what has been done when URL.openconnection()?

问题:

I want to know what has been done when URL.openconnection().
i have done some test like this:

public static void main(String[] args) {
    //    testConnection("http://www.google.com");
    testConnection("http://219.09.34.23.1");
}

private static void testConnection(final String _url) {
    new Thread(new Runnable() {
        String strurl = _url;
        long starttime = 0;
        long endtime = 0;

        public void run() {
            try {
                System.out.println("open:" + strurl);

                starttime = System.currentTimeMillis();
                System.out.println("starttime:" + starttime);

                URL url = new URL(strurl);
                HttpURLConnection conn = (HttpURLConnection) url
                        .openConnection();

                endtime = System.currentTimeMillis();
                System.out.println("openConnection endtime:" + endtime);
                System.out
                        .println("spend:" + (endtime - starttime) + " ms");

                conn.connect();
                endtime = System.currentTimeMillis();
                System.out.println("connect endtime2:" + endtime);
                System.out
                        .println("spend:" + (endtime - starttime) + " ms");

                conn.getResponseCode();
                endtime = System.currentTimeMillis();
                System.out.println("endtime3:" + endtime);
                System.out
                        .println("spend:" + (endtime - starttime) + " ms");
            } catch (MalformedURLException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();

                endtime = System.currentTimeMillis();
                System.out.println("MalformedURLException endtime:"
                        + endtime);
                System.out
                        .println("spend:" + (endtime - starttime) + " ms");
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();

                endtime = System.currentTimeMillis();
                System.out.println(" IOException endtime:" + endtime);
                System.out
                        .println("spend:" + (endtime - starttime) + " ms");
            }

        }
    }).start();
}

when i run testConnection("http://www.google.com"), all things are ok.
when i run testConnection("http://219.09.34.23.1"), "219.09.34.23.1" is a random ip maybe not exist i wrote, it print this:

open:http://219.09.34.23.1
starttime:1338978920350
openconnection endtime:1338978920355
spend:5 ms

    java.net.UnknownHostException: 219.09.34.23.1  
    at java.net.PlainSocketImpl.connect(Unknown Source)  
    at java.net.SocksSocketImpl.connect(Unknown Source)  
    at java.net.Socket.connect(Unknown Source)  
    at java.net.Socket.connect(Unknown Source)  
    at sun.net.NetworkClient.doConnect(Unknown Source)  
    at sun.net.www.http.HttpClient.openServer(Unknown Source)  
    at sun.net.www.http.HttpClient.openServer(Unknown Source)  
    at sun.net.www.http.HttpClient.<init>(Unknown Source)  
    at sun.net.www.http.HttpClient.New(Unknown Source)  
    at sun.net.www.http.HttpClient.New(Unknown Source)  
    at sun.net.www.protocol.http.HttpURLConnection.getNewHttpClient(Unknown Source)  
    at sun.net.www.protocol.http.HttpURLConnection.plainConnect(Unknown Source)  
    at sun.net.www.protocol.http.HttpURLConnection.connect(Unknown Source)  
    at Main$1.run(Main.java:37)  
    at java.lang.Thread.run(Unknown Source)

IOException endtime:1338978920393
spend:43 ms

it means it spent 5ms to run openconnection, and spent 43ms to find it is a unknownhost, my problem is, what has been done when URL.openconnection() as "219.09.34.23.1" is unknownhost? Thanks for any help!

回答1:

If you read the javadocs for URL.openConnection(), you'll find:

Returns a URLConnection instance that represents a connection to the remote object referred to by the URL.

A new instance of URLConnection is created every time when invoking the URLStreamHandler.openConnection(URL) method of the protocol handler for this URL.

It should be noted that a URLConnection instance does not establish the actual network connection on creation. This will happen only when calling URLConnection.connect().

Update

The IP you used in your "random ip" is not valid; it should consist of 4 octets, not 5. The 43ms is probably for: (1) doing a DNS lookup on the non-IP ip (2) printing the stack trace.

回答2:

Open connection is not like connect

openconnection does not let you download url content that's for sure. you have to call connect. but not really :) the following will explain:

You are not always required to explicitly call the connect method to initiate the connection. Operations that depend on being connected, like getInputStream, getOutputStream, etc, will implicitly perform the connection, if necessary.