URLConnection::getContentHandler has a benign race condition on stuffing URLConnection.handlers map with new handlers.

URLConnection::getContentHandler synchronizes on an instance rather than on the class:

private static Hashtable<String, ContentHandler> handlers = new Hashtable<>();

    /**
     * Gets the Content Handler appropriate for this connection.
     */
    synchronized ContentHandler getContentHandler()
        throws UnknownServiceException
    {
        String contentType = stripOffParameters(getContentType());
        ContentHandler handler = null;
        if (contentType == null)
            throw new UnknownServiceException("no content-type");
        try {
            handler = handlers.get(contentType);
            if (handler != null)
                return handler;
        } catch(Exception e) {
        }

        if (factory != null)
            handler = factory.createContentHandler(contentType);
        if (handler == null) {
            try {
                handler = lookupContentHandlerClassFor(contentType);
            } catch(Exception e) {
                e.printStackTrace();
                handler = UnknownContentHandler.INSTANCE;
            }
            handlers.put(contentType, handler);
        }
        return handler;
    }