9.2. The JDO Transaction Interface

The Transaction interface controls transactions in JDO. This interface consists of "getter" and "setter" methods for several Java bean-style properties, standard transaction demarcation methods, and a method to test whether there is a transaction in progress.

public boolean getNontransactionalRead ();
public void setNontransactionalRead (boolean read);
public boolean getNontransactionalWrite ();
public void setNontransactionalWrite (boolean write);
public boolean getRetainValues ();
public void setRetainValues (boolean retain);
public boolean getRestoreValues ();
public void setRestoreValues (boolean restore);
public boolean getOptimistic ();
public void setOptimistic (boolean optimistic);
public Synchronization getSynchronization ();
public void setSynchronization (Synchronization synch);

The Transaction's NonTransactionalRead, NonTransactionalWrite, RetainValues, RestoreValues, and Optimistic properties mirror those presented in the section on PersistenceManagerFactory settings. The final Transaction property, Synchronization, has not been covered yet. This property enables you to associate a javax.transaction.Synchronization instance with the Transaction. The Transaction will notify your Synchronization instance on transaction completion events, so that you can implement custom behavior on commit or rollback. See the javax.transaction.Synchronization Javadoc for details.

public void begin ();
public void commit ();
public void rollback ();

The begin, commit, and rollback methods demarcate transaction boundaries. The methods should be self-explanatory: begin starts a transaction, commit attempts to commit the transaction's changes to the data store, and rollback aborts the transaction, in which case the data store is "rolled back" to its previous state. JDO implementations will automatically roll back transactions if any fatal exception is thrown during the commit process. Otherwise, it is up to you to roll back the transaction to free its resources.

public boolean isActive ();

Finally, the isActive method returns true if the transaction is in progress (begin has been called more recently than commit or rollback), and false otherwise.

Example 9.1. Grouping Operations with Transactions

public void transferFunds (User from, User to, double amnt)
    PersistenceManager pm = JDOHelper.getPersistenceManager (from);
    Transaction trans = pm.currentTransaction ();
    trans.begin ();
        from.decrementAccount (amnt);
        to.incrementAccount (amnt);
        trans.commit ();
    catch (JDOFatalException jfe)  // trans is already rolled back
        throw jfe;
    catch (RuntimeException re)    // includes non-fatal JDO exceptions
        trans.rollback ();         // or could attempt to fix error and retry
        throw re;