J2EE^TM Developer's Guide
Entity Beans

Primary Key Class

You specify the primary key class with the Application Deployment Tool. When deploying the ProductEJB bean, for example, you would specify a java.lang.String as the primary key class. In most cases, your primary key class will be a String or some other class that belongs to the java package.

Creating a Primary Key Class

For some entity beans, you will need to define your own primary key class. For example, if a primary key is composed of multiple fields then you must create a primary key class. In the following primary key class, the productId and vendorId fields together uniquely identify an entity bean:

public class ItemKey implements java.io.Serializable {
   
   public String productId;
   public String vendorId;

   public ItemKey() { };

   public ItemKey(String productId, String vendorId) {

     this.productId = productId;
     this.vendorId = vendorId;
   }
 
   public String getProductId() {

      return productId;
   }

   public String getVendorId() {

      return vendorId;
   }
 
   public boolean equals(Object other) {

      if (other instanceof ItemKey) {
         return (productId.equals(((ItemKey)other).productId) 
                 && vendorId.equals(((ItemKey)other).vendorId));
      }

      return false;
   }

   public int hashCode() {

      return productId.hashCode();
   }
}

Class Requirements

A primary key class must meet these requirements:

The access control modifier of the class is public.
All fields are declared as public.
For container-managed persistence, the field names in the primary key clsss must match the corresponding container-managed fields in the entity bean class.
The class has a public default constructor.
The class implements the hashCode() and equals(Object other) methods.
The class is serializable.

Bean-Managed Persistence and the Primary Key Class

With bean-managed persistence, the ejbCreate method returns the primary key class:

public ItemKey ejbCreate(String productId, String vendorId,
   String description) throws CreateException {

   if (productId == null || vendorId == null) {
      throw new CreateException(
                "The productId and vendorId are required.");
   }

   this.productId = productId;
   this.vendorId = vendorId;
   this.description = description;

   return new ItemKey(productId, vendorId);
}

The ejbFindByPrimaryKey verifies the existence of the database row for the given primary key:

public ItemKey ejbFindByPrimaryKey(ItemKey primaryKey) 
   throws FinderException {

   try {
      if (selectByPrimaryKey(primaryKey))
         return primaryKey;
   . . .
}

private boolean selectByPrimaryKey(ItemKey primaryKey) 
   throws SQLException {

   String selectStatement =
         "select productid " +
         "from item where productid = ? and vendorid = ?";
   PreparedStatement prepStmt =
         con.prepareStatement(selectStatement);
   prepStmt.setString(1, primaryKey.getProductId());
   prepStmt.setString(2, primaryKey.getVendorId());
   ResultSet rs = prepStmt.executeQuery();
   boolean result = rs.next();
   prepStmt.close();
   return result;
}

Container-Managed Persistence and the Primary Key Class

For an entity bean with container-managed persistence, the ejbCreate method returns null. (With bean-managed persistence, it returns an instance of the primary key class.)

With container-managed persistence, you do not have to write the code for the ejbFindByPrimaryKey method. (For more information, see the section, The Finder Methods on page 61.)

To create an container-managed entity bean with the ItemKey class example, in the Entity Settings dialog of the New Enterprise Bean Wizard you would specify these settings:

Select Container-Managed Persistence.
Select the check boxes for all three instance variables (productId, vendorId, description).
In the PrimaryKey Class field, enter ItemKey.
Leave the Primary Key Field Name field blank.

Getting the Primary Key

A client can fetch the primary key of an entity bean by invoking the getPrimaryKey method of the EJBObject class:

Account account;
. . .
String id = (String)account.getPrimaryKey();

The entity bean retrieves its own primary key by calling the getPrimaryKey method of the EntityContext class:

EntityContext context;
. . .
String id = (String) context.getPrimaryKey();

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