hibernate-devel

[Hibernate] Hooking into Hibernate classloader

[Chris N. <ch...@si...>]

I'd like to dynamically wrap my persistent classes using CGLIB or JDK1.3
proxies to provide things like getters for metadata which aren't in the
compiled class file. I've done some searching and it appears that all of the
persistent classes are loaded by Hibernate using

  cirrus.hibernate.helpers.ReflectHelper.classForName(String)

which is implemented as:

  public static Class classForName(String name) throws ClassNotFoundException {
      try {
	  return Thread.currentThread().getContextClassLoader().loadClass(name);
      } catch (Exception e) {
	  return Class.forName(name);
      }
  }

One option would be to try to set the current context class loader before
Hibernate starts loading classes. But that is error-prone, I think. Another
way would be to add a method to set the classloader to use. But really I'd
just prefer a simple callback interface:

  public interface HibernateClassLoader {
     public Class loadClass(String name) throws ClassNotFoundException;
  }

I guess the loader would be configurable per-datastore?
If there's a better way, let me know.

Thanks,
Chris

Re: [Hibernate] Hooking into Hibernate classloader

[Gavin K. <ga...@ap...>]

I think its better to do this kind of thing by subclassing EntityPersister
and overriding instantiate(). However, that will not work properly in the
current implentation (for example getPersister( object.getClass() ) would
fail). Would you please investigate what kind of changes would be required
to make this approach work?

----- Original Message -----
From: "Chris Nokleberg" <ch...@si...>
To: <hib...@li...>
Sent: Wednesday, November 06, 2002 10:27 AM
Subject: [Hibernate] Hooking into Hibernate classloader


> I'd like to dynamically wrap my persistent classes using CGLIB or JDK1.3
> proxies to provide things like getters for metadata which aren't in the
> compiled class file. I've done some searching and it appears that all of
the
> persistent classes are loaded by Hibernate using
>
>   cirrus.hibernate.helpers.ReflectHelper.classForName(String)
>
> which is implemented as:
>
>   public static Class classForName(String name) throws
ClassNotFoundException {
>       try {
>   return Thread.currentThread().getContextClassLoader().loadClass(name);
>       } catch (Exception e) {
>   return Class.forName(name);
>       }
>   }
>
> One option would be to try to set the current context class loader before
> Hibernate starts loading classes. But that is error-prone, I think.
Another
> way would be to add a method to set the classloader to use. But really I'd
> just prefer a simple callback interface:
>
>   public interface HibernateClassLoader {
>      public Class loadClass(String name) throws ClassNotFoundException;
>   }
>
> I guess the loader would be configurable per-datastore?
> If there's a better way, let me know.
>
> Thanks,
> Chris
>
>
> -------------------------------------------------------
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Re: [Hibernate] Hooking into Hibernate classloader

[Chris N. <ch...@si...>]

On Thu, Nov 07, 2002 at 01:04:04AM +1100, Gavin King wrote:
> I think its better to do this kind of thing by subclassing EntityPersister
> and overriding instantiate(). However, that will not work properly in the
> current implentation (for example getPersister( object.getClass() ) would
> fail). Would you please investigate what kind of changes would be required
> to make this approach work?

In a nutshell, if goal is to allow instantiate to return any object as a
substitute for the original object, then Hibernate needs a way to take
that object and map back to the original object. I was toying around
with requiring the substituted object to implement a special interface:

  public interface UserProxy {
    public Object getProxiedObject();
  }

Hibernate would dereference incoming objects that implemented the
interface. For performance, maybe it would only do this after
determining that it couldn't work with original object.

But this doesn't handle things like SessionFactory.getClassMetadata,
which takes a Class. Which leads me to believe that maybe it is better
to keep this all out of Hibernate and just say that it's up to the
application to wrap the Session, SessionFactory, and other public
interfaces appropriately.

-Chris

Re: [Hibernate] Hooking into Hibernate classloader

[Max R. A. <ma...@eo...>]

> But this doesn't handle things like SessionFactory.getClassMetadata,
> which takes a Class.

Just asking out of personal interest :)
why is everything in hibernate bound up onto Class objects, why not
fully-qualifed-names in
the form of strings instead ?

That would make it more flexible and resistant to things that might not be
availble on the classpath
and e.g. make it possible to load internal mapping model and use it for
codegeneration and other
stuff which in its nature cannot rely on the classing being in the
classpath.

/max

> Which leads me to believe that maybe it is better
> to keep this all out of Hibernate and just say that it's up to the
> application to wrap the Session, SessionFactory, and other public
> interfaces appropriately.
>
> -Chris
>
>
>
>
> -------------------------------------------------------
> This sf.net email is sponsored by: See the NEW Palm
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> https://lists.sourceforge.net/lists/listinfo/hibernate-devel
>

Re: [Hibernate] Hooking into Hibernate classloader

[Gavin K. <ga...@ap...>]

Firstly, the API *should* use Class objects to reduce the possibility of
runtime errors. I forget which project I saw recently that had used strings
and was now moving to using class objects. Internally, we *could* use
classnames, but that would be slower, right? Its much quicker to take an
identityHashCode than to compute the hash of a (possibly quite long) String.
Or am I wrong on this?

As for why can't we compile mappings without the classes, thats because
various things (property types, etc) may be determined by reflection. I
still think this is a nice feature.

