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124

Chapter 6 Generating the Server Response: HTTP Status Codes

can be accomplished with them, and the following chapter discusses the

response headers. 

6.1 Specifying Status Codes

As just described, the HTTP response status line consists of an HTTP ver 

sion, a status code, and an associated message. Since the message is directly

associated with the status code and the HTTP version is determined by the

server, all a servlet needs to do is to set the status code. The way to do this is

by the 

setStatus

 method of 

HttpServletResponse

. If your response

includes a special status code and a document, be sure to call 

setStatus

before actually returning any of the content via the 

PrintWriter

. That's

because an HTTP response consists of the status line, one or more headers,

a blank line, and the actual document, in that order. The headers can

appear in any order, and servlets buffer the headers and send them all at

once, so it is legal to set the status code (part of the first line returned) even

after setting headers. But servlets do not necessarily buffer the document

itself, since users might want to see partial results for long pages. In version

2.1 of the servlet specification, the 

PrintWriter

 output is not buffered at

all, so the first time you use the 

PrintWriter

, it is too late to go back and

set headers. In version 2.2, servlet engines are permitted to partially buffer

the output, but the size of the buffer is left unspecified. You can use the

getBufferSize

 method of 

HttpServletResponse

 to determine the size,

or use 

setBufferSize

 to specify it. In version 2.2 with buffering enabled,

you can set status codes until the buffer fills up and is actually sent to the

client. If you aren't sure if the buffer has been sent, you can use the 

isCom 

mitted

 method to check.

Core Approach

Be sure to set status codes before sending any document content to the 

client.

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

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