In class sun.security.x509.X509CRLImpl, method getExtension(ObjectIdentifier) doesn't declare that IOException would be thrown, so the @throws IOException doc should be removed.

    /**
     * get an extension
     *
     * @param oid ObjectIdentifier of extension desired
     * @return Object of type {@code <extension>} or null, if not found
     * @throws IOException on error
     */
    public Object getExtension(ObjectIdentifier oid) {
        if (extensions == null)
            return null;

        // XXX Consider cloning this
        return extensions.get(OIDMap.getName(oid));
    }