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exedio persistence

Persistence Framework for Java.

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Transactions Trail

This trail assumes, that you are already familar with the general meaning of transactions. It shows you, how to start, commit and rollback transactions.

When to use transactions

The short anser is always. Whenever you want to read, write or search persistent data, this must happen within the boundaries of a transaction. These boundaries are marked by calls to the methods Model.startTransaction() and Model.commit() or Model.rollback().

Any attempt to access persistent data outside a transaction will cause an exception. There is no equivalent to what is known as auto-commit in relational databases.

Example

Since you also have to take care of exceptions, the standard way of using transactions looks like this: 

boolean failure = true;
try
{
   Main.model.startTransaction();
   // put the body of the transaction here
   Main.model.commit();
   failure = false;
}
finally
{
   if(failure)
      Main.model.rollback();
}

For your convenience, the framework provides a method Model.rollbackIfNotCommitted() which does a rollback if and only if the transaction has not been closed (committed or rolled back) before. Therefore you can make the code a little bit shorter: 

try
{
   Main.model.startTransaction();
   // put the body of the transaction here
   Main.model.commit();
}
finally
{
   Main.model.rollbackIfNotCommitted();
}

For even more convenience you may use the try-with-resources syntax introduced in Java 1.7: 

try(TransactionTry tx = Main.model.startTransactionTry("web"))
{
   // put the body of the transaction here
   tx.commit();
}

Further Reading

There is not much more to say about transactions, so you may want to proceed to any of the following trails: