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Trail: Custom Networking
Lesson: Working with URLs

## Reading from and Writing to a URLConnection

> If you've successfully used `openConnection` to initiate communications with a URL, then you have a reference to a `URLConnection` object. The `URLConnection` 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.

### Reading from a URLConnection

> The following program performs the same function as the `URLReader` 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 a `BufferedReader` on the input stream and reads from it. The bold statements highlight the differences between this example and the previous

> >     import 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();
> >         }
> >     }

> The 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 a `URLConnection` instead of reading directly from a URL might be more useful. This is because you can use the `URLConnection` 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.

### Writing to a URLConnection

> 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, a `cgi-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-side `cgi-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:

> 1. Create a `URL`.
> 2. Open a connection to the `URL`.
> 3. Set output capability on the `URLConnection`.
> 4. 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.
> 5. Write to the output stream.
> 6. 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 it `backwards`, 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`, where `string_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`:

>    >     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();
>    >         }
>    >     }

>    Let's examine the program and see how it works. First, the program processes its command-line arguments:

>    >     if (args.length != 1) {
>    >         System.err.println("Usage:  java Reverse " +
>    >                            "string_to_reverse");
>    >         System.exit(-1);
>    >     }
>    >     String stringToReverse = URLEncoder.encode(args[0]);

>    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 the `cgi-bin` script `backwards`. 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. The `URLEncoder` class methods encode the characters.

>    Next, the program creates the `URL` object--the URL for the `backwards` script on `java.sun.com`--opens a `URLConnection`, and sets the connection so that it can write to it:

>    >     URL url = new URL("http://java.sun.com/cgi-bin/backwards");
>    >     URLConnection c = url.openConnection();
>    >     c.setDoOutput(true);

>    The program then creates an output stream on the connection and opens a `PrintWriter` on it:

>    >     PrintWriter out = new PrintWriter(c.getOutputStream());

>    If the URL does not support output, `getOutputStream` method throws an `UnknownServiceException`. 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:

>    >     out.println("string=" + stringToReverse);
>    >     out.close();

>    This code writes to the output stream using the `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. The `Reverse` program constructs the input in the form required by the script by concatenating `string=` 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. The `Reverse` program does this:

>    >     BufferReader in = new BufferedReader(
>    >                           new InputStreamReader(c.getInputStream()));
>    >     String inputLine;

>    >     while ((inputLine = in.readLine()) != null)
>    >         System.out.println(inputLine);
>    >     in.close();

>    When you run the `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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