Load libs from a different classloader in a web application

Question

When a web application is loaded in Tomcat it is loaded by a specific classloader, right?
I assume that all libraries (under WEB-INF\\lib ) used by this web application are all loaded by this same classloader?
In this case, is there a way to load a library under a different classloader without any issues?
The reason I want to do this is because Axis uses some configuration properties that are bound to the classloader and would like to do requests with different properties thereby use a different classloader.
Is this possible?

Answer 1

If you want to load classes programmatically at run time, you can use URLClassLoader, but it can be quite tricky to really get it right. You would do something like this:

URL[] urls = new URL[] {
    /* URL to your axis jar */,
    /* other URLs you need */
};
URLClassLoader classLoader = new URLClassLoader(urls, getClass().getClassLoader());
Class<...> axisClass = classLoader.findClass(/* fully qualified name */);

Then you should be able to create a new instance of this class and use it.

Edit: Here is a more concrete example, albeit not using Axis because it would be too difficult to set up. I have create a JAR file that contains the following class:

public class Hello {
    public Hello(String config) {

    }

    public String getMessage() {
        return "Hello World";
    }
}

I have copied this jar file to the source folder of my test project, so I can find it using UrlClassloaderTest.class.getResource("hello.jar") . In a web app, you should probably put it into WebContent/WEB-INF (or something similar) and use the method javax.servlet.ServletContext.getRealPath("WEB-INF/hello.jar") to find it. I can then access the Hello class using the URLClassLoader and reflection:

public class UrlClassloaderTest {
    public static void main(String[] args) throws Exception {
        URL jarUrl = UrlClassloaderTest.class.getResource("hello.jar");
        URLClassLoader cl = new URLClassLoader(new URL[] { jarUrl }, UrlClassloaderTest.class.getClassLoader());

        Class helloClass = cl.loadClass("test.Hello");
        Constructor constructor = helloClass.getConstructor(String.class);
        Object helloObject = constructor.newInstance("some configuration");
        Method messageMethod = helloClass.getMethod("getMessage");
        String message = (String) messageMethod.invoke(helloObject);

        System.out.println(message);
    }
}

Note that I can not use Hello as a type here because it is not on the class path of the application, and so it is not known to the class loader of the class UrlClassLoaderTest !