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Retrieve information from existing database using JPA

I'm developing a Java Web Project using JPA (EclipseLink) to connect with our databases. I have a persistence unit where I configured the database where my data is going to persisted. The question is that I also have to access another database that is already created and populated with info, and I just want to access one table of this database to retrieve some information. How can I access it using JPA just to retrieve info (neither save nor update anything).

After setting up your entity managers you can just run a native query :
entityManager.createNativeQuery("SELECT ...");

So, you can create another persistent-unit poiting to your other database
<persistence-unit name="read-and-write-database">
...
<properties>
<!-- conection -->
<property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver" />
<property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/database-write" />
<.../>
</properties>
</persistence-unit>
<persistence-unit name="read-database">
...
<properties>
<!-- conection -->
<property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver" />
<property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/database-read" />
<.../>
</properties>
</persistence-unit>
And, in a SE environment, you would something like that to create the EntityManager:
EntityManagerFactory readWriteEntityManagerFactory = Persistence.createEntityManagerFactory("read-and-write-database");
EntityManager readWriteEntityManager = readWriteEntityManagerFactory.createEntityManager();
EntityManagerFactory readEntityManagerFactory = Persistence.createEntityManagerFactory("read-database");
EntityManager readEntityManager = readEntityManagerFactory.createEntityManager();
if you are in a container, the PersistenceContext anotation have the unitName field that you can use to define the persistente-unit
Keep in mind that by doing that, you will have two different persistence context. So, changes made in one of then, not gonna be "visiable" to the other one until the persistence of the data.
obs: sorry any typo.
I hope this helps you.
Cheers

Re-create database at runtime using EclipseLink

Is there a way to recreate the database dynamically at Run-Time in EclipseLink?
Right now I have it that if the database does not exist, it creates the database at compilation time :
<?xml version="1.0" encoding="UTF-8" ?>
<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" xmlns="http://java.sun.com/xml/ns/persistence">
<persistence-unit name="DefaultUnit" transaction-type="RESOURCE_LOCAL">
<provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
<exclude-unlisted-classes>false</exclude-unlisted-classes>
<properties>
<property name="eclipselink.ddl-generation.output-mode"
value="database" />
<property name="eclipselink.logging.level" value="INFO" />
<property name="eclipselink.logging.level.sql" value="INFO" />
<property name="javax.persistence.schema-generation.database.action"
value="create" />
</properties>
</persistence-unit>
</persistence>
However, I want to be able to Drop, and Recreate the database while the application is running. I.E when a user passes in some special flag, I want to call something that will drop the current database, and regenerate it.
I found that you can do that in Hibernate using the SchemaExport class

You can add <property name="eclipselink.ddl-generation" value="drop-and-create-tables"/> into persistence.xml.
It will delete all table and create it again. Be careful, all your earlier data will be lost.

absolutely. Properties can be added dynamically at runtime, so you can add the "eclipselink.ddl-generation" with value "drop-and-create-tables" when you create the EntityManagerFactory for the first time to have it drop and create the database. Of course, this isn't much use to a running app where you want to change things on the fly.
To get around this, EclipseLInk has a few tricks that allow dynamic changes and reloading of the persistence unit. Try
Map properties = new HashMap();
properties.put("eclipselink.ddl-generation", "drop-and-create-tables");
properties.put("eclipselink.ddl-generation.output-mode", "database");
//this causes DDL generation to occur on refreshMetadata rather than wait until an em is obtained
properties.put("eclipselink.deploy-on-startup", "true");
JpaHelper.getEntityManagerFactory(em).refreshMetadata(properties);
The org.eclipse.persistence.jpa.JPAHelper class is just used to unwrap the Factory to get a JpaEntityManagerFactory instance to make the non-JPA refreshMetadata call. Any number of properties or settings can be changed, and any new EntityManagers obtained after the refresh call will reflect those changes.

Hibernate in emeddable websphere container does not find java:comp/websphere/ExtendedJTATransaction

