This post originated from an RSS feed registered with Java Buzz by Matt Raible. Original Post: Simplifying XmlHttpRequest with JSON-RPC Feed Title: Raible Designs ~ We Build Web Apps Feed URL: http://static.raibledesigns.com/500.html Feed Description: Opinions and tips on how to build web applications using Java. Currently using Hibernate, Struts, XHTML, CSS, Ant, JUnit and XDoclet. Latest Java Buzz Posts Latest Java Buzz Posts by Matt Raible Latest Posts From Raible Designs ~ We Build Web Apps

1. Download the 0.8 release from http://oss.metaparadigm.com/jsonrpc-dist/json-rpc-java-0.8.tar.gz.
2. Add the JAR to your project and the webapps/jsonrpc/jsonrpc.js to your projects' "scripts" folder. Include this file in your SiteMesh decorator or Tiles layout. If you're not using SiteMesh or Tiles, it's high time you started.
3. JSON-RPC currently requires that you register each class you want call methods on. In our project, I registered a Spring bean (LookupHelper) that's a singleton with references to Maps in the ServletContext. Then we used JavaScript functions to call JSON-PRC and look up units for a plant, and vice versa. I'm not going to put the LookupHelper class here - you'll have to trust its methods return a single String or a comma-separated list of Strings. To register this bean with JSON-RPC, I created an HttpSessionListener and configured it in web.xml.

   /**  * UserListener class used to add/remove session attributes when  * a user first logs in.  Mainly for JavaScript Remote Scripting stuff.  *  * @author Matt Raible  */ public class UserListener implements HttpSessionListener, HttpSessionAttributeListener         { private final Log log = LogFactory.getLog(UserListener.class);     public final static String BRIDGE_KEY = "JSONRPCBridge";     /**      * Initializes LookupHelper singleton with values needed for lookup      *      * @param event the HttpSessionEvent to grab session information from      */     public void sessionCreated(HttpSessionEvent event) {         // Find the JSONRPCBridge for this session or create one         // if it doesn't exist. Note the bridge must be named BRIDGE_KEY         // in the HttpSession for the JSONRPCServlet to find it.         HttpSession session = event.getSession();         JSONRPCBridge jsonBridge = new JSONRPCBridge();         jsonBridge.setDebug(true);         session.setAttribute(BRIDGE_KEY, jsonBridge);     }     /**      * Destroys LookupHelper      *      * @param event the HttpSessionEvent to grab session information from      */     public void sessionDestroyed(HttpSessionEvent event) {         if (event.getSession() != null) {             event.getSession().removeAttribute(BRIDGE_KEY);         }     }     public void attributeAdded(HttpSessionBindingEvent event) {         if (event.getName().equals(BRIDGE_KEY)) {             HttpSession session = event.getSession();             // register LookupHelper so we can call methods on it             ApplicationContext ctx =                     WebApplicationContextUtils                     .getWebApplicationContext(session.getServletContext());             // check for null so we don't have to initialize Spring in tests             if (ctx != null) {                 log.debug("Registering lookupHelper for XmlHttpRequest...");                 JSONRPCBridge jsonBridge =                     (JSONRPCBridge) session.getAttribute(BRIDGE_KEY);                 jsonBridge.registerObject("lookupHelper",                                           ctx.getBean("lookupHelper"));             }         }     }     public void attributeRemoved(HttpSessionBindingEvent event) {         // don't care     }     public void attributeReplaced(HttpSessionBindingEvent event) {         // same as attribute added         attributeAdded(event);     } }

4. After this setup was complete, I was able to add the following JavaScript to the bottom of my JSP. These are functions that our drop-downs call to populate each other, and keep their options in synch.

   var jsonurl = "${ctx}/jsonrpc";
       var jsonrpc = null;
       var unitDropDown = document.getElementById("equipmentName");
   
       function filterUnits(plantDropDown) {
           var plantName = plantDropDown.options[plantDropDown.selectedIndex].value;
   
           if (plantName == "") {
               reloadUnits("");
               return;
           }
   
           try {
               jsonrpc = new JSONRpcClient(jsonurl);
           } catch(e) {
               alert(e);
           }
   
           // Call a Java method on the server
           var units = jsonrpc.lookupHelper.getUnitsForPlant(plantName);
           setUnits(units);
       }
   
       function reloadUnits(value) {
   
   
           if (value == "") {
               try {
                   jsonrpc = new JSONRpcClient(jsonurl);
               } catch(e) {
                   alert(e);
               }
   
               // Call a Java method on the server
               var units = jsonrpc.lookupHelper.getAllUnits();
               setUnits(units);
           }
       }
   
       function setUnits(units) {
           var unitArray = units.split(",");
   
           unitDropDown.options.length = 1;  // keep "All" option
           for (i=0; i < unitArray.length; i++) {
               unitDropDown.options[unitDropDown.options.length] =
                   new Option(unitArray[i], unitArray[i]);
           }
       }
   

The hardest part of using JSON-RPC is setting it up. We only experienced minor issues with Commons Validator, but since the JSON-RPC 0.8 release - everything has worked great, on all browsers we need to support. The only thing I don't like about this library is that you have to register objects for each user's session. I briefly looked at DWR and it looks a little cleaner - especially b/c of its Spring integration. The next time we need XMLHttpRequest, we'll probably use DRW just to compare the two.