EJB3 session beans allow you to implement data accessing business logic in transactional methods. To actually access the database, you will need EJB3 entity beans and the entity manager API. They are collectively called the Java Persistence API (JPA).

EJB3 Entity Beans are Plain Old Java Objects (POJOs) that map to relational database tables. For instance, the following entity bean class maps to a relational table named customer. The table has three columns: name, age, and signupdate. Each instance of the bean corresponds to a row of data in the table.

@Entity
public class Customer {

  String name;

  public String getName () {
    return name;
  }
  
  public void setName (String name) {
    this.name = name;
  }
  
  int age;
  
  public int getAge () {
    return age;
  }
  
  public void setAge (int age) {
    this.age = age;
  }
  
  Date signupdate;
  
  public Date getSignupdate () {
    return signupdate;
  }
  
  public void setSignupdate (Date signupdate) {
    this.signupdate = signupdate;
  }
}    
 
        

Besides simple data properties, the entity bean can also contain references to other entity beans with relational mapping annotations such as @OneToOne, @OneToMany, @ManyToMany etc. The relationships of those entity objects will be automatically set up in the database as foreign keys. For instance, the following example shows that each record in the Customer table has one corresponding record in the Account table, multiple corresponding records in the Order table, and each record in the Employee table has multiple corresponding records in the Customer table.

@Entity
public class Customer {

  ... ...
  
  Account account;
  
  @OneToOne
  public Account getAccount () {
    return account;
  }
  
  public void setAccount (Accout account) {
    this.account = account;
  }
  
  Employee salesRep;
  
  @ManyToOne
  public Employee getSalesRep () {
    return salesRep;
  }
  
  public void setSalesRep (Employee salesRep) {
    this.salesRep = salesRep;
  }
  
  Vector <Order> orders;
  
  @OneToMany
  public Vector <Order> getOrders () {
    return orders;
  }
  
  public void setOrders (Vector <Order> orders) {
    this.orders = orders;
  }


        

Using the EntityManager API, you can create, update, delete, and query entity objects. The EntityManager transparently updates the underlying database tables in the process. You can obtain an EntityManager object in your EJB3 session bean via the @PersistenceContext annotation.

@PersistenceContext
EntityManager em;

Customer customer = new Customer ();
// populate data in customer

// Save the newly created customer object to DB
em.persist (customer);

// Increase age by 1 and auto save to database
customer.setAge (customer.getAge() + 1);

// delete the customer and its related objects from the DB
em.remove (customer);

// Get all customer records with age > 30 from the DB
List <Customer> customers = em.query (
     "select c from Customer where c.age > 30");

        

The detailed use of the EntityManager API is beyond the scope of this book. Interested readers should refer to the JPA documentation or Hibernate EntityManager documentation.