How to load parent Class like this?

Question

I have a java application that use another class in a jar file. And there is a class in the application's main class extends the class in the jar file. I have packaged the application into a jar file.
the directory structure :
foder [dist] : app.jar [lib]
foder [lib]: Library.jar
(I excute "java" cmd in folder "dist" )
Now i can run it, if i use command line option -Xbootclasspath/a:
"java -Xbootclasspath/a:lib/lib.jar -jar app.jar"
(Because if i use "java -jar ", "-cp .;lib/Library.jar" option have no effect.)
But I don't konw how make it run by codes.I tried this:
Main.javapackage myapp;
java.net.URL liburl = new java.io.File("lib\\Library.jar").toURL();
java.net.URLClassLoader loader =
        new java.net.URLClassLoader(new java.net.URL[] { liburl });
Class c = loader.loadClass("mylib.parentclass");
Object    parentobj = c.newInstance();
//c = loader.loadClass("myapp.subclass");  //can't run
//Object    subobj  = c.newInstance();
subclass.javapackage myapp;
subclass extends parentclass{
}parentclass.javapackage mylib;
public class  parentclass{
}the code Object    parentobj = c.newInstance(); can excute without problem . parentclass's constructor excuted.
but
c = loader.loadClass("myapp.subclass");
Object    subobj  = c.newInstance();can not excute and without any exception throw out.
I used the "-verbose:class" option , the output is:
[Loaded java.lang.ClassFormatError from shared objects file] <---the last line
Thanks for reading and participating .
Please help me or point me to resources where I can read .
Sorry for my poor english.
thanks a lot .

Answer 1

The problem is that when a class wants to resolve a reference to another class (e.g. when a class fetches it's parent) it uses the classloader that loaded it, which is not necessarilly the classloader from which it was requested.
When you ask a standard classloader (e.g. URLClassLoader) for a class the first thing it tries is asking it's parent class loader. Only if the parent doesn't find the class does it try itself.
So, if a class is available to the system ClassLoader then even though you ask a URLClassLoader for it, it's the system ClassLoader which actually loads it and, hence, it's the system ClassLoader that class uses to resolve any references.
ClassLoaders generally form a kind or heirarchy and references can be made towards the root of the ClassLoader tree, but not in the other direction.
You would have to do something like have a small main class which sets up the classloader and make your real main class available only on the jars that the URLClassLoader adds to the classpath. That way the main class will have the URLClassLoader as its ClassLoader and will use it to get it's parent class.