the manually generated manifest in node-launcher-equinox\src\main\resources\org\apache\tuscany\sca\node\equinox\launcher
otherwise you'll spend a lot of time trying to get this to work. (we need to fix this!)

h2. Third-party libraries - JSE


h2. Third-party libraries - OSGI

Third-party libraries often rely on TCCL to load implementation classes in an extensible way.
For example, the SDO API loads the HelperContext implementation in this way. In an OSGi environment
there will not be a static dependency between the api bundle and the impl bundle so we need
to fake it. Typically we do this by setting up the TCCL appropriately before the library us

See ClassLoaderContext which help us to set up a multi-classloader configurations. 

Typically in OSGi one of the classloaders we pass in here will be the extensibiliy-equinox
bundle classloader (the ServiceDiscoverer) as this bundles has a dynamic import which allows
it to load any class in the runtime. 

h2. Tuscany Node API - JSE


h2. Tuscany Node API - OSGi
There are a small number of Tuscany Jars you need to use in the app launcher in the OSGi environment


The node API has to load the node implementation and has a dynamic import in its manifest

DynamicImport-Package: org.apache.tuscany.sca.node.impl,org.apache.tuscany.sca.extensibility

h2. SCA Client API - JSE

Factory finder impl is injected into the API class by the implementation

h2. SCA Client API - OSGi

NodeFactory maintains a NodeProxy inner class that supports cross-classloader calls. The calling
client api will have been loaded by the app classloader but the underlying node will have
been loaded by a bundle classloader. We need to bridge that gap.

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