URLConnections - Java Network Programming - page 218

The keys are the header field names. The values are lists of property values. Both names
and values are stored as strings.
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 ()
A URLConnection doesn't allow output by default, so you have to call setDoOut
put(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 5 , you saw how
to send data to server-side programs with GET . However, GET should be limited to safe
operations, such as search requests or page navigation, and not used for unsafe opera‐
tions that create or modify a resource, such as posting a comment on a web page or
ordering a pizza. Safe operations can be bookmarked, cached, spidered, prefetched, and
so on. Unsafe operations should not be.
Once you have an OutputStream , buffer it by chaining it to a BufferedOutputStream
or a BufferedWriter . You may also chain it to a DataOutputStream , an OutputStream
Writer , 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 );
OutputStream raw = uc . getOutputStream ();
OutputStream buffered = new BufferedOutputStream ( raw );
OutputStreamWriter out = new OutputStreamWriter ( buffered , "8859_1" );
out . write ( "first=Julie&middle=&last=Harting&work=String+Quartet\r\n" );
out . flush ();
out . close ();
} catch ( IOException ex ) {
System . err . println ( ex );
}
Sending data with POST is almost as easy as with GET . Invoke setDoOutput(true) and
use the URLConnection 's getOutputStream() method to write the query string rather
than attaching it to the URL. Java buffers all the data written onto the output stream
until the stream is closed. This enables it to calculate the value for the Content-length