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   <title>Chapter 5. Using container-managed persistence</title><link rel="stylesheet" href="styles.css" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/styles.css" type="text/css"><meta name="generator" content="DocBook XSL Stylesheets Vimages/callouts/"><link rel="home" href="index.html" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/index.html" title="JBoss 3.0 Documentation"><link rel="up" href="index.html" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/index.html" title="JBoss 3.0 Documentation"><link rel="previous" href="ch04s02.html" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/ch04s02.html" title="Sample configuration files"><link rel="next" href="ch05s06.html" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/ch05s06.html" title="Container Managed Persistence - CMP"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><table border="0" cellpadding="0" cellspacing="0" height="65"><tr height="65"><td rowspan="2"><img src="jboss.gif" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/jboss.gif" border="0"></td><td rowspan="2" background="gbar.gif" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/gbar.gif" width="100%" align="right" valign="top"><a href="index.html" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/index.html"><img src="doc.gif" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/doc.gif" border="0"></a><a href="index.html" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/index.html"><img src="toc.gif" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/toc.gif" border="0"></a><a href="ch04s02.html" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/ch04s02.html"><img src="prev.gif" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/prev.gif" border="0"></a><a href="ch05s06.html" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/ch05s06.html"><img src="next.gif" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/next.gif" border="0"></a></td></tr><tr></tr></table><div class="chapter"><div class="titlepage"><div><h2 class="title"><a name="cmp"></a>Chapter 5. Using container-managed persistence</h2></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><a href="ch05.html#d0e1780" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/ch05.html#d0e1780">Introduction</a></dt><dt><a href="ch05s06.html" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/ch05s06.html">Container Managed Persistence - CMP</a></dt><dt><a href="ch05s08.html" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/ch05s08.html">Creating the Beans</a></dt><dt><a href="ch05s09.html" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/ch05s09.html">Packaging and deploying the Beans</a></dt><dt><a href="ch05s10.html" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/ch05s10.html">Creating a test client </a></dt><dt><a href="ch05s11.html" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/ch05s11.html">Discussion: container-managed persistence</a></dt></dl></div><p>Author:<span class="author">Kevin Boone</span>
		<tt>&lt;<a href="mailto:k.boone@web-tomorrow.com">k.boone@web-tomorrow.com</a>&gt;</tt>  
	</p><p>JBoss 2.2 compliance checked by:<span class="author">Vincent Harcq</span>
		<tt>&lt;<a href="mailto:vincent.harcq@hubmethods.com">vincent.harcq@hubmethods.com</a>&gt;</tt>  
	</p><div class="section"><a name="d0e1780"></a><div class="titlepage"><div><h2 class="title" style="clear: both"><a name="d0e1780"></a>Introduction</h2></div></div><div class="section"><a name="d0e1783"></a><div class="titlepage"><div><h3 class="title"><a name="d0e1783"></a>What this article is about</h3></div></div><p>This article presents a step-by-step example of creating, deploying and 			  
        testing an Entity JavaBean that uses
        "container-managed persistence" with the JBoss EJB server. This        
        example is very simple, and is a variation of
        the old favourite "music CD" case study. I have chosen this example    
        because the application is likely to be
        familiar to most people without much explanation. In short, the "CD"   
        EJB models a music CD. Applications
	will want to add and delete CDs to a collection, and search the collection 		 
        for specific CDs. There is also a
        "CDCollection" EJB that lists and searches collections of CDs.</p><p>Download and install the full source code as explain in <a href="ch01s08.html" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/ch01s08.html" title="Downloading the Documentation Example Source">the section called &#8220;Downloading the Documentation Example Source&#8221;</a></p><p>The files (sources, build scripts and configuration files) needed for this chapter are under 
"documentation-example/org/jboss/docs/cmp/cd".  Take the time to review them, as well as how the classpath is set prior
to compiling/running the sources.</p><p>Be comfortable with <a href="ch01s05.html" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/ch01s05.html" title="Installing Ant">the section called &#8220;Installing Ant&#8221;</a> as you will have to run ant to build the ejb jar,
and run the test clients.</p></div><div class="section"><a name="d0e1797"></a><div class="titlepage"><div><h3 class="title"><a name="d0e1797"></a>Pre-requisites</h3></div></div><p>You will need a fully-functioning JBoss installation to follow this tutorial, 
which must have a working
database connection. I will assume that you are basically familiar with EJBs 
and know how to structure and
compile JavaBeans as Java packages. Also I will assume you know the basic 
structure of a deployment
descriptor, and the run-time descriptor that JBoss uses (jboss.xml). I will 
assume that you know how to
package and deploy EJBs using JBoss. If you don't know about these things, you 
might want to look at the chapter titled &#8220;First Steps&#8221; first.</p></div><div class="section"><a name="d0e1805"></a><div class="titlepage"><div><h3 class="title"><a name="d0e1805"></a>Persistence: review</h3></div></div><p>There are, in essence, two kinds of Enterprise JavaBean: session and entity. 
