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This covers the basic mechanics of submitting data to the server, whether based on textual input from a form or other activity such as drag and drop or mouse movements. In the following section, we’ll pick up our ObjectViewer example from chapter 4 and learn how to manage updates to the domain model in an orderly fashion.

5.5.3Managing user updates effectively

In chapter 4, we introduced the ObjectViewer, a generic piece of code for browsing complex domain models, and provided a simple example for viewing planetary data. The objects representing the planets in the solar system each contained several parameters, and we marked a couple of simple textual properties—the diameter and distance from the sun—as editable. Changes made to any properties in the system were captured by a central event listener function, which we used to write some debug information to the browser status bar. (The ability to write to the status bar is being restricted in recent builds of Mozilla Firefox. In appendix A, we present a pure JavaScript logging console that could be used to provide status messages to the user in the absence of a native status bar.) This event listener mechanism also provides an ideal way of capturing updates in order to send them to the server.

Let’s suppose that we have a script updateDomainModel.jsp running on our server that captures the following information:

■The unique ID of the planet being updated

■The name of the property being updated

■The value being assigned to the property

We can write an event handler to fire all changes to the server like so:

function updateServer(propviewer){

var planetObj=propviewer.viewer.object; var planetId=planetObj.id;

var propName=propviewer.name; var val=propviewer.value; net.ContentLoader(

'updateDomainModel.jsp',

someResponseHandler,

null,

'POST',

'planetId='+encodeURI(planetId)

+'&propertyName='+encodeURI(propName)

+'&value='+encodeURI(val)

);

}

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And we can attach it to our ObjectViewer:

myObjectViewer.addChangeListener(updateServer);

This is easy to code but can result in a lot of very small bits of traffic to the server, which is inefficient and potentially confusing. If we want to control our traffic, we can capture these updates and queue them locally and then send them to the server in batches at our leisure. A simple update queue implemented in JavaScript is shown in listing 5.13.

Listing 5.13 CommandQueue object

net.CommandQueue=function(id,url,freq){ b Create a queue object this.id=id;

net.cmdQueues[id]=this;

this.url=url; this.queued=new Array(); this.sent=new Array(); if (freq){

this.repeat(freq);

}

}

net.CommandQueue.prototype.addCommand=function(command){ if (this.isCommand(command)){

this.queue.append(command,true);

}

}

net.CommandQueue.prototype.fireRequest=function(){ c Send request to server if (this.queued.length==0){

return;

}

var data="data=";

for(var i=0;i<this.queued.length;i++){ var cmd=this.queued[i];

if (this.isCommand(cmd)){ data+=cmd.toRequestString(); this.sent[cmd.id]=cmd;

}

}

this.queued=new Array(); this.loader=new net.ContentLoader(

this.url,

net.CommandQueue.onload,net.CommandQueue.onerror,

"POST",data

);

}

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net.CommandQueue.prototype.isCommand=function(obj){ d Test object type return (

obj.implementsProp("id")

&&obj.implementsFunc("toRequestString")

&&obj.implementsFunc("parseResponse")

);

}

net.CommandQueue.onload=function(loader){ e Parse server response var xmlDoc=net.req.responseXML;

var elDocRoot=xmlDoc.getElementsByTagName("commands")[0]; if (elDocRoot){

for(i=0;i<elDocRoot.childNodes.length;i++){

elChild=elDocRoot.childNodes[i]; if (elChild.nodeName=="command"){ var attrs=elChild.attributes;

var id=attrs.getNamedItem("id").value; var command=net.commandQueue.sent[id]; if (command){

command.parseResponse(elChild);

}

}

}

}
}
net.CommandQueue.onerror=function(loader){
alert("problem sending the data to the server");
}
net.CommandQueue.prototype.repeat=function(freq){	f Poll the server
this.unrepeat();
if (freq>0){

this.freq=freq;

var cmd="net.cmdQueues["+this.id+"].fireRequest()";

this.repeater=setInterval(cmd,freq*1000);
}
}	g Switch polling off
net.CommandQueue.prototype.unrepeat=function(){
if (this.repeater){
clearInterval(this.repeater);
}
this.repeater=null;
}

The CommandQueue object (so called because it queues Command objects— we’ll get to that in a minute) is initialized bwith a unique ID, the URL of a serverside script, and, optionally, a flag indicating whether to poll repeatedly. If it

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doesn’t, then we’ll need to fire it manually every so often. Both modes of operation may be useful, so both are included here. When the queue fires a request to the server, it converts all commands in the queue to strings and sends them with the request c.

