spi service实现类加载代码

private static List<Factory> discoverFactories(ClassLoader classLoader) {
    //spi机制加载类
    final Iterator<Factory> serviceLoaderIterator =
            ServiceLoader.load(Factory.class, classLoader).iterator();

    final List<Factory> loadResults = new ArrayList<>();
    while (true) {
        try {
            // error handling should also be applied to the hasNext() call because service
            // loading might cause problems here as well
            if (!serviceLoaderIterator.hasNext()) {
                break;
            }

            loadResults.add(serviceLoaderIterator.next());
        } catch (Throwable t) {
            if (t instanceof NoClassDefFoundError) {
                LOG.debug(
                        "NoClassDefFoundError when loading a "
                                + Factory.class.getCanonicalName()
                                + ". This is expected when trying to load factory but no implementation is loaded.",
                        t);
            } else {
                throw new RuntimeException(
                        "Unexpected error when trying to load service provider.", t);
            }
        }
    }

    return loadResults;
}