最近需要把一个url的根域名找出来，看着挺简单的需求，结果写出来的代码 却有bug:

public String getSourceDomain(String sourceLink) {
		if (sourceLink == null) {
			return null;
		}
		try {
			URI url = new URI(sourceLink);
			String host = url.getHost();
			int split = host.indexOf('.');
			return "www" + host.substring(split, host.length());
		} catch (URISyntaxException e) {
			return null;
		}
}

这段代码的问题在于，如果只能处理movie.douban.com这种情况，如果是douban.com，或者test.movie.douban.com则无法处理，正确的做法

	public String getSourceDomain() {
		if (sourceLink == null) {
			return null;
		}
		try {
			URI url = new URI(sourceLink);
			String host = url.getHost();
			String[] hostStr = host.split("\\.");
			int length = hostStr.length;
			if (hostStr.length >= 2) {
				return "www." + hostStr[length - 2] + "." + hostStr[length - 1];
			} else {
				return host;
			}
		} catch (URISyntaxException e) {
			return null;
		}
	}