JPA retrieve objects from database

Question

I'm triying to solve a Java programming exercise for the universisty, but I don't know how to solve the next problem. I have the next persistence.xml file:

<?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_1_0.xsd">
  <persistence-unit name="stud" transaction-type="RESOURCE_LOCAL">
    <class>lt.vu.mif.jate.tasks.task03.jpa.model.Customer</class>
    <class>lt.vu.mif.jate.tasks.task03.jpa.model.Product</class>
    <class>lt.vu.mif.jate.tasks.task03.jpa.model.Sale</class>
    <exclude-unlisted-classes>true</exclude-unlisted-classes>
    <properties>
      <property name="hibernate.dialect" value="org.hibernate.dialect.PostgreSQLDialect"/>
      <property name="hibernate.max_fetch_depth" value="3"/>
      <property name="hibernate.cache.provider_class" value="net.sf.ehcache.hibernate.NoCacheProvider"/>
      <property name="hibernate.generate_statistics" value="false"/>
      <property name="javax.persistence.jdbc.url" value="jdbc:postgresql://****************"/>
      <property name="javax.persistence.jdbc.user" value=""/>
      <property name="javax.persistence.jdbc.password" value=""/>
      <property name="javax.persistence.jdbc.driver" value="org.postgresql.Driver"/>
      <property name="hibernate.hbm2ddl.auto" value="none"/>
    </properties>
  </persistence-unit>
</persistence>

The URL to the data base is just ommited here. I can't change this file. I have to connect to the database and get all the data from the database. I have already created the three classes needed. How can I do it? Thanks a lot.

I know is the java lenguage for make queries. But right now I don't even have a conexion with the database. — pez_betta, May 29 '15 at 09:16
What exactly is the problem here? You can't obtain a connection to the database, or you can't use the connection for anything useful yet? — Tobb, May 29 '15 at 10:31

score 0 · Accepted Answer · answered May 29 '15 at 09:42

0

Take a look at this page:

http://www.oracle.com/webfolder/technetwork/tutorials/obe/java/SettingUpJPA/SettingUpJPA.htm

and

https://docs.jboss.org/hibernate/orm/3.6/quickstart/en-US/html/hibernate-gsg-tutorial-jpa.html

There are several other tutorials etc out there. But the key part that I think you are missing is:

public static void main(String[] args) {
    EntityManagerFactory emf = Persistence.createEntityManagerFactory("stud");
    EntityManager em = emf.createEntityManager();
}

This gives you an EntityManager. You can then call:

em.getTransaction().begin();
List<Customer> result = entityManager.createQuery( "from Customer", Customer.class ).getResultList();

answered May 29 '15 at 09:42

Matt

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I have followed your advise. Now I got this excepcion: "java.lang.IllegalArgumentException: org.hibernate.hql.internal.ast.QuerySyntaxException: customer is not mapped [from customer]" Is this because I have bad anotations in my class? or maybe is something else? Thanks. – pez_betta May 29 '15 at 13:14
Normally "customer is not mapped" means that your JPA provider (in this case hibernate) has not picked up the Customer class as an entity. So it could be a problem with your annotations. Just to confirm - it is case sensitive and is looking for the entity, not the table, so make sure you have "from Customer" and not "from customer". Also, you can try "entityManager.createNativeQuery("select * from customer")" just to make sure the connection is working. In this case, it is executing native SQL, so it is looking for the table name, not entity, therefore lowercase customer. – Matt May 29 '15 at 14:14

JPA retrieve objects from database

1 Answers1