Asynchronous: The request is taking place in the background, out of sight of the user interface.

XHR: XMLHttpRequest

RPC: Remote Procedure Calling is a mechanism to allow local code to call methods on objects that exist on a remote server as if that remote object were also local.

Stub: RPC works by creating a local proxy interface that mimics the remote method signature. Example, creator = new in dwr.xml creates client-side stub.

* DWR is only useful for java web applications hosted by a servlet engine.

Since demo application worked, I wanted to leverage my DWR knowledge in my existing servlet application. I have a shopping cart application that I wanted to add ajax callback for a field and I am having some serious issues.

Since ‘has-a’ relationship exist in ItemOrder.java, so I had to do marshalling on ItemOrder and CDTypeVO in dwr.xml. Here is the code for marshalling both beans.

<allow>
<convert converter=”bean” match=”cdorder.domain.ItemOrder” />
<create creator=”new” javascript=”ItemOrder”>
<param name=”class” value=”cdorder.domain.ItemOrder”/>
</create>

<convert converter=”bean” match=”cdorder.domain.CDTypeVO” />
<create creator=”new” javascript=”CDTypeVO” >
<param name=”class” value=”cdorder.domain.CDTypeVO”/>
</create>
</allow>

This resolved all red error, but IteamOrder bean still not functioning properly. Every time I click those Execute button, I am getting a nasty pop up Error message. Not sure, what am I missing.

Here is the architecture description: I have a form which has ‘Add’ buttons. When an Add button get clicked, it submits a form and request goes to OrderPageServlet.java and here what it does as follows:

1. Get a shopping cart from session variable
2. If shopping cart is null, instantiate a empty shopping cart and put it in the session

i. cart = (ShoppingCart)session.getAttribute(“shoppingCart”);

ii. // New visitors get a fresh shopping cart.

iii. // Previous visitors keep using their existing cart.

iv. if (cart == null) {

v. cart = new ShoppingCart();

vi. session.setAttribute(“shoppingCart”, cart);

vii. }

1. Now, based on CD Id that has been passed, add item to the cart in Servlet

i. cart.addItem(cdID,type);

1. ShoppingCart has an ArrayList that keep tracks all order
2. Add item to the list in the ShopptingCart.java

i. private ArrayList itemsOrdered = new ArrayList();

ii. ItemOrder newOrder = new ItemOrder(cdTypeDAO.getSingleCdType(cdID,false));

iii. itemsOrdered.add(newOrder);

1. Here is the code snippet of ItemOrder.java

public class ItemOrder {

private CDTypeVO item;

private int numItems;

private String processedDate;

private String processedBy;

public ItemOrder(CDTypeVO item) {

setItem(item);

setNumItems(1);

}

public CDTypeVO getItem() {

return (item);

}

//@Override

// public String toString(){

// return item.toString() + ” ” + getNumItems() + ” cdID ” + getCdId() + ” cdType ” + getCDType() + ” description: ” + getDescription() + “\n”;

// }

public void setItem(CDTypeVO item) {

this.item = item;

}

public int getCdId() {

return (getItem().getCdId());

}

public String getCDType() {

return (getItem().getCdType());

}

public String getDescription() {

return (getItem().getDescription());

}

//quantity

public int getNumItems() {

return numItems;

}

public void setNumItems(int n) {

this.numItems = n;

}

public void incrementNumItems() {

setNumItems(getNumItems() + 1);

}

public void cancelOrder() {

setNumItems(0);

}

public String getProcessedDate() {

return processedDate;

}

public void setProcessedDate(String processedDate) {

this.processedDate = processedDate;

}

public String getProcessedBy() {

return processedBy;

}

public void setProcessedBy(String processedBy) {

this.processedBy = processedBy;

}

public String sayHello(String name) {

return “Hello, ” + name;