I have a Java EE application which uses Hibernate 4.2.7 as persistence provider executing Junit tests in an embeddable Websphere 8.0.0 container. Database access works fine in a real (i.e. non-embedded) Websphere 8.0.0 instance. The unit tests do work when run with OpenJPA instead of Hibernate. However, running the Junit tests with Hibernate, I get the following exception:
CNTR0020E: EJB threw an unexpected (non-declared) exception during invocation of method "getEntity" on bean "BeanId(embeddable#classes#SomeBean, null)". Exception data: org.hibernate.service.jndi.JndiException: Unable to lookup JNDI name [java:comp/websphere/ExtendedJTATransaction]
at org.hibernate.service.jndi.internal.JndiServiceImpl.locate(JndiServiceImpl.java:68)
at org.hibernate.service.jta.platform.internal.WebSphereExtendedJtaPlatform$TransactionManagerAdapter$TransactionAdapter.(WebSphereExtendedJtaPlatform.java:156)
at org.hibernate.service.jta.platform.internal.WebSphereExtendedJtaPlatform$TransactionManagerAdapter$TransactionAdapter.(WebSphereExtendedJtaPlatform.java:152)
at org.hibernate.service.jta.platform.internal.WebSphereExtendedJtaPlatform$TransactionManagerAdapter.getTransaction(WebSphereExtendedJtaPlatform.java:124)
at org.hibernate.service.jta.platform.internal.WebSphereExtendedJtaPlatform$TransactionManagerAdapter.getStatus(WebSphereExtendedJtaPlatform.java:119)
at org.hibernate.engine.transaction.internal.jta.JtaStatusHelper.getStatus(JtaStatusHelper.java:73)
at org.hibernate.engine.transaction.internal.jta.JtaStatusHelper.isActive(JtaStatusHelper.java:115)
at org.hibernate.service.jta.platform.internal.TransactionManagerBasedSynchronizationStrategy.canRegisterSynchronization(TransactionManagerBasedSynchronizationStrategy.java:56)
... stripped ...
It seems the implementation of WebsphereExtendedJtaPlatform is trying to get the current transaction via a JNDI lookup but fails because that JNDI name does not exist in the embedded container. Here's a snipped from org.hibernate.service.jta.platform.internal.WebsphereExtendedJtaPlatform:
public class TransactionAdapter implements Transaction {
private TransactionAdapter() {
if ( extendedJTATransaction == null ) {
extendedJTATransaction = jndiService().locate( "java:comp/websphere/ExtendedJTATransaction" );
}
}
... stripped ...
The class ExtendedJtaTransaction itself does exist on the class path inside com.ibm.ws.runtime.jar.
The settings in our persistence.xml look like this:
<persistence-unit name="BLA" transaction-type="JTA">
<provider>org.hibernate.ejb.HibernatePersistence</provider>
<jta-data-source>jdbc/BLA</jta-data-source>
<class>com.some.Entity</class>
<exclude-unlisted-classes>true</exclude-unlisted-classes>
<properties>
<property name="hibernate.dialect" value="org.hibernate.dialect.Oracle10gDialect" />
<property name="hibernate.archive.autodetection" value="class" />
<property name="hibernate.transaction.manager_lookup_class" value="org.hibernate.transaction.WebSphereExtendedJTATransactionLookup" />
<property name="jta.UserTransaction" value="java:comp/UserTransaction" />
<property name="hibernate.show_sql" value="true" />
<property name="hibernate.format_sql" value="true" />
<property name="hibernate.jdbc.fetch_size" value="100" />
<property name="hibernate.temp.use_jdbc_metadata_defaults" value="false" />
</properties>
Does anyone have a solution for this?
Thanks!

Transaction strategy configuration
Hibernate requires the configuration of two essential pieces in order to properly run with transactions. The first, hibernate.transaction.factory_class, defines transactional control and the second, hibernate.transaction.manager_lookup_class, defines the mechanism for registration of transaction synchronization so the persistence manager is notified at transaction end when it needs to synchronize changes with the database. For transactional control, both container-managed and bean-managed configurations are supported. The following properties must be set in Hibernate.cfg.xml when using Hibernate with WebSphere Application Server:
for container-managed transactions:
<property name="hibernate.transaction.factory_class">
org.hibernate.transaction.CMTTransactionFactory
</property>
<property name="hibernate.transaction.manager_lookup_class">
org.hibernate.transaction.WebSphereExtendedJTATransactionLookup
</property>
for bean-managed transactions:
<property name="hibernate.transaction.factory_class">
org.hibernate.transaction.JTATransactionFactory
</property>
<property name="hibernate.transaction.manager_lookup_class">
org.hibernate.transaction.WebSphereExtendedJTATransactionLookup
</property>
<property name="jta.UserTransaction">
java:comp/UserTransaction
</property >
The jta.UserTransaction property configures the factory class to obtain an instance of a UserTransaction object instance from the WebSphere container.
The hibernate.transaction.manager_lookup_class property is supported on the WebSphere platform by WebSphere Application Server V6.x and later, and on WebSphere Business Integration Server Foundation V5.1 and later. This property configures Hibernate to use the ExtendedJTATransaction interface, which was introduced in WebSphere Business Integration Server Foundation V5.1 and WebSphere Application Server V6.0. The WebSphere ExtendedJTATransaction interface establishes a pattern that is formalized in Java EE 5 via the JTA 1.1 specification.

