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partner a chance to get in before you actually make the withdrawal.
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When you wake up, complete the withdrawal and print out that you ve
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done so.
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If there wasn t enough in the first place, print a statement showing who
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you are and the fact that there wasn t enough. So what we re really trying to discover is if the following is possible: for one partner to check the account and see that there s enough, but before making the actual withdrawal, the other partner checks the account and also sees that there s
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9: Threads
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enough. When the account balance gets to 10, if both partners check it before making the withdrawal, both will think it s OK to withdraw, and the account will overdraw by 10! Here s the code:
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public class AccountDanger implements Runnable { private Account acct = new Account(); public static void main (String [] args) { AccountDanger r = new AccountDanger(); Thread one = new Thread(r); Thread two = new Thread(r); one.setName("Fred"); two.setName("Lucy"); one.start(); two.start(); } public void run() { for (int x = 0; x < 5; x++) { makeWithdrawal(10); if (acct.getBalance() < 0) { System.out.println("account is overdrawn!"); } } } private void makeWithdrawal(int amt) { if (acct.getBalance() >= amt) { System.out.println(Thread.currentThread().getName() + " is going to withdraw"); try { Thread.sleep(500); } catch(InterruptedException ex) { } acct.withdraw(amt); System.out.println(Thread.currentThread().getName() + " completes the withdrawal"); } else { System.out.println("Not enough in account for " + Thread.currentThread().getName() + " to withdraw " + acct.getBalance()) } } }
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So what happened Is it possible that, say, Lucy checked the balance, fell asleep, Fred checked the balance, Lucy woke up and completed her withdrawal, then Fred completes his withdrawal, and in the end they overdraw the account Look at the output:
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% java AccountDanger 1. Fred is going to withdraw 2. Lucy is going to withdraw 3. Fred completes the withdrawal 4. Fred is going to withdraw 5. Lucy completes the withdrawal 6. Lucy is going to withdraw 7. Fred completes the withdrawal 8. Fred is going to withdraw 9. Lucy completes the withdrawal 10. Lucy is going to withdraw 11. Fred completes the withdrawal 12. Not enough in account for Fred 13. Not enough in account for Fred 14. Lucy completes the withdrawal 15. account is overdrawn! 16. Not enough in account for Lucy 17. account is overdrawn! 18. Not enough in account for Lucy 19. account is overdrawn!
to withdraw 0 to withdraw 0
to withdraw -10 to withdraw -10
Although each time you run this code the output might be a little different, let s walk through this particular example using the numbered lines of output. For the first four attempts, everything is fine. Fred checks the balance on line 1, and finds it s OK. At line 2, Lucy checks the balance and finds it OK. At line 3, Fred makes his withdrawal. At this point, the balance Lucy checked for (and believes is still accurate) has actually changed since she last checked. And now Fred checks the balance again, before Lucy even completes her first withdrawal. By this point, even Fred is seeing a potentially inaccurate balance, because we know Lucy is going to complete her withdrawal. It is possible, of course, that Fred will complete his before Lucy does, but that s not what happens here. On line 5, Lucy completes her withdrawal and then before Fred completes his, Lucy does another check on the account on line 6. And so it continues until we get to line 8, where Fred checks the balance and sees that it s 20. On line 9, Lucy completes a withdrawal (that she had checked for earlier), and this takes the balance to 10. On line 10, Lucy checks again, sees that the balance is 10, so she knows she can do
9: Threads
a withdrawal. But she didn t know that Fred, too, has already checked the balance on line 8 so he thinks its safe to do the withdrawal! On line 11, Fred completes the withdrawal he approved on line 8. This takes the balance to zero. But Lucy still has a pending withdrawal that she got approval for on line 10! You know what s coming. On lines 12 and 13, Fred checks the balance and finds that there s not enough in the account. But on line 14, Lucy completes her withdrawal and BOOM! The account is now overdrawn by 10 something we thought we were preventing by doing a balance check prior to a withdrawal. Figure 9-4 shows the timeline of what can happen when two threads concurrently access the same object. Preventing the Account Overdraw So what can be done The solution is actually quite simple. We must guarantee that the two steps of the withdrawal checking the balance and making the withdrawal are never split apart. We need them to always be performed as one operation, even when the thread falls asleep in between step 1 and step 2! We call this an atomic operation (although the physics there is a little outdated) because the operation, regardless of the number of actual statements (or underlying byte code instructions), is completed before any other thread code that acts on the same data. You can t guarantee that a single thread will stay running throughout the entire atomic operation. But you can guarantee that even if the thread running the atomic operation moves in and out of the running state, no other running thread will be able to act on the same data. In other words, If Lucy falls asleep after checking the balance, we can stop Fred from checking the balance until after Lucy wakes up and completes her withdrawal.
FIGURE 9-4
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