}

}

1. Here is the code snippet for CDTypeVO.java Bean\
2. /*
3. * CDType.java
4. *
5. * Created on December 6, 2004, 10:02 AM
6. */
8. package cdorder.domain;
10. /**
11. *
12. * @author hhaque
13. * VO = Value Object
14. */
15. public class CDTypeVO implements java.io.Serializable{
17. private int cdId;
18. private String cdType;
19. private String description;
20. private String date;
22. /** Creates a new instance of CDType */
23. public CDTypeVO() {
24. }
26. // public String toString(){
27. //
28. // return “cdID = ” + getCdId() +
29. // ” cdType = ” + getCdType() +
30. // ” description = ” +getDescription() +” date = “+getDate() + “\n”;
31. // }
33. public int getCdId() {
34. return cdId;
35. }
36. public void setCdId(int cdId) {
37. this.cdId = cdId;
38. }
39. public String getCdType(){
40. return cdType;
41. }
42. public void setCdType(String cdType){
43. this.cdType = cdType;
44. }
45. public String getDescription(){
46. return description;
47. }
48. public void setDescription(String description){
49. this.description = description;
50. }
51. public String getDate(){
52. return date;
53. }
54. public void setDate(String date){
55. this.date = date;
56. }
58. }

When I do this

<script>

var numItemValue = dwr.util.getValue(‘numItems’);

ItemOrder.sayHello(numItemsValue, loadDataCallback); It does not do anything.

Here is my call back function:

function loadDataCallback(returnValue) {

dwr.util.setValue(‘demoReply’, returnValue);

} </script>

Here is the great trick that I have learned today. How to debug DWR? All I had to do to add debug = true in my web.xml file (http://getahead.org/dwr/server/servlet)

<servlet>
<servlet-name>dwr-invoker</servlet-name>
<servlet-class>org.directwebremoting.servlet.DwrServlet</servlet-class>
<init-param>
<param-name>debug</param-name>
<param-value>true</param-value>
</init-param>
</servlet>

Redeploy the application and in the URL I typed in http://localhost:8084/cdOrder/dwr

Keep in mind that ‘dwr’ is a keyword, it is very cool features and I love it. It helped me to speed up my development time.

Jason: DWR dynamically creates a JS file that has a stub that matches the class and methods you’re exposing. so, if you have a Demo class with the method doFoo(), it creates a stub JS class called Demo with the method doFoo() that wraps the Ajax call to the DWR servlet

End of messages received while you were offline at 9:23 AM on Wednesday
Jason: you call Demo.doFoo(<callback>) (iirc) from your page, and it uses the stub plus the plumbing from engine.js to make the XHR call, get the results, and cal your callback function

HobiOne: nice, so question is, if I have 10 beans, I have to create 10 js

Jason: well, you have to configure DWR to expose those beans, then include each JS file in your page
if I were you, i’d use annotations. much easier :)

HobiOne: I see, okay back to your answer, “configure DWR”, do you mean in dwr.xml file

Jason: yes though if you use annotations, I think you set up a different DWR servlet, then just annotate the class and methods you want to expose
it would be nice if DWR offered a KitchenSink.js that included everything else, but I don’t think it does

Sent at 9:27 AM on Wednesday
HobiOne: how about DAO stuff, do I only register beans to dwr.xml or any java classes I make in back end

Jason: well, i’d be REALLY careful what you expose to the outside world

Sent at 9:31 AM on Wednesday
Jason: i.e., i would NOT expose DAOs

HobiOne: got it, only beans, b/c from html, I dont need to talk to DAO, bean should be responsible to communicate with DAOs

A DWR Data Store for Dojo (Putting two powerful AJAX libraries to work for you):

I was trying to run DWR’s demo example and here what I missed.

I forgot to add these following pieces of code in my index.jsp page

<script type=”text/javascript” src=”dwr/engine.js”></script>
<script type=”text/javascript” src=”dwr/util.js”></script>
<script type=”text/javascript” src=”dwr/engine.js”></script>

Second, I forgot to add commons-logging-1.0.4.jar in my class path

And very last I forgot to add

<script type=”text/javascript” src=”dwr/interface/Demo.js”> </script>

Since I had a Demo.java bean

So, summary is, DWR’s http://getahead.org/dwr/getstarted has good documentation but needed little more polishing for people like novice who never used DWR. Now everything works like a champ and I can have a good night sleep.