public Iterator<S> iterator() {
    return new Iterator<S>() {
        //之所以有knownProviders是为了处理多次迭代的情况，首次迭代的时候就将其缓存到了providers，
        //如果要重新加载所有的实现类，需要调用reload
        Iterator<Map.Entry<String,S>> knownProviders
            = providers.entrySet().iterator();
        //本质是lookupIterator的方法
        public boolean hasNext() {
            if (knownProviders.hasNext())
                return true;
            return lookupIterator.hasNext();
        }

        public S next() {
            if (knownProviders.hasNext())
                return knownProviders.next().getValue();
            return lookupIterator.next();
        }

        public void remove() {
            throw new UnsupportedOperationException();
        }

    };
}

private boolean hasNextService() {
    if (nextName != null) {
        return true;
    }
    if (configs == null) {
        try {
            //PREFIX为META-INF/services/，service.getName是全限定名
            String fullName = PREFIX + service.getName();
            if (loader == null)
                configs = ClassLoader.getSystemResources(fullName);
            else
                configs = loader.getResources(fullName);
        } catch (IOException x) {
            fail(service, "Error locating configuration files", x);
        }
    }
    while ((pending == null) || !pending.hasNext()) {
        if (!configs.hasMoreElements()) {
            return false;
        }
        //将文件按行读取，每一行是list的一个元素
        pending = parse(service, configs.nextElement());
    }
    nextName = pending.next();
    return true;
}

private S nextService() {
    if (!hasNextService())
        throw new NoSuchElementException();
    String cn = nextName;
    nextName = null;
    Class<?> c = null;
    try {
    //搞到类
        c = Class.forName(cn, false, loader);
    } catch (ClassNotFoundException x) {
        fail(service,
             "Provider " + cn + " not found");
    }
    if (!service.isAssignableFrom(c)) {
        fail(service,
             "Provider " + cn  + " not a subtype");
    }
    try {
    //实例化，并强转为接口类型
        S p = service.cast(c.newInstance());
        //缓存方便以后使用
        providers.put(cn, p);
        return p;
    } catch (Throwable x) {
        fail(service,
             "Provider " + cn + " could not be instantiated",
             x);
    }
    throw new Error();          // This cannot happen
}