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If you've successfully usedopenConnection
to initiate communications with a URL, then you have a reference to aURLConnection
object. TheURLConnection
class contains many methods that let you communicate with the URL over the network.URLConnection
is an HTTP-centric class; that is, many of its methods are useful only when you are working with HTTP URLs. However, most URL protocols allow you to read from and write to the connection. This section describes both functions.
The following program performs the same function as theURLReader
program shown in Reading Directly from a URL.However, rather than getting an input stream directly from the URL, this program explicitly opens a connection to a URL and gets an input stream from the connection. Then, like
URLReader
, this program creates aBufferedReader
on the input stream and reads from it. The bold statements highlight the differences between this example and the previousThe output from this program is identical to the output from the program that opens a stream directly from the URL. You can use either way to read from a URL. However, reading from aimport java.net.*; import java.io.*; public class URLConnectionReader { public static void main(String[] args) throws Exception { URL yahoo = new URL("http://www.yahoo.com/"); URLConnection yc = yahoo.openConnection(); BufferedReader in = new BufferedReader( new InputStreamReader( yc.getInputStream())); String inputLine; while ((inputLine = in.readLine()) != null) System.out.println(inputLine); in.close(); } }URLConnection
instead of reading directly from a URL might be more useful. This is because you can use theURLConnection
object for other tasks (like writing to the URL) at the same time.Again, if the program hangs or you see an error message, you may have to set the proxy host so that the program can find the Yahoo server.
Many HTML pages contain forms-- text fields and other GUI objects that let you enter data to send to the server. After you type in the required information and initiate the query by clicking a button, your Web browser writes the data to the URL over the network. At the other end, acgi-bin
script (usually) on the server receives the data, processes it, and then sends you a response, usually in the form of a new HTML page.Many
cgi-bin
scripts use the POST METHOD for reading the data from the client. Thus writing to a URL is often called posting to a URL. Server-side scripts use the POST METHOD to read from their standard input.
Note: Some server-sidecgi-bin
scripts use the GET METHOD to read your data. The POST METHOD is quickly making the GET METHOD obsolete because it's more versatile and has no limitations on the amount of data that can be sent through the connection.A Java program can interact with
cgi-bin
scripts also on the server side. It simply must be able to write to a URL, thus providing data to the server. It can do this by following these steps:
- Create a
URL
.- Open a connection to the
URL
.- Set output capability on the
URLConnection
.- Get an output stream from the connection. This output stream is connected to the standard input stream of the
cgi-bin
script on the server.- Write to the output stream.
- Close the output stream. Hassan Schroeder, a member of the Java development team, wrote a small
cgi-bin
script named backwards and made it available at the Java Web site,http://java.sun.com/cgi-bin/backwards
. You can use this script to test the following example program. You can also put the script on your network, name itbackwards
, and test the program locally.The script at our Web site reads a string from its standard input, reverses the string, and writes the result to its standard output. The script requires input of the form
string=string_to_reverse
, wherestring_to_reverse
is the string whose characters you want displayed in reverse order.Here's an example program that runs the backwards script over the network through a
URLConnection
:Let's examine the program and see how it works. First, the program processes its command-line arguments:import java.io.*; import java.net.*; public class Reverse { public static void main(String[] args) throws Exception { if (args.length != 1) { System.err.println("Usage: java Reverse " + "string_to_reverse"); System.exit(1); } String stringToReverse = URLEncoder.encode(args[0]); URL url = new URL("http://java.sun.com/cgi-bin/backwards"); URLConnection connection = url.openConnection(); connection.setDoOutput(true); PrintWriter out = new PrintWriter( connection.getOutputStream()); out.println("string=" + stringToReverse); out.close(); BufferedReader in = new BufferedReader( new InputStreamReader( connection.getInputStream())); String inputLine; while ((inputLine = in.readLine()) != null) System.out.println(inputLine); in.close(); } }These statements ensure that the user provides one and only one command-line argument to the program, and then encodes it. The command-line argument is the string that will be reversed by theif (args.length != 1) { System.err.println("Usage: java Reverse " + "string_to_reverse"); System.exit(-1); } String stringToReverse = URLEncoder.encode(args[0]);cgi-bin
scriptbackwards
. It may contain spaces or other non-alphanumeric characters. These characters must be encoded because the string is processed on its way to the server. TheURLEncoder
class methods encode the characters.Next, the program creates the
URL
object--the URL for thebackwards
script onjava.sun.com
--opens aURLConnection
, and sets the connection so that it can write to it:The program then creates an output stream on the connection and opens aURL url = new URL("http://java.sun.com/cgi-bin/backwards"); URLConnection c = url.openConnection(); c.setDoOutput(true);PrintWriter
on it:If the URL does not support output,PrintWriter out = new PrintWriter(c.getOutputStream());getOutputStream
method throws anUnknownServiceException
. If the URL does support output, then this method returns an output stream that is connected to the standard input stream of the URL on the server side--the client's output is the server's input.Next, the program writes the required information to the output stream and closes the stream:
This code writes to the output stream using theout.println("string=" + stringToReverse); out.close();println
method. So you can see that writing data to a URL is as easy as writing data to a stream. The data written to the output stream on the client side is the input for the backwards script on the server side. TheReverse
program constructs the input in the form required by the script by concatenatingstring=
to the encoded string to be reversed.Often, when you are writing to a URL, you are passing information to a
cgi-bin
script, as in this example. This script reads the information you write, performs some action, and then sends information back to you via the same URL. So it's likely that you will want to read from the URL after you've written to it. TheReverse
program does this:When you run theBufferReader in = new BufferedReader( new InputStreamReader(c.getInputStream())); String inputLine; while ((inputLine = in.readLine()) != null) System.out.println(inputLine); in.close();Reverse
program using "Reverse Me" as an argument (including the double quote marks), you should see this output:Reverse Me reversed is: eM esreveR
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