updating multiple rows using JPA

You can either do it the object oriented way or using an update query.

Object oriented:

public void setNameOfAllEntities(String newname){
    List<MyEntity> items =
        entityManager.createQuery("from MyEntity", MyEntity.class)
            .getResultList();
    for(MyEntity entity : items){
        entity.setName(newname);
    }
}

With Update Query (untested):

public void setNameOfAllEntities(final String newname){

    final int changes =
        entityManager.createQuery("update MyEntity set name = :name")
            .setParameter("name", newname)
            .executeUpdate();

    System.out.println(changes + " rows changed");

}

Obviously, the second version performs better.

seanizer's answer is correct (+1) and a bulk update would be indeed nice for this use case. But you must take some precautions with bulk update operations. To paraphrase the JPA specification:

• bulk updates bypass optimistic locking checks (so you must manually increment the version column and/or manually validate the version column if desired)
• the persistence context is not synced with the result of bulk operations (so bulk operations should be performed in a separate transaction or at the very beginning of a transaction, before loading the state of any entity that might be affected).

My suggestion would thus be to at least increment the version column to avoid concurrency problem with other threads:

UPDATE XYZ xyz
SET xyz.name = :newname, xyz.version = xyz.version + 1 

And to perform it in a separate transaction or before loading any XYZ as previously explained.

References

• JPA 1.0 specification
  • Section 4.10 "Bulk Update and Delete Operations"