public class XmlParser {
    // this is the global state...
    private static com.example.proto.classpath.Handler cpHandler;
    static {
        cpHandler = new com.example.proto.classpath.Handler();
        URL.setURLStreamHandlerFactory(p -> proto.equalsIgnoreCase("example") ? cpHandler : null);
    }
    public static interface ResourceLocator extends Function<String, URL> {};
    public org.w3c.dom.Element parse(InputStream doc, ResourceLocator locator) {
        ResourceLocator old = cpHandler.setResourceLocator(locator);
        try {
            // do the actual parsing, not relevant here
            return doc.getDocumentElement();
        } finally {
            cpHandler.setResourceLocator(old);
        }
    }
}

public class Handler {
    private ThreadLocal<ResourceLocator> loc;

    @Override
    protected URLConnection openConnection(URL url) throws IOException {
        return loc.get().apply(url.getPath()).openConnection();
    }

    public ResourceLocator setResourceLocator(ResourceLocator loc) {
        ResourceLocator old = this.loc.get();
        this.loc.set(loc);
        return old;
    }
}