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In my day job, we decided to use a little XMLHttpRequest lovin' to populate one drop-down from another. This is my review of JSON-RPC, an open source JavaScript library and servlet for simplifying XMLHttpRequest. I considered integrate Direct Web Remoting (DWR) as well, but its java.net site was down the day I needed it. I started out with JSON-RPC 0.7, which caused some conflicts with Commons Validator client-side validation. This was fixed in the 0.8 release. JSON-RPC takes a little more setup than I care for, but it's pretty easy nonetheless:

  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.{Java2HtmlPlugin /** * 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); } } }(info)

  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(unitArrayi, unitArrayi); } }

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.



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This particular version was published on 06-Nov-2006 13:52:44 MST by MattRaible.