Can't understand EclipseLink warning

Question

I'm using EclipseLink 2.3.1 to model self referencing table with JPA 2. I get weird warning from EclipseLink when I create the EntityManager.

[EL Warning]: 2011-11-27 14:28:00.91--ServerSession(8573456)--Reverting the lazy setting on the OneToOne or ManyToOne attribute [redirectID] for the entity class [class lp.db.model.Site] since weaving was not enabled or did not occur.

I couldn't find any documentation about this warning, and I'm not sure what it means. I also want to know how to solve the problem that causes this warning to appear...

I'm new to JPA so it might be a silly thing. My program is really simple. Here is the entity definition:

@Entity
@Table(name="site") 
public class Site implements Serializable {

private static final long serialVersionUID = 1L;

    @Id
    @Column(name="site_id")
    public String siteID;

    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name="redirect_id", referencedColumnName="site_id")
    public Site redirectID;

    @Column(name="name")
    public String name;
}

Here is the persistence.xml:

<?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_2_0.xsd">
    <persistence-unit name="lpdb2" transaction-type="RESOURCE_LOCAL">
        <provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
        <class>lp.db.model.Site</class>
        <properties>
            <property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver"/>
            <property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/lpdb2"/>
            <property name="javax.persistence.jdbc.user" value="blabla"/>
        </properties>
    </persistence-unit>
</persistence>

The code that causes this warning:

Persistence.createEntityManagerFactory("lpdb2").createEntityManager();

Note that the resulting EM is fine and can be used (for example) to find elements. Also, I can traverse the graph of entities - I can find one entity in the database and then I get another entity using the redirectID field.

Answer 1

See http://wiki.eclipse.org/Introduction_to_EclipseLink_Application_Development_%28ELUG%29#Using_Weaving.

For lazy fetching to be possible on XxxToOne associations, the byte-code of the JPA entities must be modified (that's what weaving means). If it's not modified, an XxxToOne association can only be eager-fetched.

Eager fetching means that each time you load a Site from the database, its redirectID is also loaded. With lazy fetching, you load a site, and its redirect is only loaded (lazily) when you call a method on the redirectID field.