visual basic barcode printing The Join() Method in Java

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The Join() Method
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The nonstatic join() method of class Thread lets one thread join onto the end of another thread. If you have a thread B that can t do its work until another thread A has completed its work, then you want thread B to join thread A. This means that thread B will not become runnable until A has finished (and entered the dead state).
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Thread t = new Thread(); t.start(); t.join();
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9: Threads
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The preceding code takes the currently running thread (if this were in the main() method, then that would be the main thread) and joins it to the end of the thread referenced by t. This blocks the current thread from becoming runnable until after the thread referenced by t is no longer alive. You can also call one of the overloaded versions of join that takes a timeout duration, so that you re saying, wait until thread t is done, but if it takes longer than 5,000 milliseconds, then stop waiting and become runnable anyway. Figure 9-3 shows the effect of the join() method. So far we ve looked at three ways a running thread could leave the running state:
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A call to sleep()
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Guaranteed to cause the current thread to stop executing for at least the specified sleep duration (although it might be interrupted before its specified time).
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FIGURE 9-3
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The join() method
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Synchronizing Code (Exam Objective 7.3)
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A call to yield()
Not guaranteed to do much of anything, although typically it will cause the currently running thread to move back to runnable so that a thread of the same priority can have a chance.
A call to join()
Guaranteed to cause the current thread to stop executing until the thread it joins with (in other words, the thread it calls wait() on) completes. If the thread it s trying to join with is not alive, however, the current thread won t need to back out.
Besides those three, we also have the following scenarios in which a thread might leave the running state:
The thread s run() method completes. Duh. A call to wait() on an object (we don t call wait() on a thread, as we ll
see in a moment)
A thread can t acquire the lock on the object whose method code it s
attempting to run. To understand the two critical execution stoppers, you need to understand the way in which Java implements object locking to prevent multiple threads from accessing and potentially corrupting the same data. Since these are covered in the next two objectives (7.3 and 7.4), be sure you study these so you ll recognize when a running thread will stop (at least temporarily) running.
CERTIFICATION OBJECTIVE
Synchronizing Code (Exam Objective 7.3)
Write code using synchronized wait, notify, and notifyAll to protect against concurrent access problems and to communicate between threads. Can you imagine the havoc that can occur when two different threads have access to a single instance of a class, and both threads invoke methods on that object and those methods modify the state of the object In other words, what might happen if two different threads call, say, a setter method on a single object A scenario like that
9: Threads
might corrupt an object s state (by changing its instance variable values in an inconsistent way), and if that object s state is data shared by other parts of the program, well, it s too scary to even visualize. But just because we enjoy horror, let s look at an example of what might happen. The following code demonstrates what happens when two different threads are accessing the same account data. Imagine that two people each have a checkbook for a single checking account (or two people each have ATM cards, but both cards are linked to only one account). In this example, we have a class called Account that represents a bank account. To keep the code short, this account starts with a balance of 50, and can be used only for withdrawals. The withdrawal will be accepted even if there isn t enough money in the account to cover it. The account simply reduces the balance by the amount you want to withdraw:
class Account { private int balance = 50; public int getBalance() { return balance; } public void withdraw(int amount) { balance = balance - amount; } }
Now here s where it starts to get fun. Imagine a couple, Fred and Lucy, who both have access to the account and want to make withdrawals. But they don t want the account to ever be overdrawn, so just before one of them makes a withdrawal, he or she will first check the balance to be certain there s enough to cover the withdrawal. Also, withdrawals are always limited to an amount of 10, so there must be at least 10 in the account balance in order to make a withdrawal. Sounds reasonable. But that s a two-step process:
1. Check the balance. 2. If there s enough in the account (in this example, at least 10), make the withdrawal.
What happens if something separates step 1 from step 2 For example, imagine what would happen if Lucy checks the balance and sees that there s just exactly enough in the account, 10. But before she makes the withdrawal, Fred checks the balance and also sees that there s enough for his withdrawal. Since Lucy has verified
Synchronizing Code (Exam Objective 7.3)
the balance, but not yet made her withdrawal, Fred is seeing bad data. He is seeing the account balance before Lucy actually debits the account, but at this point that debit is certain to occur. Now both Lucy and Fred believe there s enough to make their withdrawals. So now imagine that Lucy makes her withdrawal, and now there isn t enough in the account for Fred s withdrawal, but he thinks there is since when he checked, there was enough! Yikes. Here s what the actual banking code looks like, with Fred and Lucy represented by two threads, each acting on the same Runnable, and that Runnable holds a reference to the one and only account instance so, two threads, one account. The logic in our code example is as follows:
1. The Runnable object holds a reference to a single account. 2. Two threads are started, representing Lucy and Fred, and each thread is given a reference to the same Runnable (which holds a reference to the actual account). 3. The initial balance on the account is 50, and each withdrawal is exactly 10. 4. In the run() method, we loop 5 times, and in each loop we
Make a withdrawal (if there s enough in the account) Print a statement if the account is overdrawn (which it should never be,
since we check the balance before making a withdrawal)
5. The makeWithdrawal() method in the test class (representing the behavior of Fred or Lucy) does the following:
Check the balance to see if there s enough for the withdrawal. If there is enough, print out the name of the one making the withdrawal. Go to sleep for 500 milliseconds just long enough to give the other
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