Do you need a database transaction for reading data?

Question

When I try to read data from the database, at least using

((Session)em.getDelegate()).createCriteria()

an exception is throws saying that a tra

Answer 1

Accoring to my experience with JPA implementation in J2EE, a Transaction manager is always needed in order to perform CRUD operation safety, by guaranteeing a rollback in order to preserve data integrity.

Enterprise applications use different resources to save data and send messages like a database or message queue. If we want to query these resources sequentially and to cancel the whole operation once a problem occurs, we have to put this query in a unit of work so that will be executed as a whole.

You could define it:

• by using related annotations (as shown in the questions); in this way, the container loads automatically the transaction manager for a given persistence context;

• by injecting manually the transaction manager, as follows:

  public class sample {
  
      @PersistenceContext
      EntityManager em;
  
      // Injected transaction manager
      @Inject
      UserTransaction utx;
  
      private static final String[] GAME_TITLES = {
          "Super Mario Brothers",
          "Mario Kart",
          "F-Zero"
      };
  
      private void clearData() throws Exception {
          utx.begin();
          em.joinTransaction();
          System.out.println("Dumping old records...");
          em.createQuery("delete from Game").executeUpdate();
          utx.commit();
      }
  
      private void insertData() throws Exception {
          utx.begin();
          em.joinTransaction();
          System.out.println("Inserting records...");
          for (String title : GAME_TITLES) {
              Game game = new Game(title);
              em.persist(game);
          }
          utx.commit();
          // clear the persistence context (first-level cache)
          em.clear();
      }
  
      // ...
  
  }
  

Spring Data, as JPA-spec implementation, may follow the same approach.

You could find more information by reading the following article: Java_Persistence/Transactions.