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("JPA 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; JPA 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();