Chapter 3. EJB Persistence Architecture

Chapter 3. EJB Persistence Architecture
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3.1. EJB Exceptions

The diagram below illustrates the relationships between the primary components of the EJB architecture.

	Note
	A number of the depicted interfaces are only required outside of an EJB3-compliant application server. In an application server, `EntityManager` instances are typically injected, rendering the `EntityManagerFactory` unnecessary. Also, transactions within an application server are handled using standard application server transaction controls. Thus, the `EntityTransaction` also goes unused.

Persistence: The javax.persistence.Persistence class contains static helper methods to obtain EntityManagerFactory instances in a vendor-neutral fashion.
EntityManagerFactory : The javax.persistence.EntityManagerFactory class is a factory for EntityManagers.
EntityManager: The javax.persistence.EntityManager is the primary EJB persistence interface used by applications. Each EntityManager manages a set of persistent objects, and has APIs to insert new objects and delete existing ones. When used outside the container, there is a one-to-one relationship between an EntityManager and an EntityTransaction. EntityManagers also act as factories for Query instances.
Entity: Entites are persistent objects that represent datastore records.
EntityTransaction: Each EntityManager has a one-to-one relation with a single javax.persistence.EntityTransaction. EntityTransactions allow operations on persistent data to be grouped into units of work that either completely succeed or completely fail, leaving the datastore in its original state. These all-or-nothing operations are important for maintaining data integrity.
Query: The javax.persistence.Query interface is implemented by each EJB vendor to find persistent objects that meet certain criteria. EJB standardizes support for queries using both the EJB Query Language (JPQL) and the Structured Query Language (SQL). You obtain Query instances from an EntityManager.

The example below illustrates how the EJB interfaces interact to execute a JPQL query and update persistent objects. The example assumes execution outside a container.

Example 3.1. Interaction of Interfaces Outside Container

// get an EntityManagerFactory using the Persistence class; typically 
// the factory is cached for easy repeated use
EntityManagerFactory factory = Persistence.createEntityManagerFactory (null);

// get an EntityManager from the factory
EntityManager em = factory.createEntityManager (PersistenceContextType.EXTENDED);

// updates take place within transactions
EntityTransaction tx = em.getTransaction ();
tx.begin ();

// query for all employees who work in our research division
// and put in over 40 hours a week average
Query query = em.createQuery ("select e from Employee e where "
    + "e.division.name = 'Research' AND e.avgHours > 40");
List results = query.getResultList ();

// give all those hard-working employees a raise
for (Object res : results)
{
    Employee emp = (Employee) res;
    emp.setSalary (emp.getSalary () * 1.1);
}

// commit the updates and free resources
tx.commit ();
em.close ();
factory.close ();

Within a container, the EntityManager will be injected and transactional handled declaratively. Thus, the in-container version of the example consists entirely of business logic:

Example 3.2. Interaction of Interfaces Inside Container

// query for all employees who work in our research division
// and put in over 40 hours a week average - note that the EntityManager em
// is injected using a @Resource annotation
Query query = em.createQuery ("select e from Employee e where "
    + "e.division.name = 'Research' and e.avgHours > 40");
List results = query.getResultList ();

// give all those hard-working employees a raise
for (Object res : results)
{
    emp = (Employee) res;
    emp.setSalary (emp.getSalary () * 1.1);
}

The remainder of this document explores the EJB interfaces in detail. We present them in roughly the order that you will use them as you develop your application.

3.1. EJB Exceptions

The diagram above depicts the EJB persistence exception architecture. All exceptions are unchecked. EJB persistence uses standard exceptions where appropriate, most notably IllegalArgumentExceptions and IllegalStateExceptions. The specification also provides a few EJB-specific exceptions in the javax.persistence package. These exceptions should be self-explanatory. See the Javadoc for additional details on EJB exceptions.

	Note
	All exceptions thrown by Kodo implement `kodo.util.ExceptionInfo` to provide you with additional error information.

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