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How do I use multiple databases with JPA?

I need two or more than two connections in my web application using jpa

To use different data sources, add multiple persistence units (say, source-1 and source-2 in persistence.xml and create multiple EntityManagerFactoryes by name):
EntityManagerFactory emf1 = Persistence.createEntityManagerFactory("source-1");
EntityManagerFactory emf2 = Persistence.createEntityManagerFactory("source-2");
or, if you're working on Spring or Java EE application server, inject them by name also:
#PersistenceUnit(name = "source-1")
EntityManagerFactory emf1;
#PersistenceContext(unitName = "source-2") // as an option
EntityManager em2;
persistence.xml will thus look like the following:
<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.1" xmlns="http://xmlns.jcp.org/xml/ns/persistence"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd">
<persistence-unit name="source-1" transaction-type="RESOURCE_LOCAL">
<properties>
<!-- source-1 properties here -->
</properties>
</persistence-unit>
<persistence-unit name="source-2" transaction-type="RESOURCE_LOCAL">
<properties>
<!-- source-2 properties here -->
</properties>
</persistence-unit>
</persistence>
Example of how to configure persistence unit, create EntityManager to manage entities and execute queries can be found here.

For single datasource jpa will use multiple connections internally.So you don't need to do anything.

DRY in persistence.xml

I'm trying to use DRY in my persistence.xml file in different persistence units.
I have the following persistence.xml (it's just one file):
<persistence-unit name="siteAPU" transaction-type="JTA">
<jta-data-source>java:/siteADS</jta-data-source>
<class>package.name.Subs</class>
<class>package.name.SubsRef</class>
<class>package.name.SubsLoc</class>
...
<persistence-unit name="siteBPU" transaction-type="JTA">
<jta-data-source>java:/siteBDS</jta-data-source>
<class>package.name.Subs</class>
<class>package.name.SubsRef</class>
<class>package.name.SubsLoc</class>
...
You can see I have two different persistence units and I have the same classes being persisted.
Is there a way I can programmatically generate the persistence.xml file and don't repeat the content inside each persistence unit?

create a orm file:
<persistence-unit name="YOU_PU" ...>
<provider>YOU_PROVIDER</provider>
<mapping-file>orm.xml</mapping-file>
Inside the ORM file you will write the entities.
There is a sample here: https://github.com/uaihebert/uaicriteria/blob/master/src/test/resources/orm.xml

DDL generation and general persistence.xml settings (OpenJPA)

Summary
I'm trying to run a Java web application JPA 2.0 example. The example application was written to run in Glassfish, using EclipseLink as JPA provider.
I would like to convert it to run in TomEE with OpenJPA as the JPA provider, but I can't any detailed tutorials for getting up and running with OpenJPA.
Problem
I'm having trouble converting persistence.xml to work with OpenJPA instead of EclipseLink. More specifically, the given persistence.xml doesn't specify:
Entity classes. Are these necessary?
The desired JPA provider. Will the container default to something?
The JDBC driver. How do I specify an "in-memory" DB (just for initial testing purposes)?
Also:
How are the DDL generation properties expressed in OpenJPA? I wasn't able to find them the OpenJPA User Guide.
Details
Below is the EclipseLink persistence.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="order" transaction-type="JTA">
<jta-data-source>jdbc/__default</jta-data-source>
<properties>
<property name="eclipselink.ddl-generation" value="drop-and-create-tables" />
<property name="eclipselink.ddl-generation.output-mode"
value="both" />
</properties>
</persistence-unit>
</persistence>
I have the following Entity classes:
order.entity.LineItem
order.entity.LineItemKey
order.entity.Order
order.entity.Part
order.entity.PartKey
order.entity.Vendor
order.entity.VendorPart
Question
Does anyone know what the equivalent persistence.xml would look like for OpenJPA?
Alternatively, if anyone could point me to an OpenJPA tutorial that covers these issues that would be just as good

If you add the openjpa.jdbc.SynchronizeMappings property as shown below OpenJPA will auto-create all your tables, all your primary keys and all foreign keys exactly to match your objects
<property name="openjpa.jdbc.SynchronizeMappings" value="buildSchema(ForeignKeys=true)"/>
Alternatively, you can use EclipseLink in TomEE by just adding the EclipseLink jars to <CATALINA_HOME>/lib/
refer here for Common PersistenceProvider properties

