Java Network Programming, Second Edition

Writing Data to a Server

Sometimes you need to write data to a URLConnection—for example, when you submit a form to a web server using POST or upload a file using PUT. The getOutputStream( ) method returns an OutputStream on which you can write data for transmission to a server:

public OutputStream getOutputStream(  )

Since a URLConnection doesn’t allow output by default, you have to call setDoOutput(true) before asking for an output stream. When you set doOutput to true for an http URL, the request method is changed from GET to POST. In Chapter 7, you saw how to send data to CGI programs with GET. GET is straightforward to work with, but it does limit the amount of data you can send. Some web servers have maximum lengths of lines they’ll accept as part of a GET request, typically 255 or 1,024. This is generally enough for a simple search request or page navigation, but not enough for a form that allows users to contribute to a bulletin board, for example. Forms that allow larger blocks of text should use POST instead. We’ll explore this more shortly.

Once you’ve got the OutputStream, you should buffer it by chaining it to a BufferedOutputStream or a BufferedWriter. You generally also chain it to a DataOutputStream, an OutputStreamWriter, or some other class that’s more convenient to use than a raw OutputStream. For example:

try { URL u = new URL("http://www.somehost.com/cgi-bin/acgi"); // open the connection and prepare it to POST URLConnection uc = u.openConnection( ); uc.setDoOutput(true); ...