Synchronized Blocks in Java

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Whereas execution of synchronized methods of an object is synchronized on the lock of the object, the synchronized block allows execution of arbitrary code to be
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CHAPTER 13: THREADS
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synchronized on the lock of an arbitrary object The general form of the synchronized statement is as follows:
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synchronized (<object
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reference expression>)
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<code block>
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The <object reference expression> must evaluate to a non-null reference value, otherwise a NullPointerException is thrown The code block is usually related to the object on which the synchronization is being done This is analagous to a synchronized method, where the execution of the method is synchronized on the lock of the current object The following code is equivalent to the synchronized pop() method at (4b) in Example 133:
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public Object pop() { synchronized (this) { // } } // Synchronized block on current object
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Once a thread has entered the code block after acquiring the lock on the specified object, no other thread will be able to execute the code block, or any other code requiring the same object lock, until the lock is relinquished This happens when the execution of the code block completes normally or an uncaught exception is thrown In contrast to synchronized methods, this mechanism allows fine-grained synchronization of code on arbitrary objects Object specification in the synchronized statement is mandatory A class can choose to synchronize the execution of a part of a method by using the this reference and putting the relevant part of the method in the synchronized block The braces of the block cannot be left out, even if the code block has just one statement
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class SmartClient { BankAccount account; // public void updateTransaction() { synchronized (account) { // (1) synchronized block accountupdate(); // (2) } } }
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In the previous example, the code at (2) in the synchronized block at (1) is synchronized on the BankAccount object If several threads were to concurrently execute the method updateTransaction() on an object of SmartClient, the statement at (2) would be executed by one thread at a time only after synchronizing on the BankAccount object associated with this particular instance of SmartClient Inner classes can access data in their enclosing context (see Section 81, p 352) An inner object might need to synchronize on its associated outer object in order to ensure integrity of data in the latter This is illustrated in the following code where the synchronized block at (5) uses the special form of the this reference to synchronize on the outer object associated with an object of the inner class This setup ensures that a thread executing the method setPi() in an inner object can only
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135: SYNCHRONIZATION
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access the private double field myPi at (2) in the synchronized block at (5) by first acquiring the lock on the associated outer object If another thread has the lock of the associated outer object, the thread in the inner object has to wait for the lock to be relinquished before it can proceed with the execution of the synchronized block at (5) However, synchronizing on an inner object and on its associated outer object are independent of each other, unless enforced explicitly, as in the following code:
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class Outer { private double myPi; protected class Inner { public void setPi() { synchronized(Outerthis) { myPi = MathPI; } } } } // (1) Top-level Class // (2) // // // // (3) Non-static member Class (4) (5) Synchronized block on outer object (6)
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Synchronized blocks can also be specified on a class lock:
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synchronized (<class
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name>class)
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<code block>
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The block synchronizes on the lock of the object denoted by the reference <class name>class This object (of type Class) represents the class in the JVM A static synchronized method classAction() in class A is equivalent to the following declaration:
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