Raible's Wiki: Sandbox

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In my day job, we decided to use a little <a href="http://raibledesigns.com/page/rd?anchor=xmlhttprequest">XMLHttpRequest</a> lovin'

to populate one drop-down from another. This is my review of <a href="http://oss.metaparadigm.com/jsonrpc/">JSON-RPC</a>,

an open source JavaScript library and servlet for simplifying XMLHttpRequest.

I considered integrate <a href="https://dwr.dev.java.net/">Direct Web Remoting</a> (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:</p>

<ol>

<li> Download the 0.8 release from <a href="http://oss.metaparadigm.com/jsonrpc-dist/json-rpc-java-0.8.tar.gz">http://oss.metaparadigm.com/jsonrpc-dist/json-rpc-java-0.8.tar.gz</a>.</li>

<li>Add the JAR to your project and the webapps/jsonrpc/jsonrpc.js to your

projects' "scripts" folder. Include this file in your <a href="http://www.opensympony.com/sitemesh">SiteMesh</a> decorator

or <a href="http://struts.apache.org/userGuide/dev_tiles.html">Tiles</a> layout. If you're not using SiteMesh or Tiles, it's high time

you started.</li>

<li>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.

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/**

* 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);

}

class MyTagLib{

def isAdmin= { attrs, body->

def user = attrs["user"]

if (user!= null && checkUserPrivs(user))

body()

}

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</li>

<li>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.

<pre>

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]);

}

</pre>

</li>

</ol>

<p>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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