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Servlets Introduction
Servlets Life Cycle
Handling the Client Request:Form Data
Handling the Client Request: HTTP Request Headers
Accessing the Standard CGI Variables
Generating the Server Response: HTTP Status Codes
Generating the Server Response: HTTP Response Headers
Handling Cookies
Session Tracking
Connecting to database in Servlets
Filters in Servlet
Servlets Summary

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Generating the Server Response: HTTP Response Headers

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A response from the Web server normally consists of a status line, one or more response headers, a blank line, and the document.

A D V E R T I S E M E N T
Setting the HTTP response headers often goes hand in hand with the setting the status codes in the status line. For example, several of the "document moved" status codes have an accompanying Location header, and the 401 (Unauthorized) code must include an accompanying WWW-Authenticate header.




Overview

The most general way to specify headers is by setHeader method of HttpServletResponse, which takes two strings: the header name and the header value. Like setting the status codes, this must be done before any document content is sent.

There are also two specialized methods to set the headers that contain dates (setDateHeader) and integers (setIntHeader). The first saves you trouble of translating a Java date in milliseconds since the epoch (as returned by System.currentTimeMillis or the getTime method applied to a Date object) into a GMT time string. The second spares you the minor inconvenience of the converting an int to a String.

Rather than setting the header outright, you can add a new header, in case a header with that name already exists. Use addHeader, addDateHeader, and addIntHeader for this. If it really 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 the specifying common headers.

  • The setContentType method sets Content-Type header, and is used by the majority of servlets.

  • The setContentLength method sets Content-Length header, useful if the browser supports persistent (keep-alive) HTTP connections.

  • The addCookie method sets the cookie (there is no corresponding setCookie, since it is normal to have multiple Set-Cookie lines).

  • And, as discussed in the previous section, the sendRedirect method sets Location header as well as setting the status code to 302.




Common Response Headers and their Meaning

Header Interpretation/Purpose
Allow What request methods (GET, POST, etc.) does the server support?
Content-Encoding

What method was used to encode the document? You need to decode it to get the type specified by the Content-Type header. Using gzip to compress the document can dramatically reduce download times for HTML files, but it is only supported by Netscape on Unix and IE 4 and 5 on Windows. On the other hand, gzipping HTML files can dramatically reduce download times, and Java's GZIPOutputStream makes it easy. So you should explicitly check if the browser supports this by looking at the Accept-Encoding header (i.e. via request.getHeader("Accept-Encoding")). That way, you can return gzipped pages to browser that know how to unzip them, but still return regular pages to other browsers.

Content-Length

How many bytes are being sent? This information is only needed if the browser is using a persistent (keep-alive) HTTP connection. If you want your servlet to take advantage of this when the browser supports it, your servlet should write the document into a ByteArrayOutputStream, look up its size when done, put that into the Content-Length field, then send the content via byteArrayStream.writeTo(response.getOutputStream()).

Content-Type

What is the MIME type of the following document? Default for servlets is text/plain, but they usually explicitly specify text/html. Setting this header is so common that there is a special method in HttpServletResponse for it: setContentType

Date

What is current time (in GMT)? Use the setDateHeader method to specify this header. That saves you the trouble of formatting the date string properly.

Expires

At what time should content be considered out of date and thus no longer cached?

Last-Modified

When was document last changed? Client can supply a date via an If-Modified-Since request header. This is treated as a conditional GET, with document only being returned if the Last-Modified date is later than the specified date. Otherwise a 304 (Not Modified) status line is returned. Again, use the setDateHeader method to specify this header.

Location

Where should client go to get document? This is usually set indirectly, along with a 302 status code, via the sendRedirect method of HttpServletResponse.

Refresh

How soon should browser ask for an updated page (in seconds)? Instead of just reloading current page, you can specify a specific page to load via setHeader("Refresh", "5; URL=http://host/path"). Note that this is commonly set via <META HTTP-EQUIV="Refresh" CONTENT="5; URL=http://host/path"> in the HEAD section of the HTML page, rather than as an explicit header from the server. This is because automatic reloading or forwarding is something often desired by HTML authors who do not have CGI or servlet access. But for servlets, setting the header directly is easier and clearer. Note that this header means "reload this page or go to the specified URL in N seconds." It does not mean "reload this page or go to the specified URL every N seconds." So you have to send a Refresh header each time, and sending a 204 (No Content) status code stops the browser from reloading further, regardless of whether you explicitly send the Refresh header or use <META HTTP-EQUIV="Refresh" ...>. Note that this header is not officially part of HTTP 1.1, but is an extension supported by both Netscape and Internet Explorer.

Server

What server am I? Servlets don't usually set this; the Web server itself does.

Set-Cookie

Specifies cookie associated with page. Servlets should not use response.setHeader("Set-Cookie", ...), but instead use the special-purpose addCookie method of HttpServletResponse. See separate section on handling cookies.

WWW-Authenticate

What authorization type and realm should client supply in their Authorization header? This header is required in responses that have a 401 (Unauthorized) status line. E.g. response.setHeader("WWW-Authenticate", "BASIC realm=\"executives\""). Note that servlets do not usually handle this themselves, but instead let password-protected Web pages be handled by the Web server's specialized mechanisms (e.g. .htaccess).



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Keywords: Handling the Client Request:Form Data,asp net client,asp net data,vb net data,control data,data net,java client,data repeater

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