Practical exercise 1 Discussion

1
Practical exercise 1Discussion

• Threads in Java
• Mutual exclusion in Java

2
The exercise

• Couriers deposit money in a bank.
• Each courier is a thread.
• Somehow, the total amount of money deposited
  differed from the amount of money actually in the
  bank.
• Find the reasons for this and correct the problem!

3
Diagnosis

• Some runs of the handed out program illustrates
  the problems

gt java Bank 5 2000000 Starting simulation of 5
couriers with 2000000 deposits each. All couriers
are done. Money in the bank 933878146 Total
money deposited 994957883 Discrepancy
61079737 Elapsed time 3976 milliseconds.
gt java Bank 5 2000000 Starting simulation of 5
couriers with 2000000 deposits each. All couriers
are done. Money in the bank 994983480 Total
money deposited 994983346 Discrepancy
-134 Elapsed time 3885 milliseconds.

• The amount of money in the bank doesnt match the
  amount of money actually deposited

4
Diagnosis (cont.)

• Bank

Courier
public class Bank .. add money to
Bank ..
public class Bank .. create Thread
courier ..
Courier 1
Bank
Courier 2
Courier 3
5
Diagnosis (cont.)

• Program Outputs

public void courierDone() int moneyInBank
nightSafeAmount vaultAmount int
moneyDeposited calculateMoneyDeposited() Syste
m.out.println("Money in the bank
"moneyInBank) System.out.println("Total money
deposited"moneyDeposited) ..

• Shared varible?

public class Bank private int
nightSafeAmount private int vaultAmount ..
6
Diagnosis (cont.)

• Where are shared varibles modified?
• public void depositMoney(int amount)

/ Deposit a given amount of money. The
money is put in the night safe. If there is
too much money in the night safe, all the
money is transferred to the vault. / public
void depositMoney(int amount)
nightSafeAmount amount if(nightSafeAmount
gt 200) // Transfer the money in the night
safe to the vault vaultAmount
nightSafeAmount nightSafeAmount 0

7
Diagnosis (cont.)

• Synchronized critical code
• synchronized keyword

public synchronized void depositMoney(int amount)
.

• Re-run the program

8
Suggestions

• Use mutual exclusion when needed
• How much synchronization?
• Even when a program contains only a single thread
  running on a single processor, a synchronized
  method call is still slower than an
  unsynchronized method call
• Questions?