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How the Banking Application is build using JTS interfaces

From an architectural point of view of JTS, the bank client is considered as an application program able to manage transactions either in a direct or indirect management mode, respectively with the interfaces org.omg.CosTransactions.TransactionFactory and org.omg.CosTransactions.Terminator or with the org.omg.CosTransactions.Current interface. Transactions created by the client in the Banking application are done in the indirect mode.

The following portion of code illustrates how a JTS 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 Current).

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;
import com.arjuna.ats.jts.OTSManager;

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("Transfert 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 
       // JBossTS class to obtain a Current instance
       Current current = OTSManager.get_current(); 
       System.out.println("Beginning a User 
                     transaction to get balance");
       current.begin();
    
       Account supplier = _bank.get_account( name_supplier );
       Account consumer = _bank.get_account( name_consumer );
       supplier.debit( famount );
       consumer.credit( famount );
    
       current.commit( );
      }
     catch (Exception e)
      {
       System.err.println("ERROR - "+e);
      }
   }
   

Since JTS is used invocations against an ORB are needed, such ORB and Object Adapter instantiation and initialisation. To ensure a better portability, the ORB Portability API provides a set of methods that can be used as described below.

public static void main( String [] args )
{
  try
   { 
    // Create an ORB instance
    myORB = ORB.getInstance("test");
    //Obtain the Root POA
    myOA = OA.getRootOA(myORB); 
    //Initialise the ORB
    myORB.initORB(args, null);
    //Initialise the POA
    myOA.initOA();
     ....
     
   }
   catch(Exception 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 org.omg.CosTransactions.Resource, then able to participate to the transaction commitment. For this aim, the Account object has to register the AccountResource object as a participant, after having obtaining the reference of the org.omg.CosTransactions.Coordinator object , itself obtained via the org.omg.CosTransactions.Control 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 getResource().balance();;
 }
   
 public void credit( float value )
 {
   getResource().credit( value );
 }
   
 public void debit( float value )
 {
   getResource().debit( value );
 }

 public AccountResource getResource()
 {
   try
   {
    if (accRes == null) {
     accRes = new AccountResource(this, _name) ;    
     Resource  ref = org.omg.CosTransactions.ResourceHelper.
      narrow(OA.getRootOA(ORB.getInstance("test")).corbaReference(accRes));
     RecoveryCoordinator recoverycoordinator = OTSManager.get_current().
      get_control().get_coordinator().register_resource(ref);
    }
  }
  catch (Exception e)
   {...}
   return accRes;
 }   
  ...
}

To be considered as a org.omg.CosTransactions.Resource, the AccountResource class shall extends the class org.omg.CosTransactions.ResourcePOA generated by the CORBA IDL compiler. The AccountRessource provides similar methods as the Account class (credit, withdraw and balance) with the appropriate methods to participate to the 2PC protocol. The following portion of code describes how the methods prepare, commit and rollback are implemented.

public class AccountResource extends org.omg.CosTransactions.ResourcePOA
{
   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 org.omg.CosTransactions.Vote prepare() 
	   throws org.omg.CosTransactions.HeuristicMixed, 
	   org.omg.CosTransactions.HeuristicHazard
{
if ( _initial_balance == _current_balance )
return org.omg.CosTransactions.Vote.VoteReadOnly;
if ( _current_balance < 0 ) return org.omg.CosTransactions.Vote.VoteRollback;
return org.omg.CosTransactions.Vote.VoteCommit;
} public void rollback() throws org.omg.CosTransactions.HeuristicCommit, org.omg.CosTransactions.HeuristicMixed, org.omg.CosTransactions.HeuristicHazard { //Nothing to do } public void commit() throws org.omg.CosTransactions.NotPrepared, org.omg.CosTransactions.HeuristicRollback, org.omg.CosTransactions.HeuristicMixed, org.omg.CosTransactions.HeuristicHazard { _account._balance = _current_balance; } public void commit_one_phase() throws org.omg.CosTransactions.HeuristicHazard { _account._balance = _current_balance; } ..... private float _initial_balance; private float _current_balance; private Account _account; }
Sample Application Source Code

Full source code for the banking application is included to provide you with a starting point for experimentation.

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