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A JDBC Implementation of the Transaction Interface
To work with JDBC transactions, it's necessary that you have access to a Connection
object. The challenge is, therefore, to let the Transaction object and the DAOs share Connections. As mentioned earlier, this is a job for the transaction handler,
which will work as follows:
- Retrieve DAO instances through a DAO factory class.
- Retrieve a Transaction instance containing Connections through the same factory class.
- Pass the transaction instance to the DAOs which may get
Connections through a getter method.
- Invoke the begin method, the DAO methods, and the end/rollback methods to form business transactions.
To match the setup from the first article, a DataSource object will be used to produce the Connections. The DataSource object is produced by the DAO/Transaction factory classes, so you'll simply let the factory class place the DataSource instance in the Transaction object. The transaction interface is therefore expanded for JDBC transactions to look like this:
package dk.hansen;
import java.sql.Connection;
import javax.sql.DataSource;
public interface JDBCTransactionIF extends TransactionIF {
// Get a JDBC connection
public Connection getConnection() throws DAOException;
// Set a datasource
public void setDataSource(DataSource ds);
}
A complete JDBCTransaction class implementing this interface can be viewed here. Some of the important parts of the class are:
In order for this mechanism to work, a DAO may only use Connections from the Transaction object. Whether a transaction is set up for several DAO methods is the responsibility of the transaction handler, which simply uses the begin, end, and rollback methods to start and stop a transaction.
The transaction handler uses the begin, end, and rollback methods for a real transaction. For non-transactional requests, like a database look-up (SQL SELECT), only set the Transaction instance on the DAO that the handler calls.
The Final Transaction Handler Class
For an example of implementing a "rename" function in the transaction handler, you'll use the code classes from "Using DAOs in Apache Struts." Please note that the DVDManager class is a DAO. The specs for the rename function is to rename a DVD's id by appending with "-2005". Since the id is the key in the database, you must implemented this function with a delete operation followed by a create. These operations must be part of a transaction to keep the database in a well-defined state.
Here are the specs for the rename function:
public void rename(String id, String append) throws DAOException
When the transaction handler starts, it should first create the DAO and transaction instances:
private TransactionIF trans;
private DVDManagerIF dao;
public TransactionHandler() throws DAOException {
DVDManagerFactoryIF factory = new JdbcDVDManagerFactory();
trans = factory.createTransaction();
dao = factory.createDVDManager();
}
You can then code the rename method like this (the important parts are bolded):
/**
* Rename a DVD by id by appending a text to its id
* @param id The id to look for
* @param append The text to append
* @throws DAOException
*/
public void rename(String id, String append) throws DAOException {
dao.setTransaction(trans);
DVD dvd = dao.getDVD(id);
if (dvd == null) throw new DAOException("DVD " + id + " not found");
try {
trans.begin();
} catch (DAOException e) {
String msg = "'begin transaction' failed.";
logger.info(msg);
throw new DAOException(msg, e);
}
try {
dao.deleteDVD(id);
dao.createDVD(id + append, dvd.getTitle());
} catch (DAOException e) {
try {
trans.rollback();
logger.info("'rollback' successful.");
} catch (DAOException e1) {
logger.info("'rollback transaction' failed", e1);
}
String msg = "rename failed.";
logger.info(msg);
throw new DAOException(msg, e);
}
try {
trans.end();
} catch (DAOException e) {
String msg = "'end transaction' failed.";
logger.info(msg);
throw new DAOException(msg, e);
}
}
Note that the initial call to getDVD may be held outside the transaction. Still, the transaction instance must be given to the DAO so it can obtain its Connection object.
When everything runs fine, write the following to the console:
dk.hansen.JdbcDVDManagerFactory - JdbcDVDManagerFactory. JDBC driver loaded
dk.hansen.JdbcDVDManagerFactory - JdbcDVDManagerFactory. Datasource loaded
dk.hansen.JDBCTransaction - Get a connection
dk.hansen.JDBCTransaction - New Connection created
dk.hansen.JDBCTransaction - Connection com.mysql.jdbc.Connection@1cb25f1 returned
dk.hansen.DatabaseDVDManager - DatabaseDVDManager.getDVD, id=ID1
dk.hansen.JDBCTransaction - Begin transaction
dk.hansen.JDBCTransaction - New Connection created
dk.hansen.JDBCTransaction - Get a connection
dk.hansen.JDBCTransaction - Connection com.mysql.jdbc.Connection@503429 returned
dk.hansen.DatabaseDVDManager - DatabaseDVDManager.getDVD, id=ID1
dk.hansen.JDBCTransaction - Get a connection
dk.hansen.JDBCTransaction - Connection com.mysql.jdbc.Connection@503429 returned
dk.hansen.DatabaseDVDManager - DatabaseDVDManager.deleteDVD, id=ID1
dk.hansen.JDBCTransaction - Get a connection
dk.hansen.JDBCTransaction - Connection com.mysql.jdbc.Connection@503429 returned
dk.hansen.DatabaseDVDManager - DatabaseDVDManager.getDVD, id=ID1-2005
dk.hansen.JDBCTransaction - Get a connection
dk.hansen.JDBCTransaction - Connection com.mysql.jdbc.Connection@503429 returned
dk.hansen.DatabaseDVDManager - DatabaseDVDManager.createDVD, id=ID1-2005
dk.hansen.JDBCTransaction - End transaction
dk.hansen.JDBCTransaction - Close connection
Note how the first getDVD call (outside the transaction) obtains a connection with the address "1cb25f1", and the following method calls (inside the transaction) all get the same connection at the address "503429".
If something goes wrong, rollback is called:
. . .
dk.hansen.JDBCTransaction - Get a connection
dk.hansen.JDBCTransaction - Connection com.mysql.jdbc.Connection@503429 returned
dk.hansen.DatabaseDVDManager - DatabaseDVDManager.deleteDVD, id=ID1
dk.hansen.JDBCTransaction - Get a connection
dk.hansen.JDBCTransaction - Connection com.mysql.jdbc.Connection@503429 returned
dk.hansen.DatabaseDVDManager - DatabaseDVDManager.getDVD, id=ID1-2005
dk.hansen.JDBCTransaction - Rollback transaction
dk.hansen.JDBCTransaction - Close connection
dk.hansen.TransactionHandler - 'rollback' successful.
dk.hansen.TransactionHandler - rename failed.
dk.hansen.DAOException: rename failed.
at dk.hansen.TransactionHandler.rename(TransactionHandler.java:86)
at dk.hansen.TransactionHandler.main(TransactionHandler.java:102)
Caused by: dk.hansen.DAOException: Id ID1-2005 is already used
at dk.hansen.DatabaseDVDManager.createDVD(DatabaseDVDManager.java:43)
at dk.hansen.TransactionHandler.rename(TransactionHandler.java:76)
... 1 more
Exception in thread "main"
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