How to Parse the Callback Events

The final step is to parse and process the actual callback events. To aid in this task, you modify the Event class that you created in How to Generate the XML so that this class can construct an Event from an XML document and create an XML Element. This change requires an additional constructor (that takes an XML document), a retrieveValues method, the addition of two member variables (vendor and publisher), accessor methods for all fields, and finally, a print method.

1. Create the Java code that implements the preceding logic.

   Note that this code is similar to the code for the RegReply class that is described in How to Parse the Registration Reply.

   public Event(Document doc)
           {
                   nvpairs = new Vector();
                   retrieveValues(doc);
           }
           public void print(PrintStream out)
           {
                   out.println("\tCLASS=" + regClass);
                   out.println("\tSUBCLASS=" + regSubclass);
                   out.println("\tVENDOR=" + vendor);
                   out.println("\tPUBLISHER=" + publisher);
                   for (int i = 0; i < nvpairs.size(); i++) {
                           NVPair tempNv = (NVPair)
                               (nvpairs.elementAt(i));
                           out.print("\t\t");
                           tempNv.print(out);
                   }
           }
   
           private void retrieveValues(Document doc)
           {
                   Node n;
                   NodeList nl;
                   String nodeName;
   
                   // Find the SC_EVENT element.
                   nl = doc.getElementsByTagName("SC_EVENT");
                   if (nl.getLength() != 1) {
                      System.out.println("Error in parsing: can't find "
                          + "SC_EVENT node.");
                      return;
                   }
   
                   n = nl.item(0);
   
                   //
                   // Retrieve the values of the CLASS, SUBCLASS,
                   // VENDOR and PUBLISHER attributes.
                   //
                   regClass = ((Element)n).getAttribute("CLASS");
                   regSubclass = ((Element)n).getAttribute("SUBCLASS");
                   publisher = ((Element)n).getAttribute("PUBLISHER");
                   vendor = ((Element)n).getAttribute("VENDOR");
   
                   // Retrieve all the nv pairs
                   for (Node child = n.getFirstChild(); child != null;
                       child = child.getNextSibling())
                   {
                          nvpairs.add(new NVPair((Element)child));
                   }
           }
   
           public String getRegClass()
           {
                   return (regClass);
           }
   
           public String getSubclass()
           {
                   return (regSubclass);
           }
   
           public String getVendor()
           {
                   return (vendor);
           }
   
           public String getPublisher()
           {
                   return (publisher);
           }
   
           public Vector getNvpairs()
           {
                   return (nvpairs);
           }
   
           private String vendor, publisher;

2. Implement the additional constructors and methods for the NVPair class that support the XML parsing.

   The changes to the Event class that are shown in Step 1 require similar changes to the NVPair class.

   public NVPair(Element elem)
           {
                   retrieveValues(elem);
           }
           public void print(PrintStream out)
           {
                   out.println("NAME=" + name + " VALUE=" + value);
           }
           private void retrieveValues(Element elem)
           {
                   Node n;
                   NodeList nl;
                   String nodeName;
   
                   // Find the NAME element
                   nl = elem.getElementsByTagName("NAME");
                   if (nl.getLength() != 1) {
                      System.out.println("Error in parsing: can't find "
                          + "NAME node.");
                      return;
                   }
                   // Get the TEXT section
                   n = nl.item(0).getFirstChild();
                   if (n == null || n.getNodeType() != Node.TEXT_NODE) {
                      System.out.println("Error in parsing: can't find "
                          + "TEXT section.");
                      return;
                   }
   
                   // Retrieve the value
                   name = n.getNodeValue();
   
                   // Now get the value element
                   nl = elem.getElementsByTagName("VALUE");
                   if (nl.getLength() != 1) {
                      System.out.println("Error in parsing: can't find "
                          + "VALUE node.");
                      return;
                   }
                   // Get the TEXT section
                   n = nl.item(0).getFirstChild();
                   if (n == null || n.getNodeType() != Node.TEXT_NODE) {
                   System.out.println("Error in parsing: can't find "
                               + "TEXT section.");
                           return;
                   }
   
                   // Retrieve the value
                   value = n.getNodeValue();
                   }
   
           public String getName()
           {
                   return (name);
           }
   
           public String getValue()
           {
                   return (value);
           }
   }

3. Implement the while loop in EventReceptionThread, which waits for event callbacks.

   EventReceptionThread is described in How to Define the Event Reception Thread.

   while(true) {
                   Socket sock = listeningSock.accept();
                   Document doc = db.parse(sock.getInputStream());
                   Event event = new Event(doc);
                   client.processEvent(event);
                   sock.close();
           }