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.