Java Enterprise in a Nutshell, Third Edition

Servlet Responses

In order to do anything useful, a servlet must send a response to each request that is made to it. In the case of an HTTP servlet, the response can include three components: a status code, any number of HTTP headers, and a response body.

The ServletResponse and HttpServletResponse interfaces include all the methods needed to create and manipulate a servlet’s output. We’ve already seen that you specify the MIME type for the data returned by a servlet using the setContentType() method of the response object passed into the servlet. With an HTTP servlet, the MIME type is generally text/html, although some servlets return binary data: a servlet that loads a GIF file from a database and sends it to the web browser should set a content type of image/gif, while a servlet that returns an Adobe Acrobat file should set it to application/pdf.

ServletResponse and HttpServletResponse each define two methods for producing output streams, getOutputStream() and getWriter(). The former returns a ServletOutputStream, which can be used for textual or binary data. The latter returns a java.io.PrintWriter object, which is used only for textual output. The getWriter() method examines the content type to determine which charset to use, so setContentType() should be called before getWriter().

HttpServletResponse also includes a number of methods for handling HTTP responses . Most of these allow you to manipulate the HTTP header fields. For example, setHeader(), setIntHeader(), and setDateHeader() ...