The queue maintains two arrays. queued is a numerically indexed array, to which new updates are appended. sent is an associative array, containing those updates that have been sent to the server but that are awaiting a reply. The objects in both queues are Command objects, obeying an interface enforced by the isCommand() function d. That is:

■It can provide a unique ID for itself.

■It can serialize itself for inclusion in the POST data sent to the server (see c).

■It can parse a response from the server (see e) in order to determine whether it was successful or not, and what further action, if any, it should take.

We use a function implementsFunc() to check that this contract is being obeyed. Being a method on the base class Object, you might think it is standard JavaScript, but we actually defined it ourselves in a helper library like this:

Object.prototype.implementsFunc=function(funcName){

return this[funcName] && this[funcName] instanceof Function;

}

Appendix B explains the JavaScript prototype in greater detail. Now let’s get back to our queue object. The onload method of the queue eexpects the server to return with an XML document consisting of <command> tags inside a central

<commands> tag.

Finally, the repeat() f and unrepeat() g methods are used to manage the repeating timer object that will poll the server periodically with updates.

The Command object for updating the planet properties is presented in listing 5.14.

Listing 5.14 UpdatePropertyCommand object

planets.commands.UpdatePropertyCommand=function(owner,field,value){ this.id=this.owner.id+"_"+field;

this.obj=owner;

this.field=field;

this.value=value;

}

planets.commands.UpdatePropertyCommand.toRequestString=function(){

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return { type:"updateProperty", id:this.id, planetId:this.owner.id, field:this.field, value:this.value

}.simpleXmlify("command");

}

planets.commands.UpdatePropertyCommand.parseResponse=function(docEl){ var attrs=docEl.attributes;

var status=attrs.getNamedItem("status").value; if (status!="ok"){

var reason=attrs.getNamedItem("message").value; alert("failed to update "

+this.field+" to "+this.value +"\n\n"+reason);

}

}

The command simply provides a unique ID for the command and encapsulates the parameters needed on the server. The toRequestString() function writes itself as a piece of XML, using a custom function that we have attached to the Object prototype:

Object.prototype.simpleXmlify=function(tagname){ var xml="<"+tagname;

for (i in this){

if (!this[i] instanceof Function){ xml+=" "+i+"=\""+this[i]+"\"";

}

}

xml+="/>"; return xml;

}

This will create a simple XML tag like this (formatted by hand for clarity):

<command type='updateProperty' id='001_diameter' planetId='mercury' field='diameter'

value='3'/>

Note that the unique ID consists only of the planet ID and the property name. We can’t send multiple edits of the same value to the server. If we do edit a property several times before the queue fires, each later value will overwrite earlier ones.

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The POST data sent to the server will contain one or more of these tags, depending on the polling frequency and how busy the user is. The server process needs to process each command and store the results in a similar response. Our CommandQueue’s onload will match each tag in the response to the Command object in the sent queue and then invoke that Command’s parseResponse method. In this case, we are simply looking for a status attribute, so the response might look like this:

<commands>

<command id='001_diameter' status='ok'/>

<command id='003_albedo' status='failed' message='value out of range'/> <command id='004_hairColor' status='failed' message='invalid property name'/>

</commands>

Mercury’s diameter has been updated, but two other updates have failed, and a reason has been given in each case. Our user has been informed of the problems (in a rather basic fashion using the alert() function) and can take remedial action.

The server-side component that handles these requests needs to be able to break the request data into commands and assign each command to an appropriate handler object for processing. As each command is processed, the result will be written back to the HTTP response. A simple implementation of a Java servlet for handling this task is given in listing 5.15.

