servlet构造response,以及获取RequestDispatcher对象

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 InternetMail 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 AssignedNumbers

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

Writing ServiceMethods

Chapter 10 • Java Servlet Technology 203

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.

To invoke a resource available on the server that is running a web component, you must first

obtain a RequestDispatcher object by using the getRequestDispatcher("URL") method. You

can get a RequestDispatcher object from either a request or the web context; however, the two

methods have slightly different behavior. The method takes the path to the requested resource

as an argument. A request can take a relative path (that is, one that does not begin with a /), but

the web context requires an absolute path. If the resource is not available or if the server has not

implemented a RequestDispatcher object for that type of resource, getRequestDispatcher

will return null. Your servlet should be prepared to deal with this condition.