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Chapter

hen a Web server responds to a request from a browser or other

Web client, the response typically consists of a status line, some

W

response headers, a blank line, and the document. Here is a

minimal example: 

HTTP/1.1 200 OK

Content Type: text/plain

Hello World

The status line consists of the HTTP version (

HTTP/1.1

 in the example

above), a status code (an integer; 200 in the above example), and a very short

message corresponding to the status code (

OK

 in the example). In most cases,

all of the headers are optional except for 

Content Type

, which specifies the

MIME type of the document that follows. Although most responses contain a

document, some don't. For example, responses to 

HEAD

 requests should

never include a document, and there are a variety of status codes that essen 

tially indicate failure and either don't include a document or include only a

short error message document. 

Servlets can perform a variety of important tasks by manipulating the sta 

tus line and the response headers. For example, they can forward the user to

other sites; indicate that the attached document is an image, Adobe Acrobat

file, or HTML file; tell the user that a password is required to access the doc 

ument; and so forth. This chapter discusses the various status codes and what

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