JPA2 run-time database connection - runtime

Question

I'm using JP2 in my current web project. My main database holds the main entities. To connect on this DB i defined a Persitence Unit with a JTA Datasource:
Persistance.xml:
<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.0" 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 http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd">
<persistence-unit name="MyPU" transaction-type="JTA">
<provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
<jta-data-source>MyDB</jta-data-source>
<exclude-unlisted-classes>false</exclude-unlisted-classes>
<properties>
<property name="eclipselink.logging.level" value="FINE"/>
<property name="eclipselink.logging.parameters" value="true"/>
<property name="eclipselink.logging.logger" value="ServerLogger"/>
<property name="eclipselink.ddl-generation" value="drop-and-create-tables"/>
</properties>
</persistence-unit>
</persistence>
and the JTA Datasource defined in sun-resources.xml:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE resources PUBLIC "-//GlassFish.org//DTD GlassFish Application Server 3.1 Resource Definitions//EN" "http://glassfish.org/dtds/glassfish-resources_1_5.dtd">
<resources>
<jdbc-connection-pool allow-non-component-callers="false" associate-with-thread="false" connection-creation-retry-attempts="0" connection-creation-retry-interval-in-seconds="10" connection-leak-reclaim="false" connection-leak-timeout-in-seconds="0" connection-validation-method="auto-commit" datasource-classname="org.postgresql.ds.PGSimpleDataSource" fail-all-connections="false" idle-timeout-in-seconds="300" is-connection-validation-required="false" is-isolation-level-guaranteed="true" lazy-connection-association="false" lazy-connection-enlistment="false" match-connections="false" max-connection-usage-count="0" max-pool-size="32" max-wait-time-in-millis="60000" name="post-gre-sql_mydb_mypool" non-transactional-connections="false" pool-resize-quantity="2" res-type="javax.sql.DataSource" statement-timeout-in-seconds="-1" steady-pool-size="8" validate-atmost-once-period-in-seconds="0" wrap-jdbc-objects="false">
<property name="serverName" value="localhost"/>
<property name="portNumber" value="5432"/>
<property name="databaseName" value="mydb"/>
<property name="User" value="myuser"/>
<property name="Password" value="mypass"/>
<property name="URL" value="jdbc:postgresql://localhost:5432/mydb"/>
<property name="driverClass" value="org.postgresql.Driver"/>
<property name="characterEncoding" value="UTF-8" />
</jdbc-connection-pool>
<jdbc-resource enabled="true" jndi-name="MyDB" object-type="user" pool-name="post-gre-sql_mydb_mypoll"/>
</resources>
And this is how i access the database on my DAO classes (witch are #ManagedBeans and #SessionScoped):
#ManagedBean(name = "pageDao")
#SessionScoped
public class PageDao implements Serializable {
#Resource
private UserTransaction utx = null;
#PersistenceUnit(unitName = "MyPU")
private EntityManagerFactory emf = null;
public EntityManager getEntityManager() {
return emf.createEntityManager();
}
public List<PageEnt> getAll() { ... }
public PageEnt getOne(long pageId) { ... }
public void addPage(PageEnt newPage) throws RollbackFailureException, PreexistingEntityException, Exception { ... }
public PageEnt update(PageEnt page) throws RollbackFailureException, NonexistentEntityException, Exception { ... }
public void remove(PageEnt page) throws RollbackFailureException, Exception { ... }
}
One of entities (customer) has properties for connecting on a separate (per-customer) database, witch are defined in run-time. These properties includes:
Databse name
Host and port
User and Password
My question are:
How do I efficiently create a database connection in run-time?
How can I create a new EntityManager from container-managed resources if there is no per-customer PersistanceUnit and Datasources defined (witch are defined at deploy-time)?
If i have to manually deal with the EntityManagerFactory (witch, as i learned in college, is a heavy and expansive object), how do I efficiently do that? Is there a good-practice or pattern?
How would the DAO pattern work? How do my DAO class will get the EntityManager?
Big thanks from Brazil.

Answer 1

It is possible to switch between multiple data sources at run time. It is provided by Spring AbstractRoutingDataSource . It is required to override the #determineCurrentLookupKey() method which will return a key to decide the specific datasource that is needed to be connected. Also there should be spring configuration that maps each of the possible keys and the corresponding data sources that are to be connected. Some thing like
<jee:jndi-lookup id="DataSource_Client1" jndi-name="DataSource_Client1" />
<jee:jndi-lookup id="DataSource_Client2" jndi-name="DataSource_Client" />
<bean id="DynamicDataSource" class="concrete implementation class name of AbstractRoutingDataSource">
<property name="targetDataSources">
<map key-type="java.lang.String">
<entry key="Client1" value-ref="DataSource_Client1" />
<entry key="Client2" value-ref="DataSource_Client2" />
</map>
</property>
</bean>
A possible reference to this Dynamic DataSource
Hope this answers one of your questions