Custom class loader

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spaceman - 25 Sep 2006 22:24 GMT

Hi,

How can I create a custom class loader which loads classes only from a
specific jar?
e.g. in my web apps web-inf/lib I have a.jar and b.jar and I'd like to
have two separate class loaders for each of them. Is it possible?
As these jars are libraries and do not have a main() etc.. or any other
entry point, how can I set the class loader for every class in the
jars?

Any advice is highly appreciated.
Thanks

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Arne Vajhøj - 26 Sep 2006 03:36 GMT

> How can I create a custom class loader which loads classes only from a
> specific jar?
[quoted text clipped - 3 lines]
> entry point, how can I set the class loader for every class in the
> jars?

WEB-INF/lib ? No !

Because they will be loaded by a parent classloader.

If the two jar files are outside of normal classloader search
path it is simple.

URLClassLoader cl = new URLClassLoader(urlofjar);
X o = (X)Class.forName(clsnam, true, cl).newInstance();

Arne

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spaceman - 26 Sep 2006 14:41 GMT

> > How can I create a custom class loader which loads classes only from a
> > specific jar?
[quoted text clipped - 3 lines]
> > entry point, how can I set the class loader for every class in the
> > jars?

Thank You.

> WEB-INF/lib ? No !

Let me be more specific. The problem I have is that both a.jar and
b.jar have some classes which are the same ( duplicated ). If I put
them toghether in WEB-INF/lib the classloader gets messed up due to the
duplicate classes.

> Because they will be loaded by a parent classloader.
>
[quoted text clipped - 3 lines]
> URLClassLoader cl = new URLClassLoader(urlofjar);
> X o = (X)Class.forName(clsnam, true, cl).newInstance();

This seem to be a good possibility however as there is no entry point
and there are a lot of classes, I'm not sure how I can control the
creation of each class and use my own class loader.
Also how can I have the web server load the jars at some point if they
are not in web-inf/lib?

> Arne

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Arne Vajhøj - 30 Sep 2006 20:28 GMT

>> WEB-INF/lib ? No !
> Let me be more specific. The problem I have is that both a.jar and
> b.jar have some classes which are the same ( duplicated ). If I put
> them toghether in WEB-INF/lib the classloader gets messed up due to the
> duplicate classes.

Yes.

You need two custom classloaders one for each jar and the jars
must not ne in path for parent classloaders.

>> URLClassLoader cl = new URLClassLoader(urlofjar);
>> X o = (X)Class.forName(clsnam, true, cl).newInstance();
[quoted text clipped - 4 lines]
> Also how can I have the web server load the jars at some point if they
> are not in web-inf/lib?

The Class.forName code above explicit specify classloader.

The URL for the classloader can be any URL including file
URL's.

Note that the deployment of you app will become a bit tricky, but
that is how it has to be.

Arne

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Custom class loader

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