package threads;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

/**
 *  This class will use an explicit lock to avoid a deadlock situation.
 */
public class ExplicitLockAccountTransfer
{
  public static void main(String[] args)
  {
    Account a1 = new Account("Account 1", 10000);
    Account a2 = new Account("Account 2", 30000);
    System.out.println("Initial Balances:");
    System.out.println(" Account 1: " + a1.getBalance());
    System.out.println(" Account 2: " + a2.getBalance());
    System.out.println("Transferring 5 from Account 1 to Account 2");
    System.out.println("Transferring 20 from Account 2 to Account 1");
    Thread threadA =
        new TransferFundsThread("Thread A", a1, a2, 5);
    Thread threadB =
        new TransferFundsThread("Thread B", a2, a1, 20);
    threadA.start();
    threadB.start();
  }
}

class Account
{
  public Account(String name, int balance)
  {
    m_name = name;
    m_balance = balance;
  }
  
  public int getBalance()
  {
    return m_balance;
  }
  
  public void setBalance(int newBalance)
  {
    m_balance = newBalance;
  }
  
  public String getName()
  {
    return m_name;
  }
  
  public Lock getLock()
  {
    return m_lock;
  }
  
  private String m_name;
  private int    m_balance;
  private Lock   m_lock = new ReentrantLock();
}

class TransferFundsThread extends Thread
{
  public TransferFundsThread(String name,
      Account from,
      Account to,
      int amount)
  {
    super(name);
    m_from   = from;
    m_to     = to;
    m_amount = amount;
  }
  
  public void run()
  {
    System.out.println("run: Transferring funds from " + m_from.getName() +
                       " to " + m_to.getName());
    transferFunds();
  }
  
  /**
   *  In order to transfer funds safely, we need to be able
   *  to hold a lock on the single lock used to synchronize
   *  access to the transferFunds method.
   */
  private void transferFunds()
  {
    while (true)
    {
      try
      {
        // Try to acquire the lock for the from account
        if ( m_from.getLock().tryLock(500, TimeUnit.MILLISECONDS) )
        {
          try
          {
            // Get the balance from the from account
            int balance = getBalance();
            
            // Try to acquire the lock for the to account
            if ( m_to.getLock().tryLock(500, TimeUnit.MILLISECONDS) )
            {
              try
              {
                doTransfer(balance);
              }
              finally
              {
                // Unlock the to lock, regardless of whether
                // an exception occurred.
                m_to.getLock().unlock();
              }
              // We succeeded, so break out of the for while loop.
              break;
            }
          }
          finally
          {
            // Unlock the from lock, regardless of whether
            // an exception occurred.
            m_from.getLock().unlock();
          }
        }
      }
      catch (InterruptedException ie)
      {
        // Do nothing; try again
      }
    }
  }
  
  /**
   * Get the balance from the to account
   */
  private int getBalance()
  {
    System.out.println(getName() +
        ": Getting balance from " +
        m_from.getName());
    int fromBalance = m_from.getBalance();
    System.out.println(getName() +
        ": Balance from " +
        m_from.getName() +
        " = " + fromBalance);
    return fromBalance;
  }
  
  /**
   * Do the actual transfer of funds
   */
  private void doTransfer(int fromBalance)
  {
    System.out.println(getName() +
        ": Getting balance from " +
        m_to.getName());
    int toBalance = m_to.getBalance();
    System.out.println(getName() +
        ": Balance from " +
        m_to.getName() +
        " = " + toBalance);
    
    if (fromBalance >= m_amount)
    {
      System.out.println(getName() +
          ": Withdrawing funds from " +
          m_from.getName());
      m_from.setBalance(fromBalance - m_amount);
      System.out.println(getName() +
          ": Depositing funds into " +
          m_to.getName());
      m_to.setBalance(toBalance + m_amount);
      
      System.out.println(getName() +
          ": New balances are:\n" +
          "  " + m_from.getName() +
          " = " +
          m_from.getBalance() + "\n" +
          "  " + m_to.getName() + " = " +
          m_to.getBalance() );
    }
  }
  
  /// Private data ///
  private Account   m_from;
  private Account   m_to;
  private int       m_amount;
}