A brilliant piece of code...

gistfile1.java

package com.sun.xml.bind.v2.model.impl;

...

public class ModelBuilder {
    ...

    static {
        try {
            XmlSchema s = null;
            s.location();
        } catch (NullPointerException e) {
            // as epxected
        } catch (NoSuchMethodError e) {
            // this is not a 2.1 API. Where is it being loaded from?
            Messages res;
            if (SecureLoader.getClassClassLoader(XmlSchema.class) == null) {
                res = Messages.INCOMPATIBLE_API_VERSION_MUSTANG;
            } else {
                res = Messages.INCOMPATIBLE_API_VERSION;
            }

            throw new LinkageError( res.format(
                Which.which(XmlSchema.class),
                Which.which(ModelBuilder.class)
            ));
        }
    }
}

It makes me both chuckle and cry every time I see that (usually when I turn on NPE breakpoints in eclipse)