URLClassLoader issues

Question

Hi,
I'm trying to create a dynamic "class finding" ClassLoader using URLClassLoader and, at run-time, adding various dependency jar files to the class loader and then creating an instance of a class with this ClassLoader.
However, the issue I am running into is that the ClassLoader is throwing both a NoClassDefFoundError and a ClassNotFoundException.
Here is the code I am using to load the classes dynamically from jar files:
ClassLoader cl = new URLClassLoader(getUrls());
Class<?> clazz = cl.loadClass("com.example.foo.JAXBDevTest");
JAXBDevTest jaxb = (JAXBDevTest) clazz.newInstance();
jaxb.testJAXBMarshalling();Here is the definition of the getUrls() static method in my test class:
private static URL[] getUrls() {
        List<URL> urls = new ArrayList<URL>();
        try {
            String jaxbHome = "/path/to/jaxb-ri/lib";
            urls.add(new File("/path/to/mappings/ib-client-mappings.jar").toURL());
            urls.add(new File(jaxbHome + "/jaxb-api.jar").toURL());
            urls.add(new File(jaxbHome + "/jaxb-impl.jar").toURL());
            urls.add(new File(jaxbHome + "/activation.jar").toURL());
            urls.add(new File(jaxbHome + "/jaxb1-impl.jar").toURL());
            urls.add(new File(jaxbHome + "/jsr173_1.0_api.jar").toURL());
     } catch (Exception e) {
            e.printStackTrace();
        return urls.toArray(new URL[urls.size()]);
    }The ObjectFactory class is instantiated in the testJAXBMarshalling method of my test object. However, without the jar containing it being on the system classpath, it isn't found. However, this quote from the ClassLoader javadoc suggests that my custom classloader would be responsible for loading it:
+"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 invokes the loadClass method of the class loader that originally created the class."+
However, this appears not to be the case as the classes that are referred to by the original class are included as URL's in the URLClassLoader instance cannot be found:
Exception in thread "main" java.lang.NoClassDefFoundError: com/example/foo/ObjectFactory
     at us.mn.hennepin.co.esb.mapping.JAXBDevTest.testJAXBMarshalling(JAXBDevTest.java:88)
     at us.mn.hennepin.co.esb.mapping.JAXBDevTest.main(JAXBDevTest.java:81)
Caused by: java.lang.ClassNotFoundException: com.example.foo.ObjectFactory
     at java.net.URLClassLoader$1.run(URLClassLoader.java:200)
     at java.security.AccessController.doPrivileged(Native Method)
     at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
     at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
     at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
     at java.lang.ClassLoader.loadClass(ClassLoader.java:252)
     at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:320)
     ... 2 moreI've spent a considerable amount of time researching this and working on it and am completely frustrated. Any help or insight is greatly appreciated.
Thanks,
John

Answer 1

jdubchak wrote:
jschell,
Thanks for your response. I'm sorry if I gave you the impression, in my original description, that I wanted URLClassLoader to handle anything other than simple jar files.
Well if you are loading jars or directories then the URLClassLoader does it exactly right.
So there is something in your code or the assumptions that lead to your code which is incorrect.
That being said, the example I presented (that causes the error) was only adding jar file to the classpath by using the URLClassLoader. To be clear it does not add to the class path. That is not possible using any method in java.
And if fact it is usually a good idea with classloaders to insure that the loaded classes can not appear in the class path.
However, even writing a custom class loader, my override of either the findClass or loadClass method is not called.
You should not be overriding those if all you are using are jars/directories.
Is it reasonable to assume, that if I write a custom class loader that loads a class from some location (possibly a jar file) that an import statement in that class that loads another class will use the same classloader? Or does it default back to the original classloader?Import statements have nothing to do with it nor even with how java runs when class loaders are not in play. Imports serve as nothing more than a hint to the compiler (not VM) where to find a class which the compiler can then use to define the fully qualified class name (FQCN) which the VM does use.
The VM decides if a class needs to be loaded. When it loads it uses the context of the call to determine the class loader. The classloader (normal ones) defer to parents to see if the class can be loaded by them.
Thus using String will end up being loaded by the System class loader. On the other hand if you use MyPackage.MyClass2 in MyPackage.MyClass1 where MyClass1 was loaded by a custom loader and MyClass2 is not loaded by a parent then the custom loader will load it (or at least attempt to.)