Strange noclassdefnotfounderror with custom classloader

[Berlin Brown]

I know the complexities of classloading are difficult and I probably
need more logic here.

My goal was to have one jar and then have a user double click on the
jar in windows and have logic to load all the other dependent jars
using java code, custom loader. I had several reasons to do this.

There are several other jars and you can see the list here:

public static final String [] JAVA_LIBRARIES = {
"lib\\log4j-1.2.15.jar",
"lib\\octane_commons.jar",
"lib\\clojure.jar",
"lib\\swt\\win32\\swt.jar",
"lib\\jfreechart\\jcommon-1.0.15.jar",
"lib\\jfreechart\\jfreechart-1.0.12.jar",
"lib\\jfreechart\\jfreechart-1.0.12-swt.jar",
"lib\\pdf\\minium.jar",
"lib\\pdf\\tagsoup-1.2.jar",
"lib\\pdf\\core-renderer.jar",
"lib",
"conf",
"src"
};

My classloader code is based on jetty's classloader and it works OK
but I keep get ting classnotdef errors. Strange ones. I can see a
clear distinction between classnotfound errors. If the file path to
the jar is invalid then I get classnotfound, easy to detect and fix.

Here is essentially the classloader code for future reference.

Classpath classpath = new Classpath();

boolean res = classpath.addComponent(libFilePath);

/// Classloader

private class Loader extends URLClassLoader {
String name;
Loader(URL[] urls, ClassLoader parent) {
super(urls, parent);
name = "StartLoader" + Arrays.asList(urls);
}

public String toString() {
return name;
}
}

//// Then set the classloader
//// where the URLs are the JAR libraries:
URL [] urls = new URL[NUMBER_OF_JARS];
for (x in urls) {
urls = new URL("THE JAR PATH");
}

ClassLoader parent = Thread.currentThread().getContextClassLoader
();

if (parent == null) {
parent = Classpath.class.getClassLoader();
}
if (parent == null) {

parent = ClassLoader.getSystemClassLoader();
}
return new Loader(urls, parent);

/////////////

The code works fine but I can't explain for when I get classpath
loader issues. I have one theory one issue.

[java] Exception in thread "main" java.lang.NoClassDefFoundError:
clojure/l
ng/IFn
[java] at java.lang.ClassLoader.defineClass1(Native Method)
[java] at java.lang.ClassLoader.defineClass(ClassLoader.java:
620)
[java] at java.security.SecureClassLoader.defineClass
(SecureClassLoader
java:124)
[java] at java.net.URLClassLoader.defineClass
(URLClassLoader.java:260)
[java] at java.net.URLClassLoader.access$100
(URLClassLoader.java:56)
[java] at java.net.URLClassLoader$1.run(URLClassLoader.java:
195)
[java] at java.security.AccessController.doPrivileged(Native
Method)
[java] at java.net.URLClassLoader.findClass
(URLClassLoader.java:188)
[java] at java.lang.ClassLoader.loadClass(ClassLoader.java:
306)
[java] at sun.misc.Launcher$AppClassLoader.loadClass
(Launcher.java:268)

[java] at java.lang.ClassLoader.loadClass(ClassLoader.java:
299)
[java] at java.lang.ClassLoader.loadClass(ClassLoader.java:
251)
[java] at java.lang.ClassLoader.loadClassInternal
(ClassLoader.java:319)

[java] at java.lang.Class.getDeclaredFields0(Native Method)
[java] at java.lang.Class.privateGetDeclaredFields(Class.java:
2259)
[java] at java.lang.Class.getDeclaredField(Class.java:1852)

I believe I Get this errror because there are multiple "classes" on
the same name in the classpath from two different jars and I guess the
classloader is confused?

other than that, i get other errors as well but can't pinpoint how I
get them.

Do you know how to diagnose these problems, should I build a more
robust classloader.

 

[Joshua Cranmer]

I believe I Get this errror because there are multiple "classes" on
the same name in the classpath from two different jars and I guess the
classloader is confused?

You get this error because the class loader can't find the class. The
Java class loaders have no problems dealing with multiple definitions of
classes (they use the first one they find).

Generally speaking, class loaders implement a successive fallback
scheme: if a class loader can't find a class, it asks its parent class
loader to find it. I'm guessing you don't have this fallback working.

Do you know how to diagnose these problems, should I build a more
robust classloader.

As far as classloader problems go, this is rather simple. All you need
to do is figure out which classloader is having problems finding the
class and then work out the fallback hierarchy to figure out what is
missing and where it needs to go.

A real classloader problem that I've had--and which you might run
into--is this fun exception:

ClassCastException: class util.Option could not be cast to class
util.Option.

 

[Roedy Green]

classnotdef

I take it you meant NoClassDefFoundError.

It has many possible causes. see
http://mindprod.com/jgloss/runerrormessages.html#NOCLASSDEFFOUNDERROR
--
Roedy Green Canadian Mind Products
http://mindprod.com

If everyone lived the way people do in Vancouver, we would need three more entire planets to support us.
~ Guy Dauncey