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From "Vincent Massol" <vmas...@octo.com>
Subject Re: WebappClassLoader question
Date Thu, 26 Jul 2001 20:20:15 GMT
Thanks Craig,

However I am still not sure this mechanism explains the problem I had. It is
not easy to describe in word so I'll write it in java code instead.

A.java
----
public class A implements HttpServlet
{
  public void doGet()
  {
    log("before error");
    B myB = new B();
    log("after error");
  }
}
----

B.java
----
public class B
{
  public B()
  {
  }
  public doSomething()
  {
    try {
      Class myClass = Class.forName("C");
      // call the method by reflection ...
    } catch (Exception e) {
      log("I would have thought here");
    }
  }
}
----

C.java
----
import junit.framework.*;

public class C extends TestCase
{
...
}
----

Now all of this is packaged in a war, classes A and B and in a jar put in
WEB-INF/lib and class C is put in WEB-INF/classes. The junit jar is *not*
put in WEB-INF/lib.

Calling the servlet A result in an error occurring between the logs "before
error" and "after error" and the log "I would have thought here" is never
printed because the error happens _before_ ... This is what I don't
understand.

Any idea ?
Thanks a lot
-Vincent

----- Original Message -----
From: "Craig R. McClanahan" <craigmcc@apache.org>
To: <tomcat-user@jakarta.apache.org>; "Vincent Massol" <vmassol@octo.com>
Sent: Thursday, July 26, 2001 6:18 PM
Subject: Re: WebappClassLoader question


On Thu, 26 Jul 2001, Vincent Massol wrote:

> Thanks Alex,
>
> I don't think the standard classloader mechanism is involved here. I
believe
> it is a 'feature' of Tomcat and more specifically of it's
WebappClassLoader.
> When you do standard java code and you have the following situation :
>
> 1st -> 2nd --- (using reflection) --> 3rd (not in classpath)
>
> then the error always happen in the 2nd class and you can put the code
> between a try catch block and you'll be able to catch the
> ClassNotFoundException. However what happens here is that the error
happens
> when calling a method on the 1st class that instanciates the 2nd class !
> This is what I don't understand.
>
> Am I dreaming or is this a behaviour of Tomcat 4 ?
>

The web app class loader in Tomcat 4 is based on java.net.URLClassLoader,
and has the same basic class loading behavior.  In particular, when a
class A is loaded, all the classes that A directly references (i.e. listed
in import statements, used as a variable declaration, and so on) are also
loaded.  This is done recursively on the referenced classes, until the
entire tree of references is resolved.  If the load for class A fails, you
will get ClassNotFoundException.  However, if the load for one of the
referenced classes fails, you will typically get NoClassDefError instead
(and the class named in the error message may or may not be the one that
is actually missing, which complicates debugging this problem tremendously
:-).

A good rule of thumb to avoid this kind of problem -- if you need to add
classes to your CLASSPATH at compile time (when rebuilding the entire
app), be sure all of those classes are visible to the web app at runtime.

Using reflection, on the other hand, lets you defer loading of "dependent"
classes until runtime, and you can deal with ClassNotFoundException errors
that you might run into.  Note however that, even if you load a class
dynamically, *that* class still contains direct references to other
classes that must all be resolved in the manner described above.


> Thanks
> -Vincent
>

Craig McClanahan

PS:  One place where the Tomcat 4 class loader *does* vary from the usual
class loader behavior is in the order of places it looks to load a
class.  The usual pattern in Java2 is to delegate to the parent class
loader first, then look locally.  Tomcat 4 does the opposite -- it checks
in /WEB-INF/classes and /WEB-INF/lib of your web application *before*
looking up the parent class loader chain.  This means that, if you have a
class in the $CATALINA_HOME/lib directory (shared across web apps), and a
version of that same class in your web app, the version in your webapp
wins.