Entity EJBs contain information
that is persistent across different client-Bean interactions, while session 
EJBs don't. For example, a class
called "Customer" that represents the customers of a particular service will 
contain persistent information
(about the customer). A class called "CustomerFinder", say, that finds 
Customer instances that match certain
criteria is likely to be a session EJB, because it does not require 
information that is maintained between
different client-server interactions.</p><p>       Session EJBs can be further divided into "stateless" and "stateful". A 
stateful session EJB has a state that is
persistent between invocations of its methods. A stateless EJB does not even 
have to retain its state between
method invocations. The stateless session EJB is therefore the type of EJB 
that exhibits the least persistence.</p><p>       The entity EJBs contain information that persists even when no clients 
are using any of the Bean's
services; the information should persist even if the server is restarted. 
There is a high degree of
correspondence between instances of an entity EJB and rows of a database 
table. In practice, all EJB servers
implement entity instances as table rows. This correspondence is so strong 
that the database notion of a "primary key"
is relevant to an entity EJB. </p><p>       The persistence of an entity EJB may be managed by the Bean itself, or 
by the server (technically by the
"container"). The latter technique is called "Container-managed persistence", 
and is the subject of the rest of
this article.</p></div><div class="section"><a name="d0e1816"></a><div class="titlepage"><div><h3 class="title"><a name="d0e1816"></a>When to Use CMP or BMP?</h3></div></div><p>Unlike what many folks believe, the choice of going BMP or CMP is not really 
one of "trade-off".  If you have
already schemas deployed you may find that the complexity of the schemas 
requires you to go with BMP or use
a BMP generating tool such as "cocobase".  These techniques are what we call 
"fake CMP" where the work of
accessing the database is left to the generated classes. </p><p>The breed of CMP that has most value is the "real CMP" or a CMP where you let 
the container manage the
persistent representation of your beans entirely.  This might not work right 
now for you if your object are
complex but should work in most simple cases. EJB2.0 also goes the extra 
length to make the persistent
engines powerful and fast with management of dependencies and relationships.  
We believe that one day you
will rely on the engines to manage the schemas, just like you rely on a 
compiler to optimize assembly code.</p></div></div></div><table border="0" cellpadding="0" cellspacing="0" height="65"><tr height="65"><td rowspan="2"><img src="gbar.gif" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/gbar.gif" width="432" height="79"></td><td rowspan="2" background="gbar.gif" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/gbar.gif" width="100%" align="right" valign="top"><a href="index.html" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/index.html"><img src="doc.gif" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/doc.gif" border="0"></a><a href="index.html" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/index.html"><img src="toc.gif" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/toc.gif" border="0"></a><a href="ch04s02.html" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/ch04s02.html"><img src="prev.gif" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/prev.gif" border="0"></a><a href="ch05s06.html" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/ch05s06.html"><img src="next.gif" tppabs="http://www.huihoo.org/jboss/online_manual/3.0/next.gif" border="0"></a></td></tr><tr></tr></table></body></html>
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