Reviews : Java Books :
Professional EJB : The EJB 2.0 Entity Model

A class that provided a client view of the local interface line item object could transport state back and forth from the client:

	public class LineItemView implements java.io.Serializable {
	
		private long id;
		private String product;
		private int quantity;
	
		public LineItemView() {}
	
		public LineItemView(long id, String product, int quantity) {
			this.id = id;
			this.product = product;
			this.quantity = quantity;
		}

		public long getId() {
			return id;

		}

		public void setId(long id) {
			this.id = id;
		}
	
		public String getProduct() {
			return product;
		}
		
		public void setProduct(String product) {
			this.product = product;
		}

		public int getQuantity() {
			return quantity;
		}

		public void setQuantity(int quantity) {
			this.quantity = quantity;
		}
	}

The implementation class for the OrderEJB component would use information in a LineItemView object to update its state, or initialize LineItemView objects and return them to the client to present a view of its data. Here is a very basic example of this technique:

	import java.util.Iterator;
	import java.util.Collection;
	import javax.ejb.EntityBean;
	import javax.ejb.EntityContext;
	import javax.ejb.CreateException;

	public abstract class OrderEJB implements EntityBean {

Some abstract accessor methods and entity bean methods have been elided from this example...

	public abstract Collection getLineItems();
	public abstract void setLineItems(Collection lineItems);

The getLineItemViews() method creates an array of LineItemView objects to return to the client:

	public LineItemView[] getLineItemViews() {

		LinkedList returnValues = new LinkedList();
		Iterator iterLineItems = getLineItems().iterator();

		while (iterLineItems.hasNext()) {
			LineItem lineItem = (LineItem) iterLineItems.next();
			LineItemView view = new LineItemView(lineItem.getId(),
				lineItem.getProduct(),
				lineItem.getQuantity());
			returnValues.add( view );
		}
	
		return (LineItemView[]) returnValues.toArray(new LineItemView[]{});
	}

The addLineItem() method uses the information in its LineItemView parameter to create a new LineItem component represented by a local interface. The implCreateLineItem() method is just a simple method that acquires a home interface and calls a create() method:

	private LineItem implCreateLineItem(int id, String product,
				int quantity) {
	
	try {
		InitialContext initial = new InitialContext();
		LineItemLocalHome home = (LineItemLocalHome)
			initial.lookup("java:comp/env/ejb/LineItemLocal");
		return home.create(id, product, quantity);	
	} catch (Exception e) {
		throw new javax.ejb.EJBException(e);	
	}
}


public void addLineItem(LineItemView liv) throws CreateException {
	try {
		LineItem l = implCreateLineItem(liv.getId(), liv.getProduct(),
					liv.getQuantity() );
		getLineItems().add(l);
	} catch (CreateException e) {
		e.printStackTrace();
		throw e;
	}
}

The updateLineItem() method uses the information in its LineItemView parameter to update an existing LineItem component (note: we would probably use an ejbSelect() method for performance reasons, rather than iterating through the collection):

	public void updateLineItem(LineItemView liv) {
		Collection col = getLineItems();
		Iterator iter = col.iterator();
		while (iter.hasNext()) {
			LineItem li = (LineItem) iter.next();
			if (li.getId() == liv.getId()) {
				li.setProduct(liv.getProduct());
				li.setQuantity(liv.getQuantity());
				return;
			}
		}
	}


	public void removeLineItem(long lineItemID) {
		Collection col = getLineItems();
		Iterator iter = col.iterator();
		while (iter.hasNext()) {
			LineItem li = (LineItem) iter.next();
			if (li.getId() == lineItemID) {
				iter.remove();
				return;
			}
		}
	}

}

Of course, other approaches are possible. For instance, it is possible to use an XML document to represent client views and server updates of arbitrary nesting and complexity. Whatever approach is appropriate for your project, it is important to remember that local interfaces and relationship collections are "implementation details" that are never directly exposed to a client.

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