com.trolltech.qt.core
Class QMutex

java.lang.Object
  extended by com.trolltech.qt.internal.QSignalEmitterInternal
      extended by com.trolltech.qt.QSignalEmitter
          extended by com.trolltech.qt.QtJambiObject
              extended by com.trolltech.qt.core.QMutex
All Implemented Interfaces:
QtJambiInterface

public class QMutex
extends QtJambiObject

The QMutex class provides access serialization between threads. The purpose of a QMutex is to protect an object, data structure or section of code so that only one thread can access it at a time (this is similar to the Java synchronized keyword). It is usually best to use a mutex with a QMutexLocker since this makes it easy to ensure that locking and unlocking are performed consistently.

For example, say there is a method that prints a message to the user on two lines:

        int number = 6;

        public void method1()
        {
            number *= 5;
            number /= 4;
        }

        public void method2()
        {
            number *= 3;
            number /= 2;
        }
If these two methods are called in succession, the following happens:
        // method1()
        number *= 5;        // number is now 30
        number /= 4;        // number is now 7

        // method2()
        number *= 3;        // number is now 21
        number /= 2;        // number is now 10
If these two methods are called simultaneously from two threads then the following sequence could result:
        // Thread 1 calls method1()
        number *= 5;        // number is now 30

        // Thread 2 calls method2().
        //
        // Most likely Thread 1 has been put to sleep by the operating
        // system to allow Thread 2 to run.
        number *= 3;        // number is now 90
        number /= 2;        // number is now 45

        // Thread 1 finishes executing.
        number /= 4;        // number is now 11, instead of 10
If we add a mutex, we should get the result we want:
        QMutex mutex = new QMutex();
        int number = 6;

        public void method1()
        {
            mutex.lock();
            number *= 5;
            number /= 4;
            mutex.unlock();
        }

        public void method2()
        {
            mutex.lock();
            number *= 3;
            number /= 2;
            mutex.unlock();
        }
Then only one thread can modify number at any given time and the result is correct. This is a trivial example, of course, but applies to any other case where things need to happen in a particular sequence.

When you call lock() in a thread, other threads that try to call lock() in the same place will block until the thread that got the lock calls unlock(). A non-blocking alternative to lock() is tryLock().

See also:
QMutexLocker, QReadWriteLock, QSemaphore, and QWaitCondition.


Nested Class Summary
static class QMutex.RecursionMode
          See also.
QMutex().
 
Nested classes/interfaces inherited from class com.trolltech.qt.QSignalEmitter
QSignalEmitter.AbstractSignal, QSignalEmitter.PrivateSignal0, QSignalEmitter.PrivateSignal1, QSignalEmitter.PrivateSignal2, QSignalEmitter.PrivateSignal3, QSignalEmitter.PrivateSignal4, QSignalEmitter.PrivateSignal5, QSignalEmitter.PrivateSignal6, QSignalEmitter.PrivateSignal7, QSignalEmitter.PrivateSignal8, QSignalEmitter.PrivateSignal9, QSignalEmitter.Signal0, QSignalEmitter.Signal1, QSignalEmitter.Signal2, QSignalEmitter.Signal3, QSignalEmitter.Signal4, QSignalEmitter.Signal5, QSignalEmitter.Signal6, QSignalEmitter.Signal7, QSignalEmitter.Signal8, QSignalEmitter.Signal9
 
Nested classes/interfaces inherited from class com.trolltech.qt.internal.QSignalEmitterInternal
com.trolltech.qt.internal.QSignalEmitterInternal.AbstractSignalInternal
 
Field Summary
 
Fields inherited from class com.trolltech.qt.internal.QSignalEmitterInternal
currentSender
 
Constructor Summary
QMutex()
          Constructs a new mutex.
QMutex(QMutex.RecursionMode mode)
          Constructs a new mutex.
 
Method Summary
 void lock()
          Locks the mutex.
 boolean tryLock()
          Attempts to lock the mutex.
 boolean tryLock(int timeout)
          This is an overloaded member function, provided for convenience.
 void unlock()
          Unlocks the mutex.
 
Methods inherited from class com.trolltech.qt.QtJambiObject
dispose, disposed, equals, finalize, reassignNativeResources, tr, tr, tr
 
Methods inherited from class com.trolltech.qt.QSignalEmitter
blockSignals, disconnect, disconnect, signalsBlocked, signalSender, thread
 
Methods inherited from class com.trolltech.qt.internal.QSignalEmitterInternal
__qt_signalInitialization
 
Methods inherited from class java.lang.Object
clone, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 
Methods inherited from interface com.trolltech.qt.QtJambiInterface
disableGarbageCollection, nativeId, nativePointer, reenableGarbageCollection, setJavaOwnership
 

Constructor Detail

QMutex

public QMutex()
Constructs a new mutex. The mutex is created in an unlocked state.

If mode is QMutex::Recursive , a thread can lock the same mutex multiple times and the mutex won't be unlocked