On Mon, 17 Dec 2001, Lauer, Oliver wrote:
>
> But why wasn't the class found in myapp/WEB-INF/lib ? Sorry, but I don't
> understand that !?
>
Class loaders are black magic :-). Here is one simple scenario to
illustrate the kinds of problems you can have.
Assume you have class A that uses Class.forName() to dynamically create
classes for you. Now assume that class A is in /common/lib, and you pass
it the name of a class that is present only in /WEB-INF/lib to create.
Because A was loaded from the "Common" classloader, it cannot see classes
in your webapp, and you get ClassNotFoundException.
As another example of problems you can encounter, assume you have classes
A and B in a JAR file, and that you have that JAR file in both
/WEB-INF/lib of your webapp, and in /common/lib. Assume that class A has
a method foo that takes a B as a parameter. Now, consider the following
sequence of events:
* Class A gets loaded from /common/lib (because it was
first referenced by some other class in /common/lib.
* You create an instance of class A and store it in your
session attributes.
* Class B gets loaded from /WEB-INF/lib (because it was
first referenced by some other class in /WEB-INF/lib.
* You create a local instance of class B.
* Your application retrieves the A object from the session
attributes and tries to call foo(), passing the instance
of B mentioned above.
* You get a ClassCastException error.
Why? Because the instance of class A (loaded from /common/lib) can only
accept an argument that is an instance of class B *also* loaded from
/common/lib). The class B loaded from /WEB-INF/lib is considered to be a
different class, even though it has the same name (and perhaps even the
same bytes).
There are thousands of ways you can trip yourself up like this. In
general, you're much better off ensuring that you have one and only one
copy of every class visible -- then, you never have to worry about it.
Craig
> > AXA eSolutions GmbH
> > AXA Konzern AG Germany
> > Oliver Lauer
> > Web Architect
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>
> -----Urspr�ngliche Nachricht-----
> Von: Craig R. McClanahan [mailto:[EMAIL PROTECTED]]
> Gesendet: Montag, 17. Dezember 2001 18:50
> An: Tomcat Users List
> Betreff: Re: Classloader question
>
>
>
>
> On Mon, 17 Dec 2001, Heikki Doeleman wrote:
>
> > Date: Mon, 17 Dec 2001 13:06:21 +0100
> > From: Heikki Doeleman <[EMAIL PROTECTED]>
> > Reply-To: Tomcat Users List <[EMAIL PROTECTED]>
> > To: "'[EMAIL PROTECTED]'" <[EMAIL PROTECTED]>
> > Subject: Classloader question
> >
> > Hi everyone,
> >
> > trying to make a web application work using Tomcat 4.0.1 I found the
> > following behaviour, which I don't quite understand:
> >
> > my application uses a library called xhive.jar
> >
> > - putting xhive.jar in [catalina]/common/lib
> > This works fine, as expected
> >
> >
> > - putting it in [myapp]/WEB-INF/lib
> > Causes a ClassDefNotFoundError. I don't know why this is so, it does not
> > seem to conform to the description in class-loader-howto.html. However I
> did
> > see someone saying that in that directory, only .class files are added to
> > the classpath, not .jars
> > (http://www.geocrawler.com/archives/3/193/2000/12/0/4841079/).
> >
> >
> > - putting it in both those places
> > Causes a ClassCastException ?? Something seems to go terribly wrong when
> the
> > jar is placed in both those directories.
> >
> > Can anyone explain more about this? I'd like to know exactly how this
> works,
> > especially I'd like to know why classes from a jar in my application's
> > WEB-INF/lib apparently are not loaded.
> >
>
> This has been discussed numerous times in the archives, but the short
> answer is this: it is fundamental to the way Java class loaders work.
> The key issues:
>
> * A class named "a.b.c.Foo" loaded from two different
> class loaders is ***not*** the same class, even if the
> bytecodes are identical. Trying to assign from one to
> the other will give you ClassCastException errors.
>
> * The "new" operator in Java tries to load the specified
> class from the same classloader that loaded the class
> containing this code. If the class of that name has
> already been loaded by the "other" classloader, you
> again get ClassNotFoundException errors.
>
> * Class loader hierarchies can delegate "up" but not "down".
> Thus, it's also easy to have the "new" operator cause
> ClassNotFoundException errors, even though you "know" that
> the class is there.
>
> Moral of the story -- you are *always* best off having one and only one
> copy of a class visible to a web application. There's ways to deal with
> some of these issues, but they get pretty intricate.
>
> > Thanks
> > Heikki Doeleman
> >
>
> Craig McClanahan
>
>
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