HttpServletResponse

The HttpServletResponse interface extends ServletResponse and provides access to the HTTP protocol-specific properties of a response. You can use the response object to do things such as

  • Set response headers
  • Write to the response body
  • Redirect the request
  • Set the HTTP status code
  • Send cookies back to the client.

Writing to Response Body

The most common thing you’ll do with a response object is written content to the response body.

  • This might be HTML to display in a browser
  • An image that the browser is retrieving
  • The contents of a file that the client is downloading.
  • It could be plain text or binary data
  • It might be just a few bytes long or it could be gigabytes long.

The getOutputStream: Returns a javax.servlet.ServletOutputStream, for sending binary data back, you must use the ServletOutputStream to send the response bytes.

The getWriter: Returns a java.io.PrintWriter, you would probably want to use the PrintWriter for returning HTML or some other character-encoded text to the client because this makes it easy to write encoded String s and char s to the response.

Also, you should never use both getOutputStream and getWriter in the same response. After a call to one, a call to the other will fail with an IllegalStateException. Set the content type or encoding

While you’re writing to the response body, it might be necessary to set the content type or encoding.

setContentType and setCharacterEncoding methods serves the purpose. You may call these methods as many times as you like; the last call to the method is the one that matters.

Note To Remember

  • If you plan to call setContentType and setCharacterEncoding along with getWriter, you must call setContentType and setCharacterEncoding beforer getWriter so that the returned writer is configured for the correct character encoding. Calls made after getWriter are ignored.

  • If you do not call setContentType and setCharacterEncoding before calling getWriter, the returned writer uses the container’s default encoding.

Setting Headers and Other Response Properties

  • The setHeader, setIntHeader, and setDateHeader: To set nearly any header value you desire.  If the existing response headers already include a header with the name you are setting, the value of that header will be overridden.

  • The addHeader, addIntHeader, and addDateHeader: These versions do not override existing header values, but instead add additional values for the given headers.

  • The getHeader, getHeaders, getHeaderNames, and containsHeader: To check which headers have already been set on the response.

  • The setStatus: To set the HTTP response status code

  • The getStatus: To determine what the current status of the response is

  • The sendError: To set the status code, indicate an optional error message to write to the response data, direct the web container to provide an error page to the client, and clear the buffer

  • The sendRedirect: To redirect the client to a different URL