Listing 5.15 CommandServlet.java

public class CommandServlet extends HttpServlet {

private Map commandTypes=null;

public void init() throws ServletException { ServletConfig config=getServletConfig();

commandTypes=new HashMap(); b Configure handlers on startup boolean more=true;

for(int counter=1;more;counter++){

String typeName=config.getInitParameter("type"+counter); String typeImpl=config.getInitParameter("impl"+counter); if (typeName==null || typeImpl==null){

more=false;

}else{

try{

Class cls=Class.forName(typeImpl); commandTypes.put(typeName,cls);

}catch (ClassNotFoundException clanfex){ this.log(

		Writing to the server	203
"couldn't resolve handler class name "
+typeImpl);
}
}
}
}
protected void doPost(
HttpServletRequest req,
HttpServletResponse resp
) throws IOException{	c Process a request
resp.setContentType("text/xml");
Reader reader=req.getReader();
Writer writer=resp.getWriter();
try{
SAXBuilder builder=new SAXBuilder(false);
Document doc=builder.build(reader);		d Process XML data

Element root=doc.getRootElement();

if ("commands".equals(root.getName())){

for(Iterator iter=root.getChildren("command").iterator(); iter.hasNext();){

Element el=(Element)(iter.next());

String type=el.getAttributeValue("type"); XMLCommandProcessor command=getCommand(type,writer); if (command!=null){

Element result=command.processXML(el); e Delegate to handler writer.write(result.toString());

}

}

}else{

sendError(writer,

"incorrect document format - " +"expected top-level command tag");

}

}catch (JDOMException jdomex){

sendError(writer,"unable to parse request document");

}

}

private XMLCommandProcessor getCommand

(String type,Writer	writer)
throws IOException{	f Match handler to command

XMLCommandProcessor cmd=null;

Class cls=(Class)(commandTypes.get(type)); if (cls!=null){

try{

cmd=(XMLCommandProcessor)(cls.newInstance()); }catch (ClassCastException castex){

sendError(writer, "class "+cls.getName()

+" is not a command");

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}catch (InstantiationException instex) { sendError(writer,

"not able to create class "+cls.getName());

}catch (IllegalAccessException illex) { sendError(writer,

"not allowed to create class "+cls.getName());

}

}else{

sendError(writer,"no command type registered for "+type);

}

return cmd;

}`

private void sendError

(Writer writer,String message) throws IOException{ writer.write("<error msg='"+message+"'/>"); writer.flush();

}

}

The servlet maintains a map of XMLCommandProcessor objects that are configured here through the ServletConfig interface b. A more mature framework might provide its own XML config file. When processing an incoming POST request c, we use JDOM to parse the XML data d and then iterate through the <command> tags matching type attributes to XMLCommandProcessors e. The map holds class definitions, from which we create live instances using reflection in the getCommand() method f.

The XMLCommandProcessor interface consists of a single method:

public interface XMLCommandProcessor { Element processXML(Element el);

}

The interface depends upon the JDOM libraries for a convenient object-based representation of XML, using Element objects as both argument and return type. A simple implementation of this interface for updating planetary data is given in listing 5.16.

Listing 5.16 PlanetUpdateCommandProcessor.java

public class PlanetUpdateCommandProcessor implements XMLCommandProcessor {

public Element processXML(Element el) {

Element result=new Element("command"); b Create XML result node String id=el.getAttributeValue("id");

find-

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result.setAttribute("id",id);
String status=null;
String reason=null;
String planetId=el.getAttributeValue("planetId");
String field=el.getAttributeValue("field");
String value=el.getAttributeValue("value");
Planet planet=findPlanet(planetId); c Access domain model
if (planet==null){
status="failed";
reason="no planet found for id "+planetId;
}else{
Double numValue=new Double(value);
Object[] args=new Object[]{ numValue };
String method = "set"+field.substring(0,1).toUpperCase()
+field.substring(1);
Statement statement=new Statement(planet,method,args);
try {	d Update domain model
statement.execute();
status="ok";

}catch (Exception e) { status="failed";

reason="unable to set value "+value+" for field "+field;

}

}

result.setAttribute("status",status); if (reason!=null){

result.setAttribute("reason",reason);

}

return result;

}

private Planet findPlanet(String planetId) {

// TODO use hibernate	e Use ORM for domain model
return null;
}

}

As well as using JDOM to parse the incoming XML, we use it here to generate XML, building up a root node b and its children programmatically in the processXML() method. We access the server-side domain model using the Planet() method c, once we have a unique ID to work with. findPlanet() isn’t implemented here, for the sake of brevity—typically an ORM such as Hibernate would be used to talk to the database behind the scenes e. We use reflection to update the domain model dand then return the JDOM object that we have constructed, where it will be serialized by the servlet.