field-based persistent objects

Amber's persistence based on JPA (the Java Persistence Architecture) supports a field-based relational model: each Java field represents a database column. The tutorial shows the configuration, classes, and client code for a single-table entity.

Demo

Amber provides a Java model for a relational database, following the Java Persistence standard.

A typical project starts planning with the relational database schema and matching the Java model to that schema. This data-driven approach contrasts with a transparent persistent object approach which starts with Java classes and then tries to create storage to match the Java model, an approach more typical of object-oriented databases. While the transparent persistence model may be appropriate for some applications, the persistence specification wisely leaves transparent persistence to other products and specifications, and concentrates on the relational database model.

In a way, Amber simply provides an extension to SQL queries, returning fully-populated Java objects instead of just returning primitive values like Strings. That somewhat understates Amber's capabilities since the Java objects are live, updating the database in a object-oriented fashion, and also provides caching. Still, viewing Amber as a SQL extension supporting relations and objects is a good starting model.

The tutorial uses "entity" to mean a persistent object.

The tutorial's design begins with its database model. The table is a collection of school courses, each with an assigned teacher. The table has an integer primary key "id" and two string data fields, "course" and "teacher".

CREATE TABLE amber_basic_courses (
  id INTEGER PRIMARY KEY auto_increment,

  course VARCHAR(250),
  teacher VARCHAR(250)
);

INSERT INTO amber_basic_courses VALUES('Potions', 'Severus Snape');
INSERT INTO amber_basic_courses VALUES('Transfiguration', 'Minerva McGonagall');

To judge the complexity of Amber, it's useful to compare the Amber Java model to the simplest possible Java model. The simple model has a single class, Course, for the table and three fields for the table's columns.

package example;

public class Course {
  private int id;
  private String course;
  private String teacher;
}

The minimal class is missing any description of its intended use as a persistent object, information needed for maintainable code. In theory, a persistent object tool could use the minimal class automatically, but without more information, the source doesn't properly describe the class behavior. Fortunately, the JDK 1.5 metadata annotations can describe the persistence information in a maintainable, self-documenting way.

Of course, those annotations might have default values and should be overridable by an optional XML configuration file, but it's necessary to annotate the intended function of the entity in the Java source itself to properly document and validate the Java code.

To find and enhance a persistent Java bean, Amber follows the following procedure.

1. <ejb-server> in the resin-web.xml configures Amber to start looking for persistence.xml in the classpath.
2. The persistence.xml in WEB-INF/classes/META-INF tells Amber to create a persistence-unit named "example".
3. The "example" persistence-unit contains a class example.CourseBean
4. Amber enhances example.CourseBean as a persistent object.

By the end of initialization time, Amber has enhanced CourseBean and made it available to the application in the persistence-unit "example".

A servlet will then lookup the CourseBean with the following procedure:

1. Obtain an EntityManager for the persistence unit either using WebBeans @In (or @Named) or using the @PersistenceUnit injection annotation.
2. Use the EntityManager to find the instance of the bean.

The minimal Java class needs the following annotations to produce a maintainable persistent object:

• A annotation to mark the class as persistent.
• An annotation naming the relational table for the class.
• An annotation to mark the primary key and any auto-generation capability.
• Annotations to mark each persistent field.
• Annotations naming the columns for the table.

The following code shows the Amber annotations for the course entity. As a quick comparison with the minimal Java class shows, Amber is close to the simplest possible implementation of the Java model which provides the necessary annotations in the list.

package example;

import javax.persistence.*;

@Entity
@Table(name="amber_basic_course")
public class Course {
  @Id
  @Column(name="id")
  @GeneratedValue
  private int _id;

  @Basic
  @Column(name="course")
  private String _course;

  @Basic
  @Column(name="teacher")
  private String _teacher;

  public String course()
  {
    return _course;
  }

  public String teacher()
  {
    return _teacher;
  }
}

The example uses the course() and teacher() methods to emphasize that the field accesses to _course and _teacher are live, i.e. they read and write the database values directly. (Under the covers, Amber uses bytecode enhancement to make this work.)

