Andrea Francia Andrea Francia - 13 days ago 6
Java Question

How to persist a property of type List<String> in JPA?

What is the smartest way to get an entity with a field of type List persisted?

Command.java



package persistlistofstring;

import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;
import javax.persistence.Basic;
import javax.persistence.Entity;
import javax.persistence.EntityManager;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Persistence;

@Entity
public class Command implements Serializable {

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
Long id;
@Basic
List<String> arguments = new ArrayList<String>();

public static void main(String[] args) {
Command command = new Command();

EntityManager em = Persistence
.createEntityManagerFactory("pu")
.createEntityManager();
em.getTransaction().begin();
em.persist(command);
em.getTransaction().commit();
em.close();

System.out.println("Persisted with id=" + command.id);
}
}





This code produces:

> Exception in thread "main" javax.persistence.PersistenceException: No Persistence provider for EntityManager named pu: Provider named oracle.toplink.essentials.PersistenceProvider threw unexpected exception at create EntityManagerFactory:
> oracle.toplink.essentials.exceptions.PersistenceUnitLoadingException
> Local Exception Stack:
> Exception [TOPLINK-30005] (Oracle TopLink Essentials - 2.0.1 (Build b09d-fcs (12/06/2007))): oracle.toplink.essentials.exceptions.PersistenceUnitLoadingException
> Exception Description: An exception was thrown while searching for persistence archives with ClassLoader: sun.misc.Launcher$AppClassLoader@11b86e7
> Internal Exception: javax.persistence.PersistenceException: Exception [TOPLINK-28018] (Oracle TopLink Essentials - 2.0.1 (Build b09d-fcs (12/06/2007))): oracle.toplink.essentials.exceptions.EntityManagerSetupException
> Exception Description: predeploy for PersistenceUnit [pu] failed.
> Internal Exception: Exception [TOPLINK-7155] (Oracle TopLink Essentials - 2.0.1 (Build b09d-fcs (12/06/2007))): oracle.toplink.essentials.exceptions.ValidationException
> Exception Description: The type [interface java.util.List] for the attribute [arguments] on the entity class [class persistlistofstring.Command] is not a valid type for a serialized mapping. The attribute type must implement the Serializable interface.
> at oracle.toplink.essentials.exceptions.PersistenceUnitLoadingException.exceptionSearchingForPersistenceResources(PersistenceUnitLoadingException.java:143)
> at oracle.toplink.essentials.ejb.cmp3.EntityManagerFactoryProvider.createEntityManagerFactory(EntityManagerFactoryProvider.java:169)
> at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:110)
> at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:83)
> at persistlistofstring.Command.main(Command.java:30)
> Caused by:
> ...

Answer

Use some JPA 2 implementation: it adds a @ElementCollection annotation, similar to the Hibernate one, that does exactly what you need. There's one example here.

Edit

As mentioned in the comments below, the correct JPA 2 implementation is

javax.persistence.ElementCollection

@ElementCollection
Map<Key, Value> collection;

See: http://docs.oracle.com/javaee/6/api/javax/persistence/ElementCollection.html

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