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Re: [equinox-dev] help understanding osgi.parentClassLoader

equinox-dev-bounces@xxxxxxxxxxx wrote on 06/01/2008 10:36:04 PM:
> Hi,
> I'm new to OSGI so please bear with me if they are basically naive questions.
> I'm using Equinox's latest version, I've gone through the mailing
> lists and OSGI's core manual especailly the Module layer section and
> found lots of very important information.
> Under Eclipse's runtime options there is
> osgi.parentClassLoader
>     the classloader type to use as the parent classloader for all
> bundles installed in the Framework. The valid types are the following:
>         * app - the application classloader.
>         * boot - the boot classloader.
>         * ext - the extension classloader.
>         * fwk - the framework classloader.
> So my questions are -
> How do you decide which type to use as your parent class loader (pcl) ?
> Basically these 4 types of classloaders load classes from different
> classpaths (boot and system), right?
> What does it mean to be an extension classloader? In the spec it says
> the extension bundles are fragments (framework or bootclasspath) but
> fragments don't have their classloaders.

Here extension class loader refers to the VM's extension class loader
which loads classes from the ext/ directory of the VM.

> System.bundle is the framework bundle which exports packages through
> pcl  using org.osgi.framework.system.packages then why there is
> specific fwk type?

The fwk type specifies the actual class loader which loads the
Equinox framework.  I would avoid this setting if possible, it only
kind of makes sense when embedding the Framework in your own
Java application and you need to expose the class loader used to
load the framework.

> If all the classes starting with java and the one's specified by
> org.osgi.framework.bootdelegation are to be loaded by pcl then what is
> boot type?

The boot type specifies the "boot" classloader of the VM.  This is
the default used by Eclipse.  This class loader is used to load the
class built into the VM (java.* and others packages, javax.xml etc.).

> what does app mean, which classloader is this referring to?

This is the application class loader setup by the VM to launch an
application.  The VM sets up the classloaders with the following


In eclipse the app class loader is used to load the boot strap
launcher (org.eclipse.equinox.launder jar).  This code creates
another classloader for the framework to launch (fwk)

> I was having some classloading issues with xerces parser and when I
> switched to app it suddenly worked, I really want to understand what
> happened.

More than likely the xerces stuff you were trying to load came from
the VM's extension classloader then.  With that said, all of this is
not really recommended in OSGi.  The parent class loader should
really only be used for java.* packages.  This allows for much more
control over what packages you get and what versions.  Depending on
the packages from the app, ext or boot class loader in the VM without
specifying constraints (Import-Package/Require-Bundle) is not a good
practice because your bundle may resolve in an environment where the
package is not available from the VM.

> Clarifications of the above questions will be very much appreciated.
> Thanks much.
> Shravan


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