Implementing an EJB 2.1 EJB QL Select Method

Example 16-3 EJB 2.1 CMP Entity Bean Implementation With Select Methods

package oracle.otnsamples.ejbql;

import javax.ejb.*;
import java.util.*;

public abstract class UserAccountBean implements EntityBean
{
    /* ----------------------------------------
    * Non-Persistent State
    * -------------------------------------- */

    protected EntityContext ctx;

    /* ------------------------------------------------------
    * Begin abstract get/set methods. Container-managed
        persistence fields are specified in the ejb-jar.xml 
        deployment descriptor.
    * ------------------------------------------------------- */

    public abstract Long getAccountnumber();
    public abstract void setAccountnumber(Long newAccountnumber);
 
    public abstract Long getCreditlimit();
    public abstract void setCreditlimit(Long newCreditlimit);
 
    /**
     * Select methods. These are implemented by the container. We can
     * customize the functionality of these methods in the deployment
     * descriptor through EJB-QL.
     *
     * These methods are NOT exposed in the bean's home interface.
     **/

    public abstract Long ejbSelectCreditLimit(Long accountnumber) throws FinderException;
    public abstract Collection ejbSelectByTopAccounts() throws FinderException;
 
    /* ------------------------------------------------------
    * Begin buisness logic methods that use select methods.
    * 
    * These methods are exposed in the bean's home interfaces.
    * ------------------------------------------------------- */

    /**
     * Method to perform post-processing operations on all the
     * UserAccounts retrieved by calling ejbSelectByTopAccounts. This
     * method further process the retrieved UserAccounts and checks
     * for the Accounts with TopCredits (credit limits) and returns the
     * collection of input number of UserAccounts.
     * Post-processing information within the EJB container itself
     * has the following two advantages : 
     *     1) It improves performance as the application can now leverage 
     *            the advantage of the vast resources available to the server.
     *     2) The data-processing code should go into the business logic
     *            and not the web-tier. This helps in maintaining the code.
     * Above are the two design considerations when deciding between ejbFind and 
     * ejbSelect methods.
     *
     * @return Collection of <input number of> Top (credited) UserAccounts
     */
    public Collection ejbHomeTopAccounts(String accountNumbers) throws FinderException
    {
        // Invoke the ejbSelect method and get all the Account Information.
        Collection collection = this.ejbSelectByTopAccounts();
        ...
        return topAccounts;
    }
 
    /**
     * Method to call ejbSelectCreditLimit and return the credit limit value
     * for the input accountnumber without post-processing.
     * Please note that this method returns a Long instead of a collection
     * that is returned normally by the EJB container. This is a major 
     * advantage of ejbSelect methods. Using these methods, we can return
     * an object from 'within' the CMP instead of 'the' CMP. This way, the 
     * application uses the server and the EJB container resources more 
     * effeciently.
     *
     * @return Credit Limit of the input UserAccount
     */
    public Long ejbHomeCreditLimit(Long accountnumber) throws FinderException
    {
        // Return the Credit Limit of the specified Account
        return this.ejbSelectCreditLimit(accountnumber);
    }
    ...
}

Example 16-4 ejb-jar.xml For EJB 2.1 EJB QL Select Methods

  <enterprise-beans>
    <entity>
      <description>Entity Bean ( CMP )</description>
      <display-name>UserAccount</display-name>
      <ejb-name>UserAccount</ejb-name>
      <local-home>oracle.otnsamples.ejbql.UserAccountLocalHome</local-home>
      <local>oracle.otnsamples.ejbql.UserAccount</local>
      <ejb-class>oracle.otnsamples.ejbql.UserAccountBean</ejb-class>
      <persistence-type>Container</persistence-type>
      <prim-key-class>java.lang.Long</prim-key-class>
      <abstract-schema-name>UserAccount</abstract-schema-name>
      <cmp-field>
        <field-name>accountnumber</field-name>
      </cmp-field>
      <cmp-field>
        <field-name>creditlimit</field-name>
      </cmp-field>
      <primkey-field>accountnumber</primkey-field>
      <query>
        <description>Selects all accounts and post-process to find top accounts</description>
        <query-method>
          <method-name>ejbSelectByTopAccounts</method-name>
        </query-method>
        <ejb-ql>select distinct object(ua) from UserAccount ua</ejb-ql>
      </query>
      <query>
        <description>Retrieves the Credit Limit for an Account</description>
        <query-method>
          <method-name>ejbSelectCreditLimit</method-name>
          <method-params>
            <method-param>java.lang.Long</method-param>
          </method-params>
        </query-method>
        <ejb-ql>
            select ua.creditlimit from UserAccount ua where ua.accountnumber = ?1
        </ejb-ql>
      </query>
    </entity>
  </enterprise-beans>