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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.optionsplantDropDown.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.optionsunitDropDown.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.