Basic CMP example showing configuration, classes, and client code for a single-table bean.

This example focuses on:

• Introduces Container Managed Persistance (CMP) fundamental concepts
• Setting up the database to work with CMP
• Developing the EntityBean classes
• Developing a Servlet to lookup and use the entity bean
• Configuring Resin to deploy the EJB and use JNDI

CMP manages tables in a relational database using a Java bean interface. Each database table corresponds to a single "entity bean". By creating an entity bean with container managed persistence, you let CMP generate the SQL to load, store, and cache entity beans from the database. Avoiding SQL is an advantage in itself, but the primary advantage is the increased flexiblity of your application code. Maintenance and code-refactoring can focus on the beans instead of changing lots of SQL statements in the program.

Files in this tutorial

WEB-INF/web.xml	web.xml configuration
WEB-INF/classes/example/CourseBean.java	The course bean
WEB-INF/classes/example/CourseServlet.java	The course servlet

Database Schema

course.sql

CREATE TABLE basic_courses ( id INTEGER PRIMARY KEY auto_increment, course VARCHAR(250), teacher VARCHAR(250) ); INSERT INTO basic_courses VALUES('Potions', 'Severus Snape'); INSERT INTO basic_courses VALUES('Transfiguration', 'Minerva McGonagall');

Bean Implementation

CourseBean.java

package example; @javax.ejb.Entity @javax.ejb.Table (name="ejb3_basic_course") public class CourseBean { private int _id; private String _course; private String _teacher; @javax.ejb.Id (generator=javax.ejb.GeneratorType.AUTO) @javax.ejb.Column (name="id") public int getId() { return _id; } public void setId(int id) { _id = id; } @javax.ejb.Basic public String getCourse() { return _course; } public void setCourse(String course) { _course = course; } @javax.ejb.Basic public String getTeacher() { return _teacher; } public void setTeacher(String teacher) { _teacher = teacher; } }

With Resin, all the Java source can be dropped in WEB-INF/classes. Resin will automatically compile any changes and regenerate the persistence classes, stubs and skeletons.

Resin Configuration

Now that we've built the bean, we need to attach it to Resin. The entity bean is deployed using the ejb-server resource.

WEB-INF/web.xml

<web-app> <!-- server configuration --> <ejb-server data-source="jdbc/resin"> <bean type="example.CourseBean"/> </ejb-server> <servlet servlet-name="basic" servlet-class="example.CourseServlet"> <init entity-manager="com.caucho.amber.ejb3.EntityManagerProxy@bc412c"/> </servlet> <servlet-mapping url-pattern="/basic" servlet-name="basic"/> </web-app>

Client Servlet

CourseServlet.java

import javax.ejb.EntityManager; public class CourseServlet extends HttpServlet { private EntityManager _manager; public void setEntityManager(EntityManager manager) { _manager = manager; } public void service(HttpServletRequest req, HttpServletResponse res) throws java.io.IOException, ServletException { PrintWriter out = res.getWriter(); res.setContentType("text/html"); CourseBean []course = new CourseBean[2]; course[0] = (CourseBean) _manager.find("CourseBean", new Integer(1)); course[1] = (CourseBean) _manager.find("CourseBean", new Integer(2)); out.println("<h3>Course Details</h3>"); for (int i = 0; i < course.length; i++) { out.println("course: " + course[i].getCourse() + "<br>"); out.println("teacher: " + course[i].getTeacher() + "<br>"); out.println("<br>"); } } }

Course Details course: Potions instructor: Severus Snape course: Transfiguration instructor: Minerva McGonagall

Conclusion

The core of EJB's database management is its management of a single table. Much of the work underlying the database management is hidden from the applicaton. Transaction management and caching happen automatically. For example, once the course has been loaded from the database, Resin-CMP does not need to query the database again until the course changes. So read-only requests, the most common, can avoid all database traffic.

More complicated applications build on the single table management. The following examples add more realistic features to this example: using queries to find all courses and creating new database rows.

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