Foreign key constraints
The next line does not create foreign keys:
<property name="openjpa.jdbc.SynchronizeMappings"
value="buildSchema(ForeignKeys=true)"/>
Only creates schema and deletes content of a database.
But if you want create foreign keys, use the following lines:
<property name="openjpa.jdbc.SynchronizeMappings"
value="buildSchema(foreignKeys=true,schemaAction='dropDB,add')"/>
<property name="openjpa.jdbc.SchemaFactory"
value="native(foreignKeys=true)" />
<property name="openjpa.jdbc.MappingDefaults"
value="ForeignKeyDeleteAction=restrict, JoinForeignKeyDeleteAction=restrict"/>
See generated SQL
In another way, if you want to see the SQL output:
<property name="openjpa.Log"
value="DefaultLevel=TRACE,SQL=TRACE" />
NOTE: In order to see the generated output in the TomEE console, you need to change the log level in the file loggin.properties with openjpa.level = FINEST
See more in http://openjpa.apache.org/faq.html

Entities not mapped after moving them into external jar package

In my Java/Seam/JbossAS app, I decided to externalize my Model classes (hibernate entities) and moved them into another project. The project produces model.jar, which is then used by the main app. The model.jar dependency is resolved by Ivy.
Building the main app with Ant works without problems. Then I copy manually the model.jar into 'mainapp.ear/lib' directory. Afterwards I deploy the app and there are no problems (although I notice that there are is no log info about found mappings). But when I want to login, I get the exception:
javax.el.ELException: javax.ejb.EJBTransactionRolledbackException:
org.hibernate.hql.ast.QuerySyntaxException: AppUser is not
mapped [select u from AppUser u where u.userName = :usernamePar]
There were no code changes in the meantime, just externalizing some of the classes into a jar. Does this mean, that I need the source code of the Model classes when compiling the main app?

The EntityManagerFactory is built for scanning entities only from the jar that has a /META-INF/persistence.xml file into.
In order to scan other jars you have to use <jar-file>:
<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 http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd"
version="2.0">
<persistence-unit name="manager1" transaction-type="JTA">
<provider>org.hibernate.ejb.HibernatePersistence</provider>
<jta-data-source>java:/DefaultDS</jta-data-source>
<mapping-file>ormap.xml</mapping-file>
<jar-file>MyApp.jar</jar-file>
<class>org.acme.Employee</class>
<class>org.acme.Person</class>
<class>org.acme.Address</class>
<shared-cache-mode>ENABLE_SELECTOVE</shared-cache-mode>
<validation-mode>CALLBACK</validation-mode>
<properties>
<property name="hibernate.dialect" value="org.hibernate.dialect.HSQLDialect"/>
<property name="hibernate.hbm2ddl.auto" value="create-drop"/>
</properties>
</persistence-unit>
</persistence>
See 2.2.1 Packaging in Hibernate doc.

Also check if your hibernate mappings are correctly placed wrt hibernate config file. Note that hibernate mapping resources or classes are relative to the location of hibernate.cfg.xml file.

Two Persistence Unit in one Persistence.xml

We created some libraries that all our projects will use, this libraries will provide the basic functionality of all our systems (login, some manage, etc). But the application itself could use another database.
What we did was to create the Persistence.xml with two persist units. And package all the core library entities in a jar called "LN-model.jar" and all of the entities of out test app in "App-model.jar". But for some reason we still obtain the following message.
Could not resolve a persistence unit corresponding to the persistence-context-ref-name [x.x.x.x.listener.InicializadorListener/em] in the scope of the module called [gfdeploy#/Users/zkropotkine/WORK/SeguridadCore/dist/gfdeploy/SeguridadCore-war_war]. Please verify your application.
Here's our Persistence.xml
<persistence version="1.0" 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_1_0.xsd">
<persistence-unit name="x" transaction-type="JTA">
<provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
<jta-data-source>jdbc/x</jta-data-source>
<jar-file>App-model.jar</jar-file>
<exclude-unlisted-classes>false</exclude-unlisted-classes>
<properties>
</properties>
</persistence-unit>
<persistence-unit name="y" transaction-type="JTA">
<provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
<jta-data-source>jdbc/y</jta-data-source>
<jar-file>LN-model.jar</jar-file>
<exclude-unlisted-classes>false</exclude-unlisted-classes>
<properties/>
</persistence-unit>
By the way we put the Persistence.xml in a jar, that we add to our Enterprise Project (EAR).

The problem is that the JPA does not know which is the persistence unit to use. when you have only one persistence unit this problem does not occur. To fix do the following:
You need to specify a persistence unit : #PersistenceContext(unitName="...") in the Ejb that do not have

You can add the annotations:
#PersistenceUnit(name = "x")
EntityManagerFactory entityManagerFactory;
#PersistenceContext(unitName = "y")
EntityManager entityManager;
Or you can create it manually:
EntityManagerFactory emfA = Persistence.createEntityManagerFactory("x", properties);
EntityManagerFactory emfB = Persistence.createEntityManagerFactory("y", properties);
For more details, please see the following link: https://docs.oracle.com/html/E25034_01/usingmultipledbs.htm
is very useful, to me helped me!