Phantom read happens when data is read more times and unexpectedly goes away or when data is read more times and unexpectedly a different value is shown. For example, in transaction A, through a query you get a set result. Before transaction A is completed, transaction B adds a row that matches the query executed in transaction A, and this row is committed before the completion of transaction A. If transaction A executes the same query again, it will get a different data. This situation is called phantom read because new data not created by transaction A has appeared in the resource. To simulate this read, we must execute a query. Consider the named query of the account entity:

@Entity
@NamedQueries({ @NamedQuery(name = "SelectAll", query = "SELECT e FROM Account e") })
public class Account {...}

Here is a runnable to execute the query in the first transaction:

@Transactional(value = REQUIRES_NEW)
public class QueryReadAccount implements Runnable {
   @Override
   public void run() {
      ...
      CountDownLatch latch = new CountDownLatch(1);
      latch.await(firstWaitTime, MILLISECONDS);
      firstResult =     entityManager.createNamedQuery("SelectAll").getResultList().size();      latch.await(secondWaitTime, MILLISECONDS);
      secondResult = entityManager.createNamedQuery("SelectAll").getResultList().size();
      ...
   }
}

Here is a runnable that inserts a new account in the second transaction:

@Transactional(value = REQUIRES_NEW)
public class QueryWriteAccount implements Runnable {
   @Override
   public void run() {
      ...
      CountDownLatch latch = new CountDownLatch(1);
      Account account = new Account();
      account.setNumber(accountNumber);
      account.add(amount);
      entityManager.persist(account);
      latch.await(waitTime, MILLISECONDS);
      result = account.getCredit();
      ...
   }
}

Now, start the two transactions with the managed executor service:

@Inject
private QueryReadAccount queryReadAccount;
@Inject
private QueryWriteAccount queryWriteAccount;
...
queryReadAccount.setFirstWaitTime(0);
queryReadAccount.setSecondWaitTime(2000);
queryWriteAccount.setAccountNumber(953);
queryWriteAccount.setAmount(456.77);
queryWriteAccount.setWaitTime(1000);
defaultExecutor.submit(queryWriteAccount);
defaultExecutor.submit(queryReadAccount);

The operations can be simplified so:

• The first transaction starts and we have a first query of all accounts
• The second transaction starts and an insert of a new account is done
• The second transaction ends
• A second read of all accounts is done in the first transaction
• The first transaction ends

The results of these operations change according the configured transaction isolation level that we will introduce in the next section.