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434

Chapter 17 Using Applets As Servlet Front Ends

That Uses Object Serialization and HTTP Tunneling) gives an example. In

the third approach, covered in Section 17.6, the applet sends 

POST

 data to a

servlet and then processes the results itself. Section 17.6 (An Applet That

Sends POST Data) gives an example. Finally, Section 17.7 serves as a

reminder that an applet can bypass the HTTP server altogether and talk

directly to a custom server program running on the applet's home machine. 

This chapter assumes that you already have some familiarity with basic

applets and focuses on the techniques to allow them to communicate with

server side programs. Readers who are unfamiliar with applets should consult

a general introduction to the Java programming language. Core Web Pro 

gramming or Core Java (both from Prentice Hall) are two good choices.

17.1 Sending Data with GET and 

Displaying the Resultant Page

The 

showDocument

 method instructs the browser to display a particular

URL. Recall that you can transmit 

GET

 data to a servlet or CGI program by

appending it to the program's URL after a question mark (?). Thus, to send

GET

 data from an applet, you simply need to append the data to the string

from which the URL is built, then create the 

URL

 object and call 

showDocu 

ment

 in the normal manner. A basic template for doing this in applets follows,

assuming that 

baseURL

 is a string representing the URL of the server side

program and that 

someData

 is the information to be sent with the request.

try {

URL programURL = new URL(baseURL + "?" + someData);

getAppletContext().showDocument(programURL);

} catch(MalformedURLException mue) { ... }

However, when data is sent by a browser, it is URL encoded, which means

that spaces are converted to plus signs (+) and nonalphanumeric characters

into a percent sign (%) followed by the two hex digits representing that charac 

ter, as discussed in Section 16.2 (The FORM Element). The preceding exam 

ple assumes that 

someData

 has already been encoded properly and fails if it

has not been. JDK 1.1 has a 

URLEncoder

 class with a static 

encode

 method

that can perform this encoding. So, if an applet is contacting a server side pro 

gram that normally receives 

GET

 data from HTML forms, the applet needs to

Home page for this book: www.coreservlets.com; Home page for sequel: www.moreservlets.com.

Servlet and JSP training courses by book's author: courses.coreservlets.com.