Java EE 7 is around for a few years already, and provides several very useful and long-awaited features, like entity graphs and better support for stored procedures and results mapping. For an overview, have a look at Thorben Janssen’s blog post. The query capabilities were also enhanced, with 3 additional keywords. All of them are available in both JPQL and Criteria API:

  • ON keyword to specify conditions for JOINs
  • FUNCTION to call arbitrary database function
  • TREAT to downcast entities to their specific type

In this post, I’ll focus on the first of them – the ON keyword in JOINs.


JPA provides essentially 2 types of locking mechanisms to help synchronize access to entities. Both mechanisms prevent a scenario, where 2 transactions overwrite data of each other without knowing it.

By entity locking, we typically want to prevent following scenario with 2 parallel transactions:

  1. Adam’s transaction reads data X
  2. Barbara’s transaction reads data X
  3. Adam’s transaction modifies data X, and changes it to XA
  4. Adam’s transaction writes data XA
  5. Barbara’s transaction modifies data X and changes it to XB
  6. Barbara’s transaction writes data XB

As a result, changes done by Adam are completely gone and overwritten by Barbara without her even noticing. A scenario like this is sometimes called dirty-read. Obviously, a desired result is that Adam writes XA, and Barbara is forced to review XA changes before writing XB.

How Optimistic Locking works


List of elements in persistence.xml

<!-- turn off 2nd level caching (optional), values:


<!-- desired provider (optional), 
 if not present, default provider will be used -->


<!-- if true, only listed classes will be treated as entities. 
  Default false. 
  Not applicable for Java SE, where every entity must be listed -->


<!-- optional declaration of used datasource. 
  If not specified, connection properties must be specified.
  Otherwise will use the referenced datasource provided by the container -->

Code language: HTML, XML (xml)

These elements can be overridden with the following properties, if a map is passed to EntityManagerFactory:

  • javax.persistence.provider to define the provider class used
  • javax.persistence.transactionType to define the transaction type used (either JTA or RESOURCE_LOCAL)
  • javax.persistence.jtaDataSource to define the JTA datasource name in JNDI
  • javax.persistence.nonJtaDataSource to define the non JTA datasource name in JNDI
  • javax.persistence.lock.timeout – pessimistic lock timeout in milliseconds (Integer or String)
  • javax.persistence.query.timeout – query timeout in milliseconds (Integer or String)
  • javax.persistence.sharedCache.mode corresponds to the share-cache-mode element defined in Section 2.2.1, “Packaging”
  • javax.persistence.validation.mode corresponds to the validation-mode element defined in Section 2.2.1, “Packaging”

See Hibernate 3.5 reference

List of standard properties


<property name="javax.persistence.jdbc.driver" 
  value="org.apache.derby.jdbc.ClientDriver"/>Code language: HTML, XML (xml)


<property name="javax.persistence.jdbc.url" 
  value="jdbc:derby://localhost:1527/chapter02DB;create=true"/>Code language: HTML, XML (xml)


<property name="javax.persistence.jdbc.user" value="APP"/>Code language: HTML, XML (xml)


<property name="javax.persistence.jdbc.password" value="APP"/>Code language: HTML, XML (xml)

Hibernate properties

Debug SQL:

<property name="hibernate.show_sql" value="true"/>Code language: HTML, XML (xml)

Schema generation (optional):

<!-- create the database schema automatically,
  values: create-drop, update -->

<property name="" 

Code language: HTML, XML (xml)


<property name="hibernate.dialect" 
  value="org.hibernate.dialect.H2Dialect" />Code language: HTML, XML (xml)

EclipseLink properties

Schema generation (optional):

<!-- create the database schema automatically, 
  values: create-tables, drop-and-create-tables -->

<property name="eclipselink.ddl-generation" 
  value="create-tables"/>Code language: HTML, XML (xml)


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