This project contains a simple example to introduce some concepts regarding JPA (Java Persistence Annotations), and the implementation we're using - Hibernate.
In the helloworld package, the Message class is annotated as an @Entity. Entities are objects that have their own lifecycles, and their own database tables. Entity types need a unique identifier. In this case, the id property is annotated with @Id to signify this as being the primary key. It is also annotated with @GeneratedValue, to specify that the id should be assigned by Hibernate when adding it to the database.
Messages can have any number of Comments. Comments are defined as @Embeddable, meaning that they are value types, rather than entity types. These objects don't have their own lifecycle, and are tied to their containing entity. The collection of comments in a message (comments) has been annotated with @ElementCollection to signal to JPA that this is a collection of value types.
In HelloJPAMain, on line 20, we're creating an EntityManagerFactory using the configuration se325.example09.helloworld. This refers to the <persistence-unit name="se325.example09.helloworld">, located in resources/META-INF/persistence.xml. In that file, we can see a <class> element defined, pointing to our Message class. We should reference all our entity types this way - or, we can use <exclude-unlisted-classes>false</exclude-unlisted-classes> to automatically include all defined entity types.
For the remainder of HelloJPAMain, we're creating an EntityManager and using it to run several database operations. Firstly, we're creating a new Message with some Comments, and persisting it to the database. Then, we're querying for a list of messages already in the database. Then, we're modifying one of the messages. For each operation, we surround it in entityManager.getTransaction().begin() and entityManager.getTransaction().commit(). More on transactions in a later example!
In this project's POM, we're adding an embedded H2 database implementation on line 49. H2 is a lightweight relational database that is commonly used for testing purposes. In our persistence.xml, lines 10 thru 13 define the connection information which Hibernate uses to connect to our H2 instance. We could replace this with connection info for any other relational database, should we choose.
On line 15, we're configuring Hibernate to drop all database tables when a new entity manager factory is created. This is a very useful setting for testing purpose, but we would obviously change it for production.