Dependent Object persistence in EJB 1.1

Question

Please take a look at the following container-managed entity EJB
code:
//REMOTE INTERFACE //----------------------
import javax.ejb.EJBObject;
import java.rmi.RemoteException;
public interface Product extends EJBObject {
public void setPrice(double price) throws RemoteException;
public double getPrice() throws RemoteException;
public String getDescription() throws RemoteException;
public void setDescription(String desc) throws RemoteException;
public ItemView getItemView() throws RemoteException;
public void setItemView(ItemView i) throws RemoteException;
//HOME INTERFACE //------------------------------
import java.util.Collection;
import java.rmi.RemoteException;
import javax.ejb.*;
public interface ProductHome extends EJBHome {
public Product create(String productId, String description,
double balance, ItemView itemView) throws RemoteException,
CreateException;
public Product findByPrimaryKey(String productId)
throws FinderException, RemoteException;
public Collection findByDescription(String description)
throws FinderException, RemoteException;
public Collection findInRange(double low, double high)
throws FinderException, RemoteException;
//EJBean //----------------------------------------------
import java.util.*;
import javax.ejb.*;
public class ProductBean implements EntityBean {
public String productId;
public String description;
public double price;
//custom java class: Item
public Item item;
private EntityContext context;
public void setPrice(double price) {
this.price = price;
public double getPrice() {
return price;
//instead of returning field Item, an object of ItemView with Item
data is returned.
public ItemView getItemView()
return new ItemView(item.getDesc(), item.getCodeNum());
public void setItemView(ItemView itemView)
this.item = new Item(itemView.getDView(), itemView.getCView
public String getDescription() { return description; }
public void setDescription(String descri) {description = descri;}
public String ejbCreate(String productId, String description,
double price, ItemView itemView) throws CreateException {
if (productId == null) {
throw new CreateException("The productId is required.");
this.productId = productId;
this.description = description;
this.price = price;
item = new Item(itemView.getDView(), itemView.getCView());
return null;
public void setEntityContext(EntityContext context) {
this.context = context;
public void ejbActivate() {
productId = (String)context.getPrimaryKey();
public void ejbPassivate() {
productId = null;
description = null;
public void ejbRemove() { }
public void ejbLoad() { }
public void ejbStore() { }
public void unsetEntityContext() { }
public void ejbPostCreate(String productId, String description,
double balance, ItemView itemView) { System.out.println
("ProductBean ejbPostCreate"); }
} // End of ProductBean
//Item //-------------
public class Item implements java.io.Serializable {
public String desc;
public int codeNum;
public Item(String desc, int CodeNum) {
desc = desc;
codeNum = codeNum;
public void setDesc(String decs) {desc = desc;}
public void setCodeNum(int i) {codeNum = codeNum;}
public String getDesc() {return desc;}
public int getCodeNum(){return codeNum;}
} //End of Item
//ItemView //-------------------------------------
public class ItemView implements java.io.Serializable {
public String dView;
public int cView;
public ItemView(String s, int i) {
dView = s;
cView = i;
public void setDView(String s) {dView = s;}
public String getDView() {return dView; }
public void setCView(int i) {cView = i;}
public int getCView() {return cView;}
Upon running the following client:
//ProductClient //-------------------------------------
import java.util.*;
import javax.naming.Context;
import javax.naming.InitialContext;
import javax.rmi.PortableRemoteObject;
public class ProductClient {
public static void main(String[] args) {
try {
Context initial = new InitialContext();
Object objref = initial.lookup("MyProduct123");
ProductHome home =
(ProductHome)PortableRemoteObject.narrow(objref,
ProductHome.class);
Product duke = home.create("1006", "Siamese atty", 23.13,
new ItemView("Hot",12));
System.out.println("Description: " + duke.getDescription());
System.out.println("Price: "+ duke.getPrice());
System.out.println("Item desc: " + duke.getItemView
().getDView());
System.out.println("Item CodeNum: " + duke.getItemView
().getCView());
} catch (Exception ex) {
System.err.println("Caught an exception." );
ex.printStackTrace();
This is expected to create a ProductBean whose Item field variable
object will have "Hot" and 12 as its values. However, the program
output is:
Description: Ceramic Catty
Price: 19.23
Item desc: null
Item CodeNum: 0
1. I am unable to understand why the Item object contained in the
ProductBean remains null.
2. I read in a sdmagazine article, that helper objects (dependent
objects) should not be exposed directly to the client. I have used
the ItemView in line with this argument although I am not clear if
this is a requirement , if so, what is the rationale behind it.
Please see
http://www.sdmagazine.com/documents/s=739/sdm0010j/0010j.htm.
3. If I do not use ItemView and expose the Item field directly to the
client, there is a runtime error NullPointerExc eption...
4. Does EJB 2.0 adopts the same approach i.e. can depenmdent objects
be directly exposed to the client or not. Or this is just a
recommendation not a requirement?
In the above mentioned article :
(http://www.sdmagazine.com/documents/s=739/sdm0010j/0010j.htm)
"EJB enables me to improve the design I presented in Figure 1. The
problem is the address attribute; it's an object and not a primitive
value. Because I need to persist addresses, I have two options: I can
implement it as a dependent value object or as a dependent object. A
dependent value object is one that is serializable, the properties of
which are saved as a single value; it's the relational database
equivalent of a binary large object (blob). On the other hand, the
properties of dependent objects are saved as individual values, one
for each property. This appears to be a significant advantage, but it
comes at a price: A dependent object must never be directly exposed
to the clients of entity beans via its remote interface because the
persistence container is free to optimize the delivery of dependent
objects and may never fully materialize them as full-fledged classes.
What you should do, as Figure 2 indicates, is expose a representation
of the dependent object if required. I have done this with the new
versions of the getAddress() and setAddress() operations in the
Customer class that now work with instances of XmlDocument instead of
Address. A little clunky, but reasonably easy to live with in
practice. Note that I didn't have to introduce any code to maintain
the relationship between the two classes to my model."
5. This is totally new to me. I don't know what the author means when
he says 'the properties of
which (dependent object) are saved as individual values, one for each
property.' Does this mean,
the dependent object can be persisted in a separate table. How is it
possible while using CMP? 6.
And 'all this comes at a price...' I can't see what's the price.
Any help is highly appreciated.
Regards,
Hasan

Answer 1

Did you forget adding "this." in the item.java.