The examples below demonstrate how to use the lifecycle methods
presented in the previous section. The examples are appropriate for
out-of-container use. Within a container,
EntityManagers are usually injected, and transactions are
usually managed. You would therefore omit the
createEntityManager and close
calls, as well as all transaction demarcation code.
Example 8.1. Persisting Objects
// create some objects
Magazine mag = new Magazine ("1B78-YU9L", "JavaWorld");
Company pub = new Company ("Weston House");
pub.setRevenue (1750000D);
mag.setPublisher (pub);
pub.addMagazine (mag);
Article art = new Article ("EJB Rules!", "Transparent Object Persistence");
art.addAuthor (new Author ("Fred", "Hoyle"));
mag.addArticle (art);
// persist
EntityManager em = emf.createEntityManager ();
em.getTransaction ().begin ();
em.persist (mag);
em.persist (pub);
em.persist (art);
em.getTransaction ().commit ();
// or we could continue using the EntityManager...
em.close ();
Example 8.2. Updating Objects
Magazine.MagazineId mi = new Magazine.MagazineId (); mi.isbn = "1B78-YU9L"; mi.title = "JavaWorld"; // updates should always be made within transactions; note that // there is no code explicitly linking the magazine or company // with the transaction; EJB automatically tracks all changes EntityManager em = emf.createEntityManager (); em.getTransaction ().begin (); Magazine mag = em.find (Magazine.class, mi); mag.setPrice (5.99); Company pub = mag.getPublisher (); pub.setRevenue (1750000D); em.getTransaction ().commit (); // or we could continue using the EntityManager... em.close ();
Example 8.3. Removing Objects
// assume we have an object id for the company whose subscriptions
// we want to delete
Object oid = ...;
// deletes should always be made within transactions
EntityManager em = emf.createEntityManager ();
em.getTransaction ().begin ();
Company pub = (Company) em.find (Company.class, oid);
for (Subscription sub : pub.getSubscriptions ())
em.remove (sub);
pub.getSubscriptions ().clear ();
em.getTransaction ().commit ();
// or we could continue using the EntityManager...
em.close ();
Example 8.4. Detaching and Merging
This example demonstrates a common client/server scenario. The client requests objects and makes changes to them, while the server handles the object lookups and transactions.
// CLIENT:
// requests an object with a given oid
Record detached = (Record) getFromServer (oid);
...
// SERVER:
// send object to client; object detaches on EM close
Object oid = processClientRequest ();
EntityManager em = emf.createEntityManager ();
Record record = em.find (Record.class, oid);
em.close ();
sendToClient (record);
...
// CLIENT:
// makes some modifications and sends back to server
detached.setSomeField ("bar");
sendToServer (detached);
...
// SERVER:
// merges the instance and commit the changes
Record modified = (Record) processClientRequest ();
EntityManager em = emf.createEntityManager ();
em.getTransaction ().begin ();
Record merged = (Record) em.merge (modified);
merged.setLastModified (System.currentTimeMillis ());
merged.setModifier (getClientIdentityCode ());
em.getTransaction ().commit ();
em.close ();