java qr code reader Modularizing the read-write lock pattern in Java

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Modularizing the read-write lock pattern
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forms a read-only operation acquire the read lock, whereas the methods that affect the state of the class must acquire the write lock. Once the operation is over, it must release the lock that it acquired. Listing 9.15 shows the Account class with the necessary modifications.
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Listing 9.15 Account.java: with the read-write lock pattern implemented
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import EDU.oswego.cs.dl.util.concurrent.*; public abstract class Account { private float _balance; private int _accountNumber; private ReadWriteLock _lock = new ReentrantWriterPreferenceReadWriteLock(); public Account(int accountNumber) { _accountNumber = accountNumber; } public void credit(float amount) { try { _lock.writeLock().acquire(); Acquiring the setBalance(getBalance() + amount); write lock } catch (InterruptedException ex) { throw new RuntimeException(ex); } finally { _lock.writeLock().release(); Releasing the } write lock } public void debit(float amount) throws InsufficientBalanceException { try { _lock.writeLock().acquire(); Acquiring the float balance = getBalance(); write lock if (balance < amount) { throw new InsufficientBalanceException( "Total balance not sufficient"); } else { setBalance(balance - amount); } } catch (InterruptedException ex) { throw new RuntimeException(ex); } finally { _lock.writeLock().release(); Releasing the } write lock }
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Creating the read-write lock
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Implementing thread safety
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public float getBalance() { try { _lock.readLock().acquire(); return _balance; } catch (InterruptedException ex) { throw new RuntimeException(ex); } finally { _lock.readLock().release(); } } public void setBalance(float balance) { try { _lock.writeLock().acquire(); _balance = balance; } catch (InterruptedException ex) { throw new RuntimeException(ex); } finally { _lock.writeLock().release(); } } }
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Acquiring the read lock
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Releasing the read lock
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Acquiring the write lock
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Releasing the write lock
We add a lock object of type ReentrantWriterPreferenceReadWriteLock in the ReadWriteLock implementation that allows the same thread to acquire a lock in a reentrant manner; that way, a thread can acquire a lock even if it is already in possession of it. The acquire() method on the locks throws InterruptedException. We simply rethrow it by wrapping it in a RuntimeException and thus avoid having to change each method to declare it will throw InterruptedException. The use of try/finally ensures the locks release even when the core operation aborts due to an exception. Clearly, the necessary code is invasive even for a class as simple as Account. You must modify each class that needs this pattern in a similar manner. Any missed method will result in potentially fatal program behavior. Further, you must make sure that the method that acquires a read lock releases only the read lock and not a write lock, and vice versa. 9.7.2 Implementation: the AspectJ way The core concept behind the AspectJ-based solution is to create an aspect that encapsulates the pattern. This way, we can avoid the need to modify each class. Further, because the pattern is reusable, we would like the aspect to be reusable as well.
Modularizing the read-write lock pattern
To achieve our goal, let s write a base aspect that captures the read-write lock pattern. To enable synchronization in this aspect, let s write a concrete aspect to extend it that provides the definition for two pointcuts: one to capture the execution of read methods and the other to capture the execution of write methods. Listing 9.16 shows the implementation of the thread-safety pattern in an abstract aspect.
Listing 9.16 ReadWriteLockSynchronizationAspect.java
import EDU.oswego.cs.dl.util.concurrent.*; public abstract aspect ReadWriteLockSynchronizationAspect perthis(readOperations() || writeOperations()) { public abstract pointcut readOperations();
Aspect association
c Read operations public abstract pointcut writeOperations(); d Write operations private ReadWriteLock _lock e Lock object = new ReentrantWriterPreferenceReadWriteLock();
before() : readOperations() { _lock.readLock().acquire(); } after() : readOperations() { _lock.readLock().release(); } before() : writeOperations() { _lock.writeLock().acquire(); } after() : writeOperations() { _lock.writeLock().release(); }
Read operation management
Write operation management
static aspect SoftenInterruptedException { declare soft : InterruptedException : call(void Sync.acquire()); } }
Softening of InterruptedException
Using the perthis() association specification, we associate an aspect instance with each worker object that matches the read or write methods defined in the concrete aspects. (See chapter 4, section 4.3.2, to learn more about aspect association.) A new aspect instance will be created for each object for which a captured
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