Tutorials : Server-Side Web Applications Using Servlets and JSP :
: Setting HTTP Status Codes

2. Specifying Status Codes

As described above, the HTTP response status line consists of an 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 the status code. The way to do that is by the setStatus method of HttpServletResponse. The setStatus method takes an 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 the 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 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 code 307 is missing altogether.

Setting the status code does not always mean that you don't need to return a document. For example, although most servers will generate a small "File Not Found" message for 404 responses, a servlet 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 the PrintWriter.

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

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This tutorial is now available as a book: Core Servlets and JavaServer Pages by Marty Hall, published by Sun Microsystems Press. Read all about it at CoreServlets.com

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