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The Transaction Object
Since a transaction handler must not have any knowledge about how transactions are actually implemented on the back-end, a standardized convention is needed for when the handler calls DAOs—to tell them whether they should participate in a specific transaction. A common solution is to introduce a transaction object which encapsulates the way transactions are implemented. The handler may now initialize the transaction object and pass it to the DAOs.
It's important to realize that a given implementation of such a transaction object normally contains code or properties that closely matches the DAOs and the given back-end implementation. A transaction object for a database back-end implementation using JDBC may for example contain a SQL Connection object.
So, this situation resembles the DAO classes and their interfaces: a Java Interface to describe the transaction object is needed, and when DAO instances are generated by the factory class a Transaction instance should also be generated.
The following figure shows how the Transaction Handler uses the Transaction and DAO instances created by a factory class.
Figure 1: The relations between classes and interfaces.
The Transaction Interface
When you work with JDBC transactions, you use methods like commit and rollback. Since this interface must work on many different back-ends, a new set of names would be appropriate, for example:
package dk.hansen
public interface TransactionIF {
/* Start a transaction */
public void begin() throws DAOException;
/* Rollback a transaction */
public void rollback() throws DAOException;
/* End a transaction */
public void end() throws DAOException;
}
The DAOException class is a simple extension of a Java Exception, capable of handling chained exceptions.
With a given implementation of this Interface, the code for a "create" method in the transaction handler will look like this:
package dk.hansen;
import org.apache.log4j.Logger;
public class TransactionHandler {
private static Logger logger = Logger.getLogger(TransactionHandler.class.getName());
public void create(...) throws DAOException {
TransactionIF trans = new MyTransaction();
/* Start a transaction */
try {
trans.begin();
} catch (DAOException e) {
String msg = "'begin transaction' failed.";
logger.info(msg);
throw new DAOException(msg, e);
}
try {
// Calls to DAOs here...
someDao.setTransaction(trans);
...
} catch (DAOException e) {
try {
trans.rollback();
} catch (DAOException e1) {
logger.info("'rollback transaction' failed.");
}
String msg = "create failed.";
logger.info(msg);
throw new DAOException(msg, e);
}
/* Stop the transaction */
try {
trans.end();
} catch (DAOException e) {
String msg = "'end transaction' failed.";
logger.info(msg);
throw new DAOException(msg, e);
}
}
}
The TransactionHandler does three things:
- It creates an instance of the Transaction class.
- It uses the begin, end, and rollback methods.
- It passes the instance to the DAOs.
It's evident that the TransactionHandler knows nothing about how transactions are implemented. It simply follows the conventions that have been set up.
To see if this setup actually works, you need to make a "real" implementation of the TransactionIF interface, using, for example, JDBC transactions.
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