Some ideas on why getContentLength() is always -1 in JAVA download

If Content Length is not described in the header file

# #There is no solution yet

If Content Long is described in the header file

Option 1:

Disguise as a browser

 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");

Add proxy to the code

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);    
}

Option 2:

Add the following attributes to prevent the server from gzip compression:

Java Doc has a description of this:

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");

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