Reuse JPA entity classes from a Jar file

Background

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Let’s say you had your application in one single Maven project as a prototype and decided to finalize the functionality into a finished (or almost, e.g., Alpha or Beta version) product. When you have JPA entities and you separated them into a single jar file, your persistence.xml needs to change and use the <class> tag in that file.

Of course, the JPA entities jar file has to be in the classpath.

Software Environment

Windows 7 Professional SP1
Eclipse – Kepler Release
Java 1.7 (1.7.0_67 – Windows x86)
MySQL 5.6.16 – Community Server (GPL)
MySQL Java Connector 5.1.34
Hibernate 4.3.7 (Entity Manager)

JPA Entity Class

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Your entities can keep their JPA-related annotations. Here is a sample entity class:

@Entity
@Table(name = "PERSON")
public class Person implements Serializable {
    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "PERSON_ID")
    private Integer personId;

    @Column(name = "PERSON_NAME")
    private String personName;
}

@Entity

@Table(name = "PERSON")

public class Person implements Serializable {

private static final long serialVersionUID = 1L;

@Id

@GeneratedValue(strategy = GenerationType.IDENTITY)

@Column(name = "PERSON_ID")

private Integer personId;

@Column(name = "PERSON_NAME")

private String personName;

}

Modify persistence.xml

Although your classes may be in a separate jar file, they are still in the same application and it referenced in persistence.xml using <class> tags.

<?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="TurretaPersistenceUnit" transaction-type="RESOURCE_LOCAL">
        <provider>org.hibernate.ejb.HibernatePersistence</provider>
        <class>com.turreta.domain.Person</class>
        <properties>
            <property name="hibernate.dialect" value="org.hibernate.dialect.Oracle10gDialect"/>
            <property name="hibernate.show_sql" value="true"/>
            <property name="hibernate.cache.use_query_cache" value="false"/>
            <property name="hibernate.cache.use_second_level_cache" value="true"/>
            <property name="hibernate.cache.provider_class" value="org.hibernate.cache.EhCacheProvider"/>
        </properties>        
    </persistence-unit>    
</persistence>