@javax.persistence.Table(name="amber_basic_course")

@Id
@Column(name="id")
@GeneratedValue
private int _id;

@Basic 
@Column (name="course")
private String _course;

With Amber, you create a new database row by creating the model object and then calling persist() on the EntityManager. The EntityManager is the main facade for creating, querying and removing JPA objects. Normally, the application will use injection to get an EntityManager.

import javax.persistence.EntityManager;
import javax.webbeans.In;

public class MyServlet {
  @In EntityManager _manager;

  public void create()
  {
    _manager.getTransaction().begin();
    try {
      _manager.persist(new Course("Potions", "Severus Snape"));
    } finally {
      _manager.getTransaction().commit();
    }
  }
}

As mentioned above, the servlet gets its EntityManager using WebBeans injection annotated by @In since there is only one EntityManager in the application.

The persist() must be enclosed in a transaction, as required by the JPA specification. The transaction ensures that the database update is safe and consistent. In this example, we've used an explicit transaction. Many applications will use session beans to handle the transactions automatically.

Once the Course is in the database, we can read it by its primary key using the EntityManagerfind() method.

import javax.persistence.EntityManager;
import javax.webbeans.In;

public class MyServlet {
  @In EntityManager _manager;

  public void find(PrintWriter out)
  {
    Course course = _manager.find(Course.class, 1);

    out.println(course.course + " " + course.teacher());
  }
}

Notice that the read doesn't require a transaction. In fact, Amber generally generally caches reads and only accesses the database when the cache expires or the data changes.

Of course, the find method is generally only useful if you already know the primary key. Usually, your application will need to query the database using some other criteria, like the name of the Course.

The client servlet queries the database for all courses and lists them. It uses the EntityManager API to create a Query and uses the Query to obtain the results.

import javax.persistence.*;
import javax.webbeans.In;

public class CourseServlet extends HttpServlet {
  @In
  private EntityManager _manager;

  public void service(HttpServletRequest req, HttpServletResponse res)
    throws java.io.IOException, ServletException
  {
    PrintWriter out = res.getWriter();

    res.setContentType("text/html");

    out.println("<h3>Course Details</h3>");

    Query query = _manager.createQuery("SELECT o FROM Course o");
    
    for (Course course : (List<Course>) query.listResults()) {
      out.println("course: " + course.course() + "<br>");
      out.println("teacher: " + course.teacher() + "<br>");
      out.println("<br>");
    }
  }
}

<h3>Course Details</h3>
course: Potions
instructor: Severus Snape

course: Transfiguration
instructor: Minerva McGonagall

@PersistenceContext(name="example")

Query query = _manager.createQuery("SELECT o FROM Course o");

List list = query.listResults();

Finally, the application will use remove() in the EntityManager object to remove the entry.

import javax.persistence.EntityManager;
import javax.webbeans.In;

public class MyServlet {
  @In EntityManager _manager;

  public void remove(Course course)
  {
    _manager.getTransaction().begin();
    try {
      _manager.remove(course);
    } finally {
      _manager.getTransaction().commit();
    }
  }
}

The remove() must be enclosed in a transaction, just like the persist() call. Again, the transaction ensures that the database update is safe and consistent.

The Resin configuration is fairly straightforward. Resin needs to start the ejb-server, configure the JDBC data-source, and list the beans that will be used.

<web-app>
  <!-- server configuration -->
  <persistence-manager data-source="jdbc/resin"/>

  <servlet servlet-name="basic" servlet-class="example.CourseServlet"/>

  <servlet-mapping url-pattern="/basic" servlet-name="basic"/>
</web-app>

The <persistence-manager> configures Amber support.

The persistence.xml lives in META-INF/persistence.xml. Since we're developing in WEB-INF/classes, the file will be in WEB-INF/classes/persistence.xml.

<persistence xmlns="http://java.sun.com/xml/ns/persistence" version="1.0">
  <persistence-unit name="example">
    <class>example.CourseBean</class>

    <exclude-unlisted-classes/>
  </persistence-unit>
</persistence>

Demo