Reading Data from a Server

Here is the minimal set of steps needed to retrieve data from a URL using a URLConnection object:

  1. Construct a URL object.
  2. Invoke the URL object's openConnection( ) method to retrieve a URLConnection object for that URL.
  3. Invoke the URLConnection's getInputStream( ) method.
  4. Read from the input stream using the usual stream API.

The getInputStream() method returns a generic InputStream that lets you read and parse the data that the server sends. 

public InputStream getInputStream( )

uses the getInputStream() method to download a web page.

Example 15-1. Download a web page with a URLConnection

import java.net.*;
import java.io.*;
public class SourceViewer2 {
 public static void main (String[] args) {
 if (args.length > 0) {
 try {
 //Open the URLConnection for reading
 URL u = new URL(args[0]);
 URLConnection uc = u.openConnection( );
 InputStream raw = uc.getInputStream( );
 InputStream buffer = new BufferedInputStream(raw); // chain the InputStream to a Reader
 Reader r = new InputStreamReader(buffer);
 int c;
 while ((c = r.read( )) != -1) {
 System.out.print((char) c);
 } }
 catch (MalformedURLException ex) {
 System.err.println(args[0] + " is not a parseable URL");
 }
 catch (IOException ex) {
 System.err.println(ex);
 }
 } // end if
 } // end main
} // end SourceViewer2

It is no accident that this program is almost the same as Example 7-5. The openStream( ) method of the URL class just returns an InputStream from its own URLConnection object. The output is identical as well, so I won't repeat it here. The differences between URL and URLConnection aren't apparent with just a simple input stream as in this example. The biggest differences between the two classes are:

  • URLConnection provides access to the HTTP header.
  • URLConnection can configure the request parameters sent to the server.
  • URLConnection can write data to the server as well as read data from the server.