Matt RaibleMatt Raible is a Java Champion and Developer Advocate at Okta. developer.okta.com

The Angular Mini-Book The Angular Mini-Book is a guide to getting started with Angular. You'll learn how to develop a bare-bones application, test it, and deploy it. Then you'll move on to adding Bootstrap, Angular Material, continuous integration, and authentication.

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This book shows you how to build an app with JHipster, and guides you through the plethora of tools, techniques and options you can use. Furthermore, it explains the UI and API building blocks so you understand the underpinnings of your great application.

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10+ YEARS


Over 10 years ago, I wrote my first blog post. Since then, I've authored books, had kids, traveled the world, found Trish and blogged about it all.

Simplifying XmlHttpRequest with JSON-RPC

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.

    /**
     * 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 =
                        (JSONRPCBridgesession.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.

    <script type="text/javascript">
    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]);
        }
    }
    </script>

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 DWR just to compare the two.

Posted in Java at Mar 03 2005, 08:44:40 AM MST 8 Comments

Off to Vegas

I'm leaving for Vegas in a couple hours to attend TheServerSide Symposium. It should be a good time, mostly because it's in Vegas - but also because there's going to be a lot of folks to network with and some good sessions to attend. My only goal for this whole 3-day vacation is to attend 5 sessions. I should be able to get most of those in today, so I can sleep in and goof off tomorrow. Of course, since it's Vegas, I might still be up tomorrow when the sessions start. ;-)

Thanks to SourceBeat for the free trip to Vegas!

Posted in Java at Mar 03 2005, 03:47:59 AM MST 3 Comments