URL组成：
protocol://host:port/path?query
从URL中拿到host：
public static String getHostFromURL(String url){
if(url==null)
return null;
//remove 'http(s)://'
int index=url.indexOf("//");
if(index>=0)
url=url.substring(index+"//".length());
//remove path of URL
index=url.indexOf("/");
if(index>=0)
url=url.substring(0,index);
return url;
}
