135: SYNCHRONIZATION in Java

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135: SYNCHRONIZATION
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Synchronized Methods
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If the methods of an object should only be executed by one thread at a time, then the declaration of all such methods should be specified with the keyword synchronized A thread wishing to execute a synchronized method must first obtain the object s lock (ie, hold the lock) before it can enter the object to execute the method This is simply achieved by calling the method If the lock is already held by another thread, the calling thread waits No particular action on the part of the program is necessary A thread relinquishes the lock simply by returning from the synchronized method, allowing the next thread waiting for this lock to proceed Synchronized methods are useful in situations where methods can manipulate the state of an object in ways that can corrupt the state if executed concurrently A stack implementation usually defines the two operations push and pop as synchronized, ensuring that pushing and popping of elements are mutually exclusive operations If several threads were to share a stack, then one thread would, for example, not be able to push an element on the stack while another thread was popping the stack The integrity of the stack is maintained in the face of several threads accessing the state of the same stack This situation is illustrated by Example 133 The code in Example 133 is intentionally non-generic in order to avoid generic considerations getting in the way The main() method in class Mutex creates a stack at (6), which is used by the two threads created at (7) and (8) The two threads continually push and pop the stack The non-synchronized push() and pop() methods at (2a) and (4a) intentionally sleep at (3) and (5), respectively, between an update and the use of the value in the field topOfStack This setup increases the chances for the state of the stack being corrupted by one of the threads, while the other one is sleeping The output from the program in Example 133 bears this out when the methods are not declared synchronized Non-synchronized updating of the value in the field topOfStack between the two threads is a disaster waiting to happen This is an example of what is called a race condition It occurs when two or more threads simultaneously update the same value and, as a consequence, leave the value in an undefined or inconsistent state From the output shown in Example 133, we can see that the main thread exits right after creating and starting the threads The threads push and pop the stack The stack state eventually gets corrupted, resulting in an ArrayOutOfBoundsException in the Pusher thread The uncaught exception results in the demise of the Pusher thread, but the Popper thread continues Running the program in Example 133 with the synchronized version of the push() and pop() methods at (2b) and (4b), respectively, avoids the race condition The method sleep() does not relinquish any lock that the thread might have on the current object It is only relinquished when the synchronized method exits, guaranteeing mutually exclusive push and pop operations on the stack
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CHAPTER 13: THREADS
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Example 133 Mutual Exclusion class StackImpl { private Object[] stackArray; private int topOfStack; public StackImpl(int capacity) { stackArray = new Object[capacity]; topOfStack = -1; } public boolean push(Object element) { // (2a) non-synchronized //public synchronized boolean push(Object element) { // (2b) synchronized if (isFull()) return false; ++topOfStack; try { Threadsleep(1000); } catch (Exception e) { } // (3) Sleep a little stackArray[topOfStack] = element; return true; } public Object pop() { // (4a) non-synchronized //public synchronized Object pop() { // (4b) synchronized if (isEmpty()) return null; Object obj = stackArray[topOfStack]; stackArray[topOfStack] = null; try { Threadsleep(1000); } catch (Exception e) { } // (5) Sleep a little topOfStack--; return obj; } public boolean isEmpty() { return topOfStack < 0; } public boolean isFull() { return topOfStack >= stackArraylength - 1; } } //_______________________________________________________________________________ public class Mutex { public static void main(String[] args) { final StackImpl stack = new StackImpl(20); // (6) Shared by the threads // (1)
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(new Thread("Pusher") { // (7) Thread no 1 public void run() { for(;;) { Systemoutprintln("Pushed: " + stackpush(2008)); } } })start(); (new Thread("Popper") { // (8) Thread no 2 public void run() { for(;;) { Systemoutprintln("Popped: " + stackpop()); } }
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135: SYNCHRONIZATION })start(); Systemoutprintln("Exit from main()"); } }
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Possible output from the program when run with (2a) and (4a):
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Exit from main() Pushed: true Popped: 2008 Popped: 2008 Popped: null Popped: null javalangArrayIndexOutOfBoundsException: -1 at StackImplpush(Mutexjava:15) at Mutex$1run(Mutexjava:41) Popped: null Popped: null
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While a thread is inside a synchronized method of an object, all other threads that wish to execute this synchronized method or any other synchronized method of the object will have to wait This restriction does not apply to the thread that already has the lock and is executing a synchronized method of the object Such a method can invoke other synchronized methods of the object without being blocked The non-synchronized methods of the object can always be called at any time by any thread Static methods synchronize on the class lock Acquiring and relinquishing a class lock by a thread in order to execute a static synchronized method is analogous to that of an object lock for a synchronized instance method A thread acquires the class lock before it can proceed with the execution of any static synchronized method in the class, blocking other threads wishing to execute any static synchronized methods in the same class This does not apply to static, non-synchronized methods, which can be invoked at any time A thread acquiring the lock of a class to execute a static synchronized method has no effect on any thread acquiring the lock on any object of the class to execute a synchronized instance method In other words, synchronization of static methods in a class is independent from the synchronization of instance methods on objects of the class A subclass decides whether the new definition of an inherited synchronized method will remain synchronized in the subclass
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