Generating the Server Response: HTTP Status Codes

Generating the Server Response: HTTP Status Codes :

When a Web server responds to the request from the browser or other Web client, the response typically consists of a status line, some response headers, a blank line, and the document

Specifying Status Codes

As described above, the HTTP response status line consists of the HTTP version, 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 the servlet needs to do is to set status code. The way to do that is by the setStatus method of the HttpServletResponse.

The setStatus method takes the int (the status code) as an argument, but instead of using explicit numbers, it is clearer and more reliable to use the constants defined in HttpServletResponse. The name of each constant is derived from standard HTTP 1.1 message for each constant, all uppercase with a prefix of SC(for Status Code) and spaces changed to underscores. Thus, since the message for 404 is Not Found, the equivalent constant in the HttpServletResponse is SC_NOT_FOUND There are two exceptions however. For some odd reason the constant for code 302 is derived from the HTTP 1.0 message, not the HTTP 1.1 message, and the constant for the code 307 is missing altogether.

Setting the status code does not always mean that you don't need to the return a document. For example, although most servers will generate the small "File Not Found" message for 404 responses, a servlets might want to customize this response. However, if you do this, you need to be sure to call response.setStatus before sending any content via PrintWriter.

Although the general method of setting status codes is simply to the call response.setStatus(int), there are two common cases where a shortcut method in the HttpServletResponse is provided. The sendError method generates the 404 response along with a short message formatted inside an HTML document. And the sendRedirect method generates the 302 response along with a Location header indicating the URL of the new document.


HTTP 1.1 Status Codes and Their Meaning

Following is the list of all the available HTTP 1.1 status codes, along with their associated message and interpretation. You should be cautious in using status codes that are available only in HTTP 1.1, since many browsers still only support HTTP 1.0. If you do use the status codes specific to HTTP 1.1, in most cases you want to either explicitly check the HTTP version of request (via the getProtocol method of the HttpServletRequest) or reserve it for situations when no HTTP 1.0 status code would be particularly meaningful to the client anyhow.

100 Continue Continue with partial request. (New in HTTP 1.1)

101 Switching Protocols Server will comply with Upgrade header and change to different protocol. (New in HTTP 1.1)

200 OK Everything's fine; document follows for GET and POST requests. This is the default for servlets; if you don't use setStatus, you'll get this.

201 Created Server created a document; the Location header indicates its URL.

202 Accepted Request is being acted upon, but processing is not completed.

203 Non-Authoritative Information Document is being returned normally, but some of the response headers might be incorrect since a document copy is being used. (New in HTTP 1.1)

204 No Content No new document; browser should continue to display previous document. This is a useful if the user periodically reloads a page and you can determine that the previous page is already up to date. However, this does not work for pages that are automatically reloaded via the Refresh response header or the equivalent header, since returning this status code stops future reloading. JavaScript-based automatic reloading could still work in such a case, though

205 Reset Content No new document, but browser should reset document view. Used to force browser to clear CGI form fields. (New in HTTP 1.1)

206 Partial Content Client sent a partial request with a Range header, and server has fulfilled it. (New in HTTP 1.1)

300 Multiple Choices Document requested can be found several places; they'll be listed in the returned document. If server has a preferred choice, it should be listed in the Location response header.

301 Moved Permanently Requested document is elsewhere, and the URL for it is given in the Location response header. Browsers should automatically follow the link to the new URL.

302 Found Similar to 301, except that the new URL should be interpreted as a temporary replacement, not a permanent one. Note: the message was "Moved Temporarily" in HTTP 1.0, and the constant in HttpServletResponse is SC_MOVED_TEMPORARILY, not SC_FOUND.Very useful header, since browsers automatically follow the link to the new URL. This status code is so useful that there is a special method for it, sendRedirect. Using response.sendRedirect(url) has a couple of advantages over doing response.setStatus(response.SC_MOVED_TEMPORARILY) and response.setHeader("Location", url). First, it is easier. Second, with sendRedirect, the servlet automatically builds a page containing the link (to show to older browsers that don't automatically follow redirects). Finally, sendRedirect can handle relative URLs, automatically translating them to absolute ones. Note that this status code is sometimes used interchangeably with 301. some servers will send 301 and others will send 302. Technically, browsers are only supposed to automatically follow the redirection if the original request was GET. See the 307 header for details.