> ----- Original Message -----
> From: "Alex Fernández" <afm229@tid.es>
> To: <tomcat-user@jakarta.apache.org>
> Sent: Wednesday, July 25, 2001 3:44 PM
> Subject: Re: WebappClassLoader question
>
>
> > Hi Vincent!
> >
> > I've run into the same situation a couple of times, when one class uses
> > a second class, and this second class uses a third one that is not
> > present.
> >
> > 1st -> 2nd -> 3rd (missing)
> >
> > One would think that instantiating the 2nd should give an error, but
> > that loading the 2nd and/or instantiating the 1st should be ok. In fact,
> > all of the behaviors raise exceptions.
> >
> > The following paragraph in ClassLoader javadoc might be of help:
> >
> > "The methods and constructors of objects created by a class loader may
> > reference other classes. To determine the class(es) referred to, the
> > Java virtual machine calls the loadClass method of the class loader that
> > originally created the class."
> >
> > Or, to find out what the JVM is doing, the spec is here:
> > http://java.sun.com/docs/books/vmspec/
> >
> > Hope it helps.
> >
> > Un saludo,
> >
> > Alex.
> >
> > > Vincent Massol wrote:
> > >
> > > Hi,
> > >
> > > Here is the situation :
> > >
> > > * I have a class that makes use of JUnit (by extending the JUnit
> > > TestCase class). Let's call it ServletTestCase
> > > * I have a second class that is used to call a method in
> > > ServletTestCase, let's call it MyProxyClass
> > > * I have a third class (a servlet) that does _not_ make use of JUnit.
> > > Let's call it ServletTestRedirector. This class actually instanciate
> > > MyProxyClass and calls one of its method.
> > > * I package these classes in a war file and I _don't_ include
> > > junit.jar in this war file
> > >
> > > When I access the servlet, I get a ClassNotFoundException on a JUnit
> > > class. So far it is normal ...
> > > When I debugged it, I have actually found that the error was happening
> > > when ServletTestRedirector was instancianting MyProxyClass (which does
> > > _not_ make use of JUnit) and before it was calling its method.
> > >
> > > Here is the stack trace I got :
> > >
> > > java.lang.NoClassDefFoundError: junit/framework/TestCase
> > >  at java.lang.ClassLoader.defineClass0(Native Method)
> > >  at java.lang.ClassLoader.defineClass(ClassLoader.java:486)
> > >  at
> > >
java.security.SecureClassLoader.defineClass(SecureClassLoader.java:111)
> > >  at
> > >
>
org.apache.catalina.loader.WebappClassLoader.findClassInternal(WebappClassLo
> ader.java:1475)
> > >  at
> > >
>
org.apache.catalina.loader.WebappClassLoader.findClass(WebappClassLoader.jav
> a:836)
> > >  at
> > >
>
org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.jav
> a:1215)
> > >  at
> > >
>
org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.jav
> a:1098)
> > >  at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:313)
> > >  at
> > >
>
org.apache.commons.cactus.server.ServletTestRedirector.doPost(ServletTestRed
> irector.java:143)
> > > Here is what I imagined is happening (tell me if this correct or wrong
> > > !) :
> > >
> > > As MyProxyClass is within the war file, the WebappClassLoader gets
> > > called to load it. The WebappClassLoader, in trying to find out the
> > > correct class, actually loads some other class in memory, and thus the
> > > ServletTestCase, which fails to load because the junit jar is not in
> > > the classpath.
> > >
> > > Is that correct ?
> > > Don't you find it strange that the error about the missing class is
> > > reported when calling a class that has nothing to do with the problem
> > > ? It gets very hard to catch errors ...
> > >
> > > For example, in MyProxyClass, the code that calls the ServletTestCase
> > > method is as follows :
> > >
> > >         ServletTestCase testInstance = null;
> > >         try {
> > >             testClass = Class.forName(theClassName);
> > >             Constructor constructor = testClass.getConstructor(new
> > > Class[] { String.class });
> > >             testInstance =
> > > (ServletTestCase)constructor.newInstance(new Object[] { theMethod });
> > >         } catch (Exception e) {
> > >             logger.debug("Error instanciating class [" + theClassName
> > > + "]", e);
> > >             e.printStackTrace();
> > >             throw new ServletException("Error instanciating class [" +
> > > theClassName + "]", e);
> > >         }
> > > And there is never any exception caught here .... because the error
> > > happens earlier in the call stack, when the ServletTestRedirector
> > > instanciates MyProxyClass ...
> > >
> > > ... or am I missing something ? :)
> > >
> > > Thanks
> > > -Vincent Massol
> > >
> >
>
>




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