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386

Chapter 16 Using HTML Forms

URL of a program that will process the data, and when the user submits the

form (usually by pressing a button), the names and values of the controls are

sent to the designated URL as a string of the form 

Name1=Value1&Name2=Value2...NameN=ValueN

This string can be sent to the designated program in one of two ways. The

first, which uses the HTTP 

GET

 method, appends the string to the end of the

specified URL, after a question mark. The second way data can be sent is by

the HTTP 

POST

 method. Here, the 

POST

 request line, the HTTP request

headers, and a blank line are first sent to the server, and then the data string

is sent on the following line. 

For example, Listing 16.1 (HTML code) and Figure 16 1 (typical result)

show a simple form with two textfields. The HTML elements that make up this

form are discussed in detail in the rest of this chapter, but for now note a couple

of things. First, observe that one text field has a name of 

firstName

 and the

other has a name of 

lastName

. Second, note that the GUI controls are consid 

ered text level (inline) elements, so you need to use explicit HTML formatting

to make sure that the controls appear next to the text describing them. Finally,

notice that the 

FORM

 element designates 

http://localhost:8088/Some 

Program

 as the URL to which the data will be sent. 

Before submitting the form, I start a server program called 

EchoServer

on port 8088 of my local machine. 

EchoServer

, shown in Section 16.12, is a

mini  Web server  used for debugging. No matter what URL is specified and

what data is sent to it, it merely returns a Web page showing all the HTTP

information sent by the browser. As shown in Figure 16 2, when the form is

submitted with 

Joe

 in the first textfield and 

Hacker

 in the second, the

browser simply requests the URL 

http://localhost:8088/Some 

Program?firstName=Joe&lastName=Hacker

. Listing 16.2 (HTML code)

and Figure 16 3 (typical result) show a variation that uses 

POST

 instead of

GET

. As shown in Figure 16 4, submitting the form with textfield values of

Joe

 and 

Hacker

 results in the line 

firstName=Joe&lastName=Hacker

being sent to the browser on a separate line after the HTTP request headers

and a blank line.

That's the general idea behind HTML forms: GUI controls gather data

from the user, each control has a name and a value, and a string containing all

the name/value pairs is sent to the server when the form is submitted.

Extracting the names and values on the server is straightforward in servlets:

that was covered in Chapter 3 (Handling the Client Request: Form Data).

The remainder of this chapter covers options in setting up forms and the var 

ious GUI controls you can put in them. 

Second edition of this book: www.coreservlets.com; Sequel: www.moreservlets.com.

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