visual basic barcode printing Problems with concurrent access in Java

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Problems with concurrent access
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So how do you protect the data You must do two things:
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Mark the variables private Synchronize the code that modifies the variables
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Remember, you protect the variables in the normal way using an access control modifier. It s the method code that you must protect, so that only one thread at a time can be executing that code. You do this with the synchronized keyword. We can solve all of Fred and Lucy s problems by adding one word to the code. We mark the makeWithdrawal() method synchronized as follows:
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private synchronized 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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Now we ve guaranteed that once a thread (Lucy or Fred) starts the withdrawal process (by invoking makeWithdrawal()), the other thread cannot enter that method until the first one completes the process by exiting the method. The new output shows the benefit of synchronizing the makeWithdrawal() method:
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% java AccountDanger Fred is going to withdraw Fred completes the withdrawal Lucy is going to withdraw Lucy completes the withdrawal Fred is going to withdraw Fred completes the withdrawal
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9: Threads
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Lucy is going to withdraw Lucy completes the withdrawal Fred is going to withdraw Fred completes the withdrawal Not enough in account for Lucy Not enough in account for Fred Not enough in account for Lucy Not enough in account for Fred Not enough in account for Lucy
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to to to to to
withdraw withdraw withdraw withdraw withdraw
0 0 0 0 0
Notice that now the threads, Lucy and Fred, always check the account balance and complete the withdrawal before the other thread can check the balance.
Synchronization and Locks
How does synchronization work With locks. Every object in Java has a built-in lock that only comes into play when the object has synchronized method code. Since there is only one lock per object, if one thread has picked up the lock, no other thread can enter the synchronized code (which means any synchronized method of that object) until the lock has been released. Typically, releasing a lock means the thread holding the lock (in other words, the thread currently in the synchronized method) exits the synchronized method. At that point, the lock is free until some other thread enters a synchronized method on that object. You need to remember the following key points about locking and synchronization:
Only methods can be synchronized, not variables. Each object has just one lock. Not all methods in a class must be synchronized. A class can have both synchronized
and nonsynchronized methods.
If two methods are synchronized in a class, only one thread can be accessing
one of the two methods. In other words, once a thread acquires the lock on an object, no other thread can enter any of the synchronized methods in that class (for that object).
If a class has both synchronized and nonsynchronized methods, multiple
threads can still access the nonsynchronized methods of the class! If you have methods that don t access the data you re trying to protect, then you don t
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need to mark them as synchronized. Synchronization is a performance hit, so you don t want to use it without a good reason.
If a thread goes to sleep, it takes its locks with it. A thread can acquire more than one lock. For example, a thread can enter
a synchronized method, thus acquiring a lock, and then immediately invoke a synchronized method on a different object, thus acquiring that lock as well. As the stack unwinds, locks are released again. Also, if a thread acquires a lock and then attempts to call a synchronized method on that same object, no problem. The JVM knows that this thread already has the lock for this object, so the thread is free to call other synchronized methods on the same object, using the lock the thread already has.
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