Changing Persistence Unit dynamically - JPA

Persistence units in persistence.xml are created during building the application. As I want to change the database url at runtime, is there any way to modify the persistence unit at runtime? I supposed to use different database other than pre-binded one after distributed.
I'm using EclipseLink (JPA 2.1)

Keep the persistence unit file (Persistence.xml) as it's. You can override the properties in it as follows.
EntityManagerFactory managerFactory = null;
Map<String, String> persistenceMap = new HashMap<String, String>();
persistenceMap.put("javax.persistence.jdbc.url", "<url>");
persistenceMap.put("javax.persistence.jdbc.user", "<username>");
persistenceMap.put("javax.persistence.jdbc.password", "<password>");
persistenceMap.put("javax.persistence.jdbc.driver", "<driver>");
managerFactory = Persistence.createEntityManagerFactory("<current persistence unit>", persistenceMap);
manager = managerFactory.createEntityManager();

You can use Persistence.createEntityManagerFactory(Map) to pass properties to choose the database URL and other settings.

In Long-lived Session Architecture you should create a Plug-in-Framework.
Therefore you need to create a different Thread-Group and Class-Repository.
This might be your Class-Loader-Tree
System-Class-Loader (usually a URLClassLoader, contains the Entitys)
JPA-Class-Loader
Load your jpa.jar with persistence.xml inside, specify the Database-Configuration from Application-Class-Loader
Instanciate your entityManager/session-factory.
Load any plugin you need to work with the DataBase. Execute Unit-Tests (;D) and Plugin-Integration-Tests.

If you are using Thorntail framework, you can wire-up the persistence.xml file to fetch runtime variables from "project-defaults.yml" file.
<persistence-unit name="java:jboss/datasources/my-postgres-ds">
<provider>org.hibernate.ejb.HibernatePersistence</provider>
<class>com.package.jpa.EntityClass1</class>
<class>com.package.jpa.EntityClass2</class>
<class>com.package.jpa.EntityClass3</class>
<properties>
<property name="hibernate.archive.autodetection" value="class"/>
<property name="hibernate.show_sql" value="true"/>
<property name="hibernate.dialect" value="org.hibernate.dialect.PostgreSQLDialect"/>
<property name="hibernate.connection.url"
value="${thorntail.datasources.data-sources.my-postgres-ds.connection-url}"/>
<property name="hibernate.connection.username"
value="${thorntail.datasources.data-sources.my-postgres-ds.user-name}"/>
<property name="hibernate.connection.password"
value="${thorntail.datasources.data-sources.my-postgres-ds.password}"/>
<property name="hibernate.default_schema" value="public"/>
<property name="hibernate.hbm2ddl.auto" value="update"/>
</properties>
</persistence-unit>
Take note of the dynamic DB values in ${...} as the point to values in the project-default.yml file.
Then you project-defaults.yml file will have an entry like this:
thorntail:
http:
port: 8989
datasources:
data-sources:
my-postgres-ds:
driver-name: my-postgres-driver
connection-url: "jdbc:postgresql://localhost:5432/my-db-name"
user-name: my-user-name
password: "my-password#"
jdbc-drivers:
my-postgres-driver:
driver-module-name: org.postgresql
driver-xa-datasource-class-name: org.postgresql.xa.PGXADataSource
I expect this should also work for Spring boot using application.properties and persistence.xml

javax.persistence.PersistenceException: No Persistence provider for EntityManager named customerManager

I am new to JPA & Hibernate. After reading some online materials I now understand what Hibernate is and how it can be used with JPA.
Now, I am trying to run this JPA & Hibernate tutorial. I've done everything they mention in this tutorial.
I don't have Oracle DB, only MySQL. So I made some changes to persistence.xml using my understanding of JPA & Hibernate (I don't know if it's correct or not... Seems to me it is.)
Here is my persistence.xml
<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_1_0.xsd" version="1.0">
<persistence-unit name="customerManager" transaction-type="RESOURCE_LOCAL">
<provider>org.hibernate.ejb.HibernatePersistence</provider>
<class>Customer</class>
<properties>
<property name="hibernate.dialect" value="org.hibernate.dialect.MySQLInnoDBDialect"/>
<property name="hibernate.connection.driver_class" value="com.mysql.jdbc.Driver"/>
<property name="hibernate.show_sql" value="true"/>
<property name="hibernate.connection.username" value="root"/>
<property name="hibernate.connection.password" value="1234"/>
<property name="hibernate.connection.url" value="jdbc:mysql://localhost:3306/general"/>
<property name="hibernate.max_fetch_depth" value="3"/>
</properties>
</persistence-unit>
</persistence>
But I don't seem to get the output they describe. It's giving me:
Customer id before creation:null
log4j:WARN No appenders could be found for logger (org.hibernate.cfg.annotations.Version).
log4j:WARN Please initialize the log4j system properly.
Exception in thread "main" javax.persistence.PersistenceException: No Persistence provider for EntityManager named customerManager
at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:55)
at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:33)
at CustomerDAO.create(CustomerDAO.java:8)
at CustomerDAO.main(CustomerDAO.java:22)
Any suggestions will be appreciated.
Update:
I have made the changes that are asked to done. But, still getting the asme error lines!!!
They didnt mentioned anything about orm.xml in that tutorial. may it be a problem causer!!!

