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144

Chapter 7 Generating the Server Response: HTTP Response Headers

the header value. As with setting status codes, you must specify headers

before returning the actual document. With servlets version 2.1, this means

that you must set the headers before the first use of the 

PrintWriter

 or raw

OutputStream

 that transmits the document content. With servlets version

2.2 (the version in J2EE), the 

PrintWriter

 may use a buffer, so you can set

headers until the first time the buffer is flushed. See Section 6.1 (Specifying

Status Codes) for details.

Core Approach

Be sure to set response headers before sending any document content to 

the client.

In addition to the general purpose 

setHeader

 method, 

HttpServlet 

Response

 also has two specialized methods to set headers that contain dates

and integers:

setDateHeader(String header, long milliseconds)

This method saves you the trouble of translating a Java date in 

milliseconds since 1970 (as returned by 

System.currentTimeMillis

, 

Date.getTime

, or 

Calendar.getTimeInMillis

) into a GMT time string. 

setIntHeader(String header, int headerValue)

This method spares you the minor inconvenience of converting 

an 

int

 to a 

String

 before inserting it into a header.

HTTP allows multiple occurrences of the same header name, and you

sometimes want to add a new header rather than replace any existing header

with the same name. For example, it is quite common to have multiple

Accept

 and 

Set Cookie

 headers that specify different supported MIME

types and different cookies, respectively. With servlets version 2.1, 

set 

Header

, 

setDateHeader

 and 

setIntHeader

 always add new headers, so

there is no way to  unset  headers that were set earlier (e.g., by an inherited

method). With servlets version 2.2, 

setHeader

, 

setDateHeader

, and

setIntHeader

  replace any existing headers of the same name, whereas

addHeader

, 

addDateHeader

, and 

addIntHeader

 add a header regardless of

whether a header of that name already exists. If it matters to you whether a

specific header has already been set, use 

containsHeader

 to check. 

Finally, 

HttpServletResponse

 also supplies a number of convenience

methods for specifying common headers. These methods are summarized as

follows.

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

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