Hi,
I trying to make JPA work for me. The delivered example works without
problems, I can also successfully modify this example. The example was based
on http://openjpa.apache.org/quick-start.html.
But if I create a new project the Entity Manager returns only a NULL value.
Error:
Exception in thread "main" java.lang.NullPointerException
at hellojpa.Main.main(Main.java:44)
I use the same files as in the working example. All jars are included in the
build path. I assume that the persistence.xml is not found but to my
knowledge the classpath is correctly maintained. The path .../bin contains
the directory META-INF and here the file persistence.xml.
Any advice? I must be missing something very simple but cannot figure it
out. For example coding please see below.
Best regards, Lars
File structure:
src
- hellojpa
- Main.java
- Message.java
- META-INF
- persistence.xml
Same structure in the bin directory.
-----------persistence.xml..............
<?xml version="1.0" encoding="UTF-8"?>
<!--
Licensed to the Apache Software Foundation (ASF) under one
or more contributor license agreements. See the NOTICE file
distributed with this work for additional information
regarding copyright ownership. The ASF licenses this file
to you under the Apache License, Version 2.0 (the
"License"); you may not use this file except in compliance
with the License. You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing,
software distributed under the License is distributed on an
"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
KIND, either express or implied. See the License for the
specific language governing permissions and limitations
under the License.
-->
<persistence xmlns="http://java.sun.com/xml/ns/persistence";
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"; version="1.0">
<!--
A persistence unit is a set of listed persistent entities as well
the configuration of an EntityManagerFactory. We configure each
example in a separate persistence-unit.
-->
<persistence-unit name="hellojpa"
transaction-type="RESOURCE_LOCAL">
<!--
The default provider can be OpenJPA, or some other product.
This element is optional if OpenJPA is the only JPA provider
in the current classloading environment, but can be specified
in cases where there are multiple JPA implementations available.
-->
<provider>
org.apache.openjpa.persistence.PersistenceProviderImpl
</provider>
<!-- We must enumerate each entity in the persistence unit -->
<class>hellojpa.Message</class>
<properties>
<property name="openjpa.ConnectionURL"
value="jdbc:derby:C:/DerbyDatabases/hellojpa-database5;create=true" />
<property name="openjpa.ConnectionDriverName"
value="org.apache.derby.jdbc.EmbeddedDriver" />
<property name="openjpa.ConnectionUserName" value="" />
<property name="openjpa.ConnectionPassword" value="" />
<!--
Tell OpenJPA to automatically create tables in the database
for entities. Note that this should be disabled when
running against a production database, since you probably
don't want to be altering the schema at runtime.
-->
<property name="openjpa.jdbc.SynchronizeMappings"
value="buildSchema" />
</properties>
</persistence-unit>
</persistence>
-----------Main.java
/*
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you under the Apache License, Version 2.0 (the
* "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing,
* software distributed under the License is distributed on an
* "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
* KIND, either express or implied. See the License for the
* specific language governing permissions and limitations
* under the License.
*/
package hellojpa;
import java.util.*;
import javax.persistence.*;
/**
* A very simple, stand-alone program that stores a new entity in the
* database and then performs a query to retrieve it.
*/
public class Main {
@SuppressWarnings("unchecked")
public static void main(String[] args) {
System.out.println(System.getProperty("java.class.path"));
// Create a new EntityManagerFactory using the System properties.
// The "hellojpa" name will be used to configure based on the
// corresponding name in the META-INF/persistence.xml file
EntityManagerFactory factory = Persistence.
createEntityManagerFactory("hellojpa", System.getProperties());
// Create a new EntityManager from the EntityManagerFactory. The
// EntityManager is the main object in the persistence API, and is
// used to create, delete, and query objects, as well as access
// the current transaction
EntityManager em = factory.createEntityManager();
// Begin a new local transaction so that we can persist a new entity
em.getTransaction().begin();
// Create and persist a new Message entity
em.persist(new Message("Hello Persistence!"));
// Commit the transaction, which will cause the entity to
// be stored in the database
em.getTransaction().commit();
// It is always good practice to close the EntityManager so that
// resources are conserved.
em.close();
// Create a fresh, new EntityManager
EntityManager em2 = factory.createEntityManager();
// Perform a simple query for all the Message entities
Query q = em2.createQuery("select m from Message m");
// Go through each of the entities and print out each of their
// messages, as well as the date on which it was created
for (Message m : (List<Message>) q.getResultList()) {
System.out.println(m.getMessage()
+ " (created on: " + m.getCreated() + ")");
}
// Again, it is always good to clean up after ourselves
em2.close();
factory.close();
}
}
-----------
