利用java下载实现getContentLength()一直为-1的方法

conn.setRequestProperty("User-Agent", " Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36"); 

String host = "127.0.0.1";  
   String port = "8888";  
   setProxy(host, port); 
   public static void setProxy(String host, String port) {  
  System.setProperty("proxySet", "true");  
  System.setProperty("proxyHost", host);  
  System.setProperty("proxyPort", port);  
} 

By default, this implementation of HttpURLConnection requests that servers use gzip compression. Since getContentLength() returns the number of bytes transmitted, you cannot use that method to predict how many bytes can be read from getInputStream(). Instead, read that stream until it is exhausted: whenread() returns -1. 

conn.setRequestProperty("Accept-Encoding", "identity");