----- Original Message -----
From: "Max Rydahl Andersen" <ma...@eo...>
Cc: "hibernate list" <hib...@li...>
Sent: Thursday, November 07, 2002 7:23 PM
Subject: Re: [Hibernate] Hooking into Hibernate classloader


>
> > But this doesn't handle things like SessionFactory.getClassMetadata,
> > which takes a Class.
>
> Just asking out of personal interest :)
> why is everything in hibernate bound up onto Class objects, why not
> fully-qualifed-names in
> the form of strings instead ?
>
> That would make it more flexible and resistant to things that might not be
> availble on the classpath
> and e.g. make it possible to load internal mapping model and use it for
> codegeneration and other
> stuff which in its nature cannot rely on the classing being in the
> classpath.
>
> /max
>
> > Which leads me to believe that maybe it is better
> > to keep this all out of Hibernate and just say that it's up to the
> > application to wrap the Session, SessionFactory, and other public
> > interfaces appropriately.
> >
> > -Chris
> >
> >
> >
> >
> > -------------------------------------------------------
> > This sf.net email is sponsored by: See the NEW Palm
> > Tungsten T handheld. Power & Color in a compact size!
> > http://ads.sourceforge.net/cgi-bin/redirect.pl?palm0001en
> > _______________________________________________
> > hibernate-devel mailing list
> > hib...@li...
> > https://lists.sourceforge.net/lists/listinfo/hibernate-devel
> >
>
>
>
> -------------------------------------------------------
> This sf.net email is sponsored by: See the NEW Palm
> Tungsten T handheld. Power & Color in a compact size!
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> https://lists.sourceforge.net/lists/listinfo/hibernate-devel

Re: [Hibernate] Hooking into Hibernate classloader

[Juozas B. <ba...@ce...>]

Is Reflect helper used to load classes ?

It can be a problem on some app servers :
 public static Class classForName(String name) throws ClassNotFoundException
{
  try {
   return Thread.currentThread().getContextClassLoader().loadClass(name);
  }
  catch (Exception e) {
   return Class.forName(name);
  }

sometimes Thread.currentThread().getContextClassLoader() !=
Application.classLoader;
I think it is on some tomcat versions.
It is good to have optional classLoader parameter in SessionFactory,
 to load metadata and persistent classes. Default can be:
"this.getClass().getClassLoader();"
(the same  to load application and SessionFactory implementation ).



----- Original Message -----
From: "Gavin King" <ga...@ap...>
To: "Max Rydahl Andersen" <ma...@eo...>
Cc: <hib...@li...>
Sent: Thursday, November 07, 2002 9:37 AM
Subject: Re: [Hibernate] Hooking into Hibernate classloader


> Firstly, the API *should* use Class objects to reduce the possibility of
> runtime errors. I forget which project I saw recently that had used
strings
> and was now moving to using class objects. Internally, we *could* use
> classnames, but that would be slower, right? Its much quicker to take an
> identityHashCode than to compute the hash of a (possibly quite long)
String.
> Or am I wrong on this?
>
> As for why can't we compile mappings without the classes, thats because
> various things (property types, etc) may be determined by reflection. I
> still think this is a nice feature.
>
> ----- Original Message -----
> From: "Max Rydahl Andersen" <ma...@eo...>
> Cc: "hibernate list" <hib...@li...>
> Sent: Thursday, November 07, 2002 7:23 PM
> Subject: Re: [Hibernate] Hooking into Hibernate classloader
>
>
> >
> > > But this doesn't handle things like SessionFactory.getClassMetadata,
> > > which takes a Class.
> >
> > Just asking out of personal interest :)
> > why is everything in hibernate bound up onto Class objects, why not
> > fully-qualifed-names in
> > the form of strings instead ?
> >
> > That would make it more flexible and resistant to things that might not
be
> > availble on the classpath
> > and e.g. make it possible to load internal mapping model and use it for
> > codegeneration and other
> > stuff which in its nature cannot rely on the classing being in the
> > classpath.
> >
> > /max
> >
> > > Which leads me to believe that maybe it is better
> > > to keep this all out of Hibernate and just say that it's up to the
> > > application to wrap the Session, SessionFactory, and other public
> > > interfaces appropriately.
> > >
> > > -Chris
> > >
> > >
> > >
> > >
> > > -------------------------------------------------------
> > > This sf.net email is sponsored by: See the NEW Palm
> > > Tungsten T handheld. Power & Color in a compact size!
> > > http://ads.sourceforge.net/cgi-bin/redirect.pl?palm0001en
> > > _______________________________________________
> > > hibernate-devel mailing list
> > > hib...@li...
> > > https://lists.sourceforge.net/lists/listinfo/hibernate-devel
> > >
> >
> >
> >
> > -------------------------------------------------------
> > This sf.net email is sponsored by: See the NEW Palm
> > Tungsten T handheld. Power & Color in a compact size!
> > http://ads.sourceforge.net/cgi-bin/redirect.pl?palm0001en
> > _______________________________________________
> > hibernate-devel mailing list
> > hib...@li...
> > https://lists.sourceforge.net/lists/listinfo/hibernate-devel
>
>
>
> -------------------------------------------------------
> This sf.net email is sponsored by: See the NEW Palm
> Tungsten T handheld. Power & Color in a compact size!
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> _______________________________________________
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