Just for completeness. There is another situation causing this error:
missing META-INF/services/javax.persistence.spi.PersistenceProvider
file.
For Hibernate, it's located in hibernate-entitymanager-XXX.jar, so, if hibernate-entitymanager-XXX.jar is not in your classpath, you will got this error too.
This error message is so misleading, and it costs me hours to get it correct.
See JPA 2.0 using Hibernate as provider - Exception: No Persistence provider for EntityManager.

Your persistence.xml is not valid and the EntityManagerFactory can't get created. It should be:
<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_1_0.xsd" version="1.0">
<persistence-unit name="customerManager" transaction-type="RESOURCE_LOCAL">
<provider>org.hibernate.ejb.HibernatePersistence</provider>
<class>Customer</class>
<properties>
<property name="hibernate.dialect" value="org.hibernate.dialect.MySQLInnoDBDialect"/>
<property name="hibernate.connection.driver_class" value="com.mysql.jdbc.Driver"/>
<property name="hibernate.show_sql" value="true"/>
<property name="hibernate.connection.username" value="root"/>
<property name="hibernate.connection.password" value="1234"/>
<property name="hibernate.connection.url" value="jdbc:mysql://localhost:3306/general"/>
<property name="hibernate.max_fetch_depth" value="3"/>
</properties>
</persistence-unit>
</persistence>
(Note how the <property> elements are closed, they shouldn't be nested)
Update: I went through the tutorial and you will also have to change the Id generation strategy when using MySQL (as MySQL doesn't support sequences). I suggest using the AUTO strategy (defaults to IDENTITY with MySQL). To do so, remove the SequenceGenerator annotation and change the code like this:
#Entity
#Table(name="TAB_CUSTOMER")
public class Customer implements Serializable {
private static final long serialVersionUID = 1L;
#Id
#GeneratedValue(strategy=GenerationType.AUTO)
#Column(name="CUSTOMER_ID", precision=0)
private Long customerId = null;
...
}
This should help.
PS: you should also provide a log4j.properties as suggested.

I had the same problem today. My persistence.xml was in the wrong location. I had to put it in the following path:
project/src/main/resources/META-INF/persistence.xml

I was facing the same issue. I realised that I was using the Wrong provider class in persistence.xml
For Hibernate it should be
<provider>org.hibernate.ejb.HibernatePersistence</provider>
And for EclipseLink it should be
<provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>

If you use Hibernate 5.2.10.Final, you should change
<provider>org.hibernate.ejb.HibernatePersistence</provider>
to
<provider>org.hibernate.jpa.HibernatePersistenceProvider</provider>
in your persistence.xml
According to Hibernate 5.2.2: No Persistence provider for EntityManager

If you are using Maven you may have both src/{main,test}/resources/META-INF/persistence.xml. This is a common setup: test your JPA code with h2 or Derby and deploy it with PostgreSQL or some other full DBMS. If you're using this pattern, do make sure the two files have different unit names, else some versions of the Persistence class will try to load BOTH (because of course your test-time CLASSPATH includes both classes and test-classes); this will cause conflicting definitions of the persistence unit, resulting in the dreaded annoying message that we all hate so much!
Worse: this may "work" with some older versions of e.g., Hibernate, but fail with current versions. Worth getting it right anyway...

A bit too late but I got the same issue and fixed it switching schemalocation into schemaLocation in the persistence.xml file (line 1).

I have seen this error , for me the issue was there was a space in the absolute path of the persistance.xml , removal of the same helped me.

I was also facing the same issue when I was trying to get JPA entity manager configured in Tomcat 8. First I has an issue with the SystemException class not being found and hence the entityManagerFactory was not being created. I removed the hibernate entity manager dependency and then my entityManagerFactory was not able to lookup for the persistence provider. After going thru a lot of research and time got to know that hibernate entity manager is must to lookup for some configuration. Then put back the entity manager jar and then added JTA Api as a dependency and it worked fine.

my experience tells me that missing persistence.xml,will generate the same exception too.
i caught the same error msg today when i tried to run a jar package packed by ant.
when i used jar tvf to check the content of the jar file, i realized that "ant" forgot to pack the persistnece.xml for me.
after I manually repacked the jar file ,the error msg disappered.
so i believe maybe you should try simplely putting META-INF under src directory and placing your persistence.xml there.