class ExternalEntityResolver implements EntityResolver {
    private Map<String, InputSource> entities;

    public ExternalEntityResolver() {
        entities = new HashMap<>();
        entities.put("xhtml1-strict.dtd", new InputSource(getClass().getClassLoader().getResourceAsStream("entities/xhtml1-strict.dtd")));
        entities.put("xhtml-lat1.ent", new InputSource(getClass().getClassLoader().getResourceAsStream("entities/xhtml-lat1.ent")));
        entities.put("xhtml-special.ent", new InputSource(getClass().getClassLoader().getResourceAsStream("entities/xhtml-special.ent")));
        entities.put("xhtml-symbol.ent", new InputSource(getClass().getClassLoader().getResourceAsStream("entities/xhtml-symbol.ent")));
    }

    @Override
    public InputSource resolveEntity(String publicId, String systemId) throws SAXException, IOException {
        for (Map.Entry<String, InputSource> entity : entities.entrySet()) {
            if (systemId.contains(entity.getKey())) {
                if (entity.getValue().getByteStream() == null) {
                    throw new IOException("Local entity [" + entity.getKey() + "] not found!");
                } else {
                    return entity.getValue();
                }
            }
        }
        //use default behaviour
        return null;
    }
}