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<h2 class="first-section-title"><a name="383"></a><a name="ch12lev1sec2"></a>Using Entity Beans</h2><p class="first-para">For readers who use entity beans, this section illustrates how to incorporate them into a layered architecture. This is not intended as comprehensive entity bean coverage. For more in-depth information on using entity beans, see <a href="LiB0088.html#427" target="_parent" class="chapterjump">Alur et al. (2001)</a>.</p>
<p class="para">Using the layered architecture as discussed throughout this book, entity bean usage occurs in data access objects and makes native JDBC code within data access objects unnecessary. In other words, each method in a DAO uses entity beans to retrieve and store data, not native JDBC. For example, consider a method on <span class="fixed">PurchaseOrderDAO</span> called <span class="fixed">getPOsForCustomer</span> that will look up all orders for a given customer. <a class="internaljump" href="#ch12list02">Listing 12.2</a> illustrates how the method uses an entity bean to retrieve information for the purchase orders and place them in <span class="fixed">PurchaseOrderVO</span> objects so that they can be used throughout the application.</p>
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<span class="example-title"><span class="example-titlelabel">Listing 12.2: </span>Using Entity Beans for Data Access Objects</span><a name="384"></a><a name="ch12list02"></a>
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<pre class="literallayout">
1: public PurchaseOrderVO[] getPOsForCustomer (
2: String customerId)
3: {
4: PurchaseOrderVO[] po = null;<a name="385"></a><a name="IDX-158"></a>
5: try
6: {
7: // Look up Entity Bean reference
8: Context ctx = new InitialContext();
9: PurchaseOrderHome poHome = (PurchaseOrderHome)
10: PortableRemoteObject.narrow(
11: ctx.lookup(
12: "java:comp/env/BookSamples/PurchaseOrder"),
13: PurchaseOrderHome.class);
14: // —> End of Entity Bean Lookup
15:
16: PurchaseOrder poEjb = null;
17: Iterator poIt = null;
18: Collection poC =
19: poHome.findByCustomerId(customerId);
20: if (poC.size() > 0)
21: {
22: po = new PurchaseOrderVO[poC.size()];
23: int i = 0;
24: poIt = poC.iterator();
25: while (poIt.hasNext())
26: {
27: poEjb = (PurchaseOrder)poIt.next();
28: po[i] = new PurchaseOrderVO();
29: po[i].setCustomerId(poEjb.getCustomerId());
30: po[i].setOrderNbr(poEjb.getOrderNbr());
31: po[i].setOrderDate(poEjb.getDateCreated());
32: po[i].setShipDate(poEjb.getDateShipped());
33:
34: i++;
35: }
36: }
37:
38: }
39: catch (Throwable t)
40: {
41: throw new SampleException
42: ( "Error searching PO. cust=" +
43: customerId, t);
44: }
45:
46: return po;
47: }
</pre>
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<p class="para">
<i class="emphasis">Source:</i> /src/book/sample/dao/db/PurchaseOrderDAO.java</p>
<p class="para">Note that this example uses container-managed persistence and local interfaces from the 2.0 EJB specification.</p>
<p class="para">The example in <a class="internaljump" href="#ch12list02">listing 12.2</a> illustrates that in a layered architecture, DAOs <a name="386"></a><a name="IDX-159"></a>can have the same role within the application no matter what technology the objects use to interact with a database. Data access objects can be converted from using native JDBC to using entity beans (and vice versa) without affecting the rest of the application.</p>
<p class="last-para">As a performance enhancement, you may want to consider placing the context and JNDI look-up (lines 7 through 14 of <a class="internaljump" href="#ch12list02">listing 12.2</a>) in the constructor so that you don't repeat these operations for each call. I don't make this a general recommendation because it can cause availability issues in clustered environments.</p>
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