Strange noclassdefnotfounderror with custom classloader

From:

Berlin Brown <berlin.brown@gmail.com>

Newsgroups:

comp.lang.java.programmer

Date:

Sat, 20 Jun 2009 06:44:26 -0700 (PDT)

Message-ID:

<8402b544-12e5-4937-a397-c61f6bed6a3b@p4g2000vba.googlegroups.com>

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[i] = 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.