HttpURLConnection handshake and request send

Question

I found the 2 piece of code below from internet, and I am using it in my app.

One thing I really don't understand is, why there is no HttpUrlConnection.connect() being called to establish the Http Connection (handshake) and there is no any function being called to send the requst to the server? Can anyone explain? How can the code be skipped but the response still be obtained?

// HTTP GET request
    private void sendGet() throws Exception {

        String url = "http://www.google.com/search?q=mkyong";

    URL obj = new URL(url);
    HttpURLConnection con = (HttpURLConnection) obj.openConnection();

    // optional default is GET
    con.setRequestMethod("GET");

    //add request header
    con.setRequestProperty("User-Agent", USER_AGENT);

    int responseCode = con.getResponseCode();
    System.out.println("\nSending 'GET' request to URL : " + url);
    System.out.println("Response Code : " + responseCode);

    BufferedReader in = new BufferedReader(
            new InputStreamReader(con.getInputStream()));
    String inputLine;
    StringBuffer response = new StringBuffer();

    while ((inputLine = in.readLine()) != null) {
        response.append(inputLine);
    }
    in.close();

    //print result
    System.out.println(response.toString());

}

========================================

    URL obj = new URL("http://mkyong.com");
    URLConnection conn = obj.openConnection();
    Map<String, List<String>> map = conn.getHeaderFields();

    System.out.println("Printing Response Header...\n");

    for (Map.Entry<String, List<String>> entry : map.entrySet()) {
        System.out.println("Key : " + entry.getKey() 
                           + " ,Value : " + entry.getValue());
    }

    System.out.println("\nGet Response Header By Key ...\n");
    String server = conn.getHeaderField("Server");

    if (server == null) {
        System.out.println("Key 'Server' is not found!");
    } else {
        System.out.println("Server - " + server);
    }

            System.out.println("\n Done");

    } catch (Exception e) {
    e.printStackTrace();
    }

Answer 1

URLConnection#connect() says

Operations that depend on being connected, like getContentLength, will implicitly perform the connection, if necessary.

This includes getOutputStream() and getResponseCode() . So when you call getResponseCode() , connect() is implicitly called for you.

Answer 2

You don't have to explicitly call the connect function,

see java doc

http://docs.oracle.com/javase/7/docs/api/java/net/URL.html#openConnection%28%29

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().

If for the URL's protocol (such as HTTP or JAR), there exists a public, specialized URLConnection subclass belonging to one of the following packages or one of their subpackages: java.lang, java.io, java.util, java.net, the connection returned will be of that subclass. For example, for HTTP an HttpURLConnection will be returned, and for JAR a JarURLConnection will be returned.