Constructing Responses

A response contains data passed between a server and the client. All responses implement the ServletResponse interface. This interface defines methods that allow you to

• Retrieve an output stream to use to send data to the client. To send character data, use the PrintWriter returned by the response’s getWriter method. To send binary data in a Multipurpose Internet Mail Extensions (MIME) body response, use the ServletOutputStream returned by getOutputStream. To mix binary and text data, as in a multipart response, use a ServletOutputStream and manage the character sections manually.

• Indicate the content type (for example, text/html) being returned by the response with the setContentType(String) method. This method must be called before the response is committed. A registry of content type names is kept by the Internet Assigned Numbers Authority (IANA) at http://www.iana.org/assignments/media-types/.

• Indicate whether to buffer output with the setBufferSize(int) method. By default, any content written to the output stream is immediately sent to the client. Buffering allows content to be written before anything is sent back to the client, thus providing the servlet with more time to set appropriate status codes and headers or forward to another web resource. The method must be called before any content is written or before the response is committed.

• Set localization information, such as locale and character encoding.

HTTP response objects, javax.servlet.http.HttpServletResponse, have fields representing HTTP headers, such as the following:

• Status codes, which are used to indicate the reason a request is not satisfied or that a request has been redirected.

• Cookies, which are used to store application-specific information at the client. Sometimes, cookies are used to maintain an identifier for tracking a user’s session (see Session Tracking).