20.3 Another Simple Servlet

You may recall Example 11-18, an error handler utility class that displays Java exceptions to the user in a dialog box and allows them to be reported to a server through a URL. The servlet shown in Example 20-2 is the server-side portion of that example: it handles an HTTP POST request from the ErrorHandler class, deserializes the exception, and returns a response. (The response claims that the exception has been recorded, when in fact it is simply discarded. Saving the exception to a database is left as an exercise; we'll see an example later in the chapter of a servlet that does communicate with a database.)

The code for this example is simple. It demonstrates the use of ServletRequest.getInputStream( ) to read the body of an HTTP POST request and the use of HttpServletResponse.sendError( ) to send an HTTP error.

Example 20-2. ErrorHandlerServlet.java

package je3.servlet; import java.io.*; import javax.servlet.*; import javax.servlet.http.*; import java.sql.*; /**  * This servlet is the server-side companion to the  * ErrorHandler.reportThrowable( ) utility method developed elsewhere in this   * this book; it responds to the HTTP POST request initiated by that method.  **/ public class ErrorHandlerServlet extends HttpServlet {     // This servlet only supports HTTP POST requests     public void doPost(HttpServletRequest request,                        HttpServletResponse response) throws IOException     {         ObjectInputStream in =             new ObjectInputStream(request.getInputStream( ));         try {             Throwable throwable = (Throwable) in.readObject( );                          // Exercise: save the throwable to a database, along with             // the current time, and the IP address from which it was reported.                          // Our response will be displayed within an HTML document, but             // it is not a complete document itself.  Declare it plain text,             // but it is okay to include HTML tags in it.             response.setContentType("text/plain");             PrintWriter out = response.getWriter( );             out.println("Thanks for reporting your <tt>" +                         throwable.getClass( ).getName( ) + "</tt>.<br>" +                         "It has been filed and will be investigated.");         }         catch(Exception e) {             // Something went wrong deserializing the object; most likely             // someone tried to invoke the servlet manually and didn't provide             // correct data.  We send an HTTP error because that is the             // easiest thing to do.  Note, however that none of the HTTP error             // codes really describes this situation adequately.             response.sendError(HttpServletResponse.SC_GONE,                                "Unable to deserialize throwable object");         }     } }

20.3.1 Testing the Error Handler

If you use the je3.war file, the ErrorHandlerServlet will be at this URL:

http://localhost:8080/je3/ErrorHandler

To test it, you can't just enter that URL into a web browser. Doing so would issue an HTTP GET request, which the servlet does not support. Instead, run the ErrorHandler$Test program shown in Example 11-18, and submit a report that way. You should see the servlet's response in the dialog box.