Java Examples In A Nutshell (3rd Edition) [Electronic resources] نسخه متنی

5.2 Using a URLConnection

The URLConnection
class establishes a connection to a URL. The openStream(
) method of URL we used in Example 5-1 is merely a convenience method that creates a
URLConnection object and calls its
getInputStream( ) method. By using a
URLConnection object directly instead of relying
on openStream( ), you have much more control over
the process of downloading the contents of a URL.

Example 5-2 is a simple
program that shows how to use a URLConnection to
obtain the content type, size, last-modified date, and other
information about the resource referred to by a URL. If the URL uses
the HTTP protocol, it also demonstrates how to use the
HttpURLConnection subclass to obtain additional
information about the connection.

Note
the use of the java.util.Date class to convert a
timestamp (a long that contains the number of
milliseconds since midnight, January 1, 1970 GMT) to a human-readable
date and time string.

Example 5-2. GetURLInfo.java

package je3.net;
import java.net.*;
import java.io.*;
import java.util.Date;
/**
* A class that displays information about a URL.
**/
public class GetURLInfo {
/** Use the URLConnection class to get info about the URL */
public static void printinfo(URL url) throws IOException {
URLConnection c = url.openConnection( );  // Get URLConnection from URL
c.connect( );                             // Open a connection to URL
// Display some information about the URL contents
System.out.println("  Content Type: " + c.getContentType( ));
System.out.println("  Content Encoding: " + c.getContentEncoding( ));
System.out.println("  Content Length: " + c.getContentLength( ));
System.out.println("  Date: " + new Date(c.getDate( )));
System.out.println("  Last Modified: " +new Date(c.getLastModified( )));
System.out.println("  Expiration: " + new Date(c.getExpiration( )));
// If it is an HTTP connection, display some additional information.
if (c instanceof HttpURLConnection) {
HttpURLConnection h = (HttpURLConnection) c;
System.out.println("  Request Method: " + h.getRequestMethod( ));
System.out.println("  Response Message: " +h.getResponseMessage( ));
System.out.println("  Response Code: " + h.getResponseCode( ));
}
}
/** Create a URL, call printinfo( ) to display information about it. */
public static void main(String[  ] args) {
try { printinfo(new URL(args[0])); }
catch (Exception e) {
System.err.println(e);
System.err.println("Usage: java GetURLInfo <url>");
}
}
}