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EclipseLink Solutions Guide for EclipseLink
Release 2.5
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Implementing the Solution

This section contains the following tasks to enable shared caching in a JPA environment:

Task 1: Set up the Database and Tables

The solution presumes that you are working with an Oracle 11gR2 (11.2) or higher database that contains the tables that you are interested in.

Task 2: Grant User Permissions

Among other permissions, the database user must be granted the CHANGE NOTIFICATION privilege. To do this, you must have a DBA privilege, such as SYS, or have your database administrator apply it:

grant change notification to user

The following example illustrates granting the change notification privilege to user SCOTT.

...
define user="SCOTT"
define pass="tiger"
grant create session, alter session to &&user
/
grant resource, connect to &&user                               
/
grant select any dictionary to &&user
/
grant select any table to &&user
/
grant change notification to &&user
/
...

Task 3: Set the Classpath

Ensure that the eclipselink.jar EclipseLink library, the ojdbc6.jar JDBC library, the persistence.jar JPA library, and the domain classes are present on the classpath.

Task 4: Identify Classes that will Participate in Change Notification

By default, all entities in the domain will participate in change notification. There are several different ways to limit the entities that will participate. For example, the entity classes can be indicated by the <entity class...> element in the orm.xml file, indicated with the <exclude-unlisted-classes> element in the persistence.xml file, or contained in a JAR file.


NoteNote:

The <exclude-unlisted-classes> element is not intended for use in the Java SE environment.


Entity classes can also be excluded by using a Cache annotation attribute in the Java files. For more information, see Exclude Classes from Change Notification (Optional).

Another way to identify the entity classes is to use the <class> element in the persistence.xml file. The following example indicates that the Order, OrderLine, and Customer classes in the model package will participate in change notification. For an example of a complete persistence.xml file, see Example 20-1.

...
<class>model.Order</class>
<class>model.OrderLine</class>
<class>model.Customer</class>
...

Task 5: Add the Database Event Listener

Use the eclipselink.cache.database-event-listener property to identify the database event listener. The org.eclipse.persistence.platform.database.oracle.dcn.OracleChangeNotificationListener class is the listener for Continuous Query Notification. This allows the EclipseLink cache to be invalidated by database events.

The following example illustrates the eclipselink.cache.database-event-listener property configured with the OracleChangeNotificationListener class. For an example of a complete persistence.xml file, see Example 20-1.

...
   <properties>
      <property name="eclipselink.cache.database-event-listener" value="org.eclipse.persistence.platform.database.oracle.dcn.OracleChangeNotificationListener"/>
   </properties>
...

Note that you can also use:

<property name="eclipselink.cache.database-event-listener" value="DCN"> 

Example 20-1 illustrates an example of a complete persistence.xml file. The classes that will participate in change notification are the Order, OrderLine, and Customer classes from the model package. The eclipselink.cache.database-event-listener property is set to the full path of the OracleChangeNotificationListener class.


NoteNote:

A <provider> tag is optional if running in a container where EclipseLink is the default provider.


Example 20-1 Sample persistence.xml File

<?xml version="1.0" encoding="UTF-8"?>
<persistence xmlns="http://java.sun.com/xml/ns/persistence"
                xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                xsi:schemaLocation="http://java.sun.com/xml/ns/persistence persistence_2_0.xsd"
                version="2.0">
    <persistence-unit name="acme" transaction-type="RESOURCE_LOCAL">
        <provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
        <class>model.Order</class>
        <class>model.OrderLine</class>
        <class>model.Customer</class>
        <exclude-unlisted-classes>false</exclude-unlisted-classes>
        <properties>
            <property name="eclipselink.cache.database-event-listener" value="DCN"/>
        </properties>
    </persistence-unit>
</persistence>

Task 6: Edit the Java Files

Typically, to participate in change notification, no changes are needed to the Java classes which correspond to database tables. However, setting optimistic locking with the @Version annotation is strongly suggested.

If you want to exclude classes that are listed in the persistence unit, you can tag them in the Java files. EclipseLink tracks changes only to the primary table. If you want changes to secondary tables to also be tracked, you can indicate this in the Java files.

Set Optimistic Locking

Oracle strongly suggests that you use optimistic locking: writes on stale data will fail and automatically invalidate the cache. Include an @Version annotation in your entity; the version column in the primary table will always be updated, and the older version of the object will always be invalidated.

In Example 20-2 the @Version annotation is defined for the entity Customer. Note that getters and setters are defined for the version variable.

Example 20-2 Defining the @Version Annotation

...
@Entity
@Table(name="DBE_CUSTOMER")
public class Customer implements Serializable {
    @Id
    @GeneratedValue(generator="CUST_SEQ")
    @TableGenerator(name="CUST_SEQ")
    @Column(name="CUST_NUMBER")
    private long id;

 @Version
    private long version;
 
 ...
    public long getVersion() {
        return version;
    }
 
    public void setVersion(long version) {
        this.version = version;
    }
...

Exclude Classes from Change Notification (Optional)

Use the databaseChangeNotificationType attribute of the Cache annotation to identify the classes for which you do not want change notifications. To exclude a class from change notification, set the attribute to DatabaseChangeNotificationType.NONE, as illustrated in the following example.

...
@Entity
@Cache(databaseChangeNotificationType=DatabaseChangeNotificationType.NONE)
public class Order {
...

Track Changes in Secondary Tables (Optional)

EclipseLink tracks changes only to the primary table. If any updates occur in a secondary table, EclipseLink will not invalidate the object. If you want changes to secondary tables to be tracked as well, add the @Version annotation to the entity.

CQN listens only for events from the primary table. It does not track changes in secondary tables, or relationships tables. The reason for this is that Oracle CQN only tracks the ROWID, so there is no correlation from the ROWID of the primary, secondary and relationship tables. Thus, to receive events when a secondary or relationship table changes, the version in the primary table must change so that the event is returned.

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