Prentice Hall and Sun Microsystems. Personal use only; do not redistribute.

17.3 Sending Data with GET and Processing the Results Directly

439

most CGI programs are already set up to return HTML documents. How 

ever, if you are developing both the client and the server sides of the process,

it seems a bit wasteful to always send back an entire HTML document; in

some cases, it would be nice to simply return data to an applet that is already

running. The applet could then present the data in a graph or some other

custom display. This approach is sometimes known as HTTP tunneling since a

custom communication protocol is embedded within HTTP: proxies, encryp 

tion, server redirection, connections through firewalls, and all.

There are two main variations to this approach. Both make use of the 

URL 

Connection

 class to open an input stream from a URL. The difference lies in

the type of stream they use. The first option is to use a 

BufferedInput 

Stream

 or some other low level stream that lets you read binary or ASCII

data from an arbitrary server side program. That approach is covered in the

first subsection. The second option is to use an 

ObjectInputStream

 to

directly read high level data structures. That approach, covered in the second

subsection, is available only when the server side program is also written in

the Java programming language.

Reading Binary or ASCII Data

An applet can read the content sent by the server by first creating a 

URLCon 

nection

 derived from the URL of the server side program and then attach 

ing a 

BufferedInputStream

 to it. Seven main steps are required to

implement this approach on the client, as described below. I'm omitting the

server side code since the client code described here works with arbitrary

server side programs or static Web pages. 

Note that many of the stream operations throw an 

IOException

, so the

following statements need to be enclosed in a 

try

/

catch

 block. 

1. Create a 

URL

 object referring to applet's home host. You 

can pass an absolute URL string to the 

URL

 constructor (e.g., 

"http://host/path"

), but since browser security restrictions 

prohibit connections from applets to machines other than the 

home server, it makes more sense to build a URL based upon 

the hostname from which the applet was loaded. 

URL currentPage = getCodeBase();

String protocol = currentPage.getProtocol();

String host = currentPage.getHost();

int port = currentPage.getPort();

String urlSuffix = "/servlet/SomeServlet";

URL dataURL = new URL(protocol, host, port, urlSuffix);

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