CRUD (create-read-update-delete) is a repetitive code in Java EE projects but it can be isolated in a unique class through the usage of JPA annotations and Generics - a class I call CRUDEntityFacade. This is not a new pattern and the goal of this blog entry is just to prepare you to read my next entries about JAXB and JPA together. I am doing really nice things with Jersey and Glassfish, and before exposing complicated inventions I decided to register the basics of the persistence layer (also very nice for my own future reference).

Complete sample projects

The code I am publishing here is just to avoid you to checkout a full project and to dig to inspect my CRUD strategy. But if you want to see this technique in action, please be my guest to checkout one of my open-source projects. The projects are built by Maven and were created in Eclipse - but you should be able to run it on your preferred IDE without any problems. Both projects require minimum Java 6.

username is your java.net login, or you can use guest without password to get a read only copy.

The generic CRUD implemented with generics and JPA annotations

  1. Defining the persistence interface containing the persistence operations we want to share among the entities. Note that I replicated the runtime exceptions because it is an interface and we expect interfaces to be the most documented and self-understandable artifacts in our project.

    public interface FootprintEntityFacade { T create(T entity) throws EntityExistsException, IllegalStateException, IllegalArgumentException, TransactionRequiredException; T read(Serializable primaryKey) throws IllegalStateException, IllegalArgumentException; T update(T entity) throws IllegalStateException, IllegalArgumentException, TransactionRequiredException; void deleteO(T entity) throws IllegalStateException, IllegalArgumentException, TransactionRequiredException, PersistenceException; }
  2. To define a superclass of all entities. This is an important step, since we want to use generics we must have a unique type to pass in our interface implementation. The mapped superclass is also useful to share the ID attribute.

     @MappedSuperclass public abstract class AbstractFootprintEntity implements Serializable { @Transient public static final long serialVersionUID = 196919661993L; @Id @GeneratedValue(strategy = GenerationType.AUTO) private long id; 
    // getters & setters
     } 
  3. Then we need some entities. I will use an example of the Footprint entity that represents Events (JUG meetings, courses or conferences).

     @Entity public class FpEvent extends AbstractFootprintEntity { @Transient private static final long serialVersionUID = 196919661993L; @Column(nullable = false) private String name; @Column(nullable = true) private String website; @Version private long updatedTime; 
    // getters & setters
     } 
  4. Now an important step, the realization of our generic CRUD interface to our Event Entity. In theory this empty interface is not necessary, but my experiments proved that this is the best way to go. It opens a chance for the customization of the persistence interface and - the main reason - it avoids conflicts between different entity instances using a same interface. You will have 1 empty interface for each Entity in your project, an oddity I couldn't get rid off - if you know how to avoid it, please tell me.

     @Local public interface EventFacadeLocal extends FootprintEntityFacade<FpEvent> { } 
  5. Now we just need to implement the CRUD. A special note about the empty constructor, that uses reflection to get the class of the generic type - this is the hidden trick that makes the magic possible.

     @Stateless public class CRUDEntityFacade<T extends AbstractFootprintEntity> implements FootprintEntityFacade<T> { private transient final Class entityClass; @SuppressWarnings("unchecked") public CRUDEntityFacade() { entityClass = (Class) ((java.lang.reflect.ParameterizedType) this .getClass().getGenericSuperclass()).getActualTypeArguments()[0]; } @PersistenceContext(name = "footprint") protected transient EntityManager manager; public T create(final T entity) throws EntityExistsException, IllegalStateException, IllegalArgumentException, TransactionRequiredException { manager.persist(entity); manager.flush(); return entity; } public T read(final Serializable primaryKey) throws IllegalStateException, IllegalArgumentException { return manager.find(entityClass, primaryKey); } public T update(final T entity) throws IllegalStateException, IllegalArgumentException, TransactionRequiredException { manager.merge(entity); manager.flush(); return entity; } public void delete(final T entity) throws IllegalStateException, IllegalArgumentException, TransactionRequiredException, PersistenceException { manager.remove(entity); manager.flush(); } } 
  6. Now we need to create a concrete implementation of the facade interface. Again an empty type, where you can add your customs methods if any. You will never instantiate this type manually, but it will be used by the generics mechanism to have different types for different facades.

     @Stateless public class EventFacade extends CRUDEntityFacade<FpEvent> implements EventFacadeLocal { } 
  7. It is done, now we can persist any entity of the type AbstractFootprintEntity (remember to create the empty sub-interface for each new entity you want to persist). Below you find an example of the CRUD usage in a Jersey resource:

     @Path("/event") public class EventResource { @EJB private EventFacadeLocal eventFacade; @Produces( { MediaType.APPLICATION_XML }) @Consumes(MediaType.APPLICATION_XML) @POST @Path("/test2") public FpEvent postJAXBElement(FpEvent e) { return eventFacade.create(e); } }

In my next entries I will show you how to use dual annotation (JAXB + JPA) in order to minimize the impedance mismatch between the persistence layer and the serialization of element used in the services endpoints - REST or SOAP.

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