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plug-in loading classes question [message #448375]

Mon, 24 April 2006 07:46

Eclipse User

Hi!

I've got a question which relates to loading classes in different plug-ins:

I've got

"PluginA" with package "a.b.c"
"PluginB" with package "a.b.c"

Both have got a class "Hello" in the package "a.b.c"

==> PluginA.a.b.c.Hello
==> PluginB.a.b.c.Hello

"PluginA.a.b.c.Hello" has a method called "helloWorld"
"PluginB.a.b.c.Hello" has a method called "helloUnderWorld"

Now PluginA is loading first.

When PluginB is calling the method "helloUnderWorld" an exception is thrown:
"noSuchMethodError".

I thought that every plugin has its own classloader. So, why does the
"PluginB.a.b.c.Hello" class not override the class "PluginA.a.b.c.Hello",
thus the method "helloUnderWorld" can be found?

Am I missing something in this context, because how will you guarantee, if
you've 100 plugins and no plugin knows the source of any other, that this
problem will not occure?

Thx, Christian

Re: plug-in loading classes question [message #448426 is a reply to message #448375]

Mon, 24 April 2006 09:32

Eclipse User

You would only see that kind of behaviour if plugin B has a dependency
on plugin A.

> I thought that every plugin has its own classloader.
> So, why does the "PluginB.a.b.c.Hello" class not override the class
> "PluginA.a.b.c.Hello", thus the method "helloUnderWorld" can be found?

If B depends on A, I would imagine that the plugin B classloader asks A
first (gets the class back) and doesn't look in it's own plugin.

Later,
PW

Re: plug-in loading classes question [message #448428 is a reply to message #448426]

Mon, 24 April 2006 10:25

Eclipse User

Right, there is a dependency, but how can I tell the plugin to look in its
own classes first?

"Paul Webster" <pwebster@ca.ibm.com> schrieb im Newsbeitrag
news:e2ik23$9mk$1@utils.eclipse.org...
> You would only see that kind of behaviour if plugin B has a dependency
> on plugin A.
>
> > I thought that every plugin has its own classloader.
> > So, why does the "PluginB.a.b.c.Hello" class not override the class
> > "PluginA.a.b.c.Hello", thus the method "helloUnderWorld" can be found?
>
> If B depends on A, I would imagine that the plugin B classloader asks A
> first (gets the class back) and doesn't look in it's own plugin.
>
> Later,
> PW

Re: plug-in loading classes question [message #448431 is a reply to message #448428]

Mon, 24 April 2006 10:43

Eclipse User

Christian Gesswagner wrote:
> Right, there is a dependency, but how can I tell the plugin to look in its
> own classes first?

you cannot. That's not the way plugin classloading works, and you can't
fool around with it.

An example of eclipse NLS stuff:

org.eclipse.ui.workbench has the WorkbenchMessages NLS class. It also
defines the messages.properties class, which provides it's basic messages.

Then they use a fragment, hosted off of org.eclipse.ui.workbench, to
provide the messages_de.properties in the same package. Fragments are
sucked into the classpath of the host plugin exactly as if they were
part of the host plugin. Then it can find the language when it's changed.

You can't replace something that's already in the classpath, but you can
add the extra bits.

Later,
PW

Re: plug-in loading classes question [message #448441 is a reply to message #448428]

Mon, 24 April 2006 18:20

Eclipse User

The best and most sensible way to avoid getting both yourself and the
classloader confused is to NOT have two plugins containing classes with
exactly the same class and package name.

Regards
Neil

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