From an architectural point of view of JTA, the bank client is considered as
an application program able to manage transactions via the javax.transaction.UserTransaction interface. The following portion of code illustrates how a JTA transaction is
started and terminated when the client asks to transfer money from one account to another. This also describes what are JBossTS packages that need to be used
in order to obtain appropriate objects instances (such UserTransaction).
Note: The code below is a simplified view of the BankClient.java program. Only the transfer operation is illustrated; other operations manage transactions in the same way.
(see for details the BankClient.java)
package com.arjuna.demo.jta.localbank;
public class BankClient
{
private Bank _bank;
// This operation is used to make a transfer
//from an account to another account
private void makeTransfer()
{
System.out.print("Take money from : ");
String name_supplier = input();
System.out.print("Put money to : ");
String name_consumer = input();
System.out.print("Transfer amount : ");
String amount = input();
float famount = 0;
try
{
famount = new Float( amount ).floatValue();
}
catch ( java.lang.Exception ex )
{
System.out.println("Invalid float number, abort operation...");
return;
}
try
{
//the following instruction asks a specific ArjunaTA
//class to obtain a UserTransaction instance
javax.transaction.UserTransaction userTran =
com.arjuna.ats.jta.UserTransaction.userTransaction();
System.out.println("Beginning a User transaction to get balance");
userTran.begin();
Account supplier = _bank.get_account( name_supplier );
Account consumer = _bank.get_account( name_consumer );
supplier.debit( famount );
consumer.credit( famount );
userTran.commit( );
}
catch (Exception e)
{
System.err.println("ERROR - "+e);
}
}
......
}
The Bank object has mainly two operations: creating an account, which is added in the account list,
and returning an Account object. No transactional instruction is performed by the Bank object
package com.arjuna.demo.jta.localbank;
public class Bank {
private java.util.Hashtable _accounts;
public Bank()
{
_accounts = new java.util.Hashtable();
}
public Account create_account( String name )
{
Account acc = new Account(name);
_accounts.put( name, acc );
return acc;
}
public Account get_account(String name)
throws NotExistingAccount
{
Account acc = ( Account ) _accounts.get( name );
if ( acc == null )
throw new NotExistingAccount("The Account requested does not exist");
return acc;
}
}
The Account object provides mainly three methods balance,
credit and withdraw.
However, in order to provide the transactional behaviour, rather than to modify
the current account directly (according to credit or withdraw) this task is
delegated to an AccountResource object that is able, according to the transaction
outcome, to set the account value either to its initial state or its final state.
The AccountResource object is in fact an object that implements the javax.transactions.XAResource,
then able to participate to the transaction commitment. For this aim, the Account
object has to register or enlist the AccountResource object as a participant
after having obtaining the reference of the javax.transaction.Transaction object
via the javax.transaction.TransactionManager object
package com.arjuna.demo.jta.localbank;
public class Account
{
float _balance;
AccountResource accRes = null;
public Account(String name)
{
_name = name;
_balance = 0;
}
public float balance()
{
return getXAResource().balance();;
}
public void credit( float value )
{
getXAResource().credit( value );
}
public void debit( float value )
{
getXAResource().debit( value );
}
public AccountResource getXAResource()
{
try
{
javax.transaction.TransactionManager transactionManager =
com.arjuna.ats.jta.TransactionManager.transactionManager();
javax.transaction.Transaction currentTrans =
transactionManager.getTransaction();
if (accRes == null) {
currentTrans.enlistResource(
accRes = new AccountResource(this, _name) );
}
currentTrans.delistResource( accRes, XAResource.TMSUCCESS );
}
catch (Exception e)
{
System.err.println("ERROR - "+e);
}
return accRes;
}
...
}
The AccountResource class that implements the javax.transaxtion.XAResource
interface provides similar methods as the Account class (credit, withdraw and
balance) but also all methods specified by the javax.transaxtion.XAResource.
The following portion of code describes how the methods prepare,
commit and rollback
are implemented.
public class AccountResource implements XAResource
{
public AccountResource(Account account, String name )
{
_name = name;
_account = account;
_initial_balance = account._balance;
_current_balance = _initial_balance;
}
public float balance()
{
return _current_balance;
}
public void credit( float value )
{
_current_balance += value;
}
public void debit( float value )
{
_current_balance -= value;
}
public void commit(Xid id, boolean onePhase) throws XAException
{
//The value of the associated Account object is modified
_account._balance = _current_balance;
}
public int prepare(Xid xid) throws XAException
{
if ( _initial_balance == _current_balance ) //account not modified
return (XA_RDONLY);
if ( _current_balance < 0 )
throw new XAException(XAException.XA_RBINTEGRITY);
//If the integrity of the account is corrupted then vote rollback
return (XA_OK); //return OK
}
public void rollback(Xid xid) throws XAException
{
//Nothing is done
}
private float _initial_balance;
private float _current_balance;
private Account _account;
}
}
Sample Application Source Code
Full source code for the banking application with JTA is included to provide
you with a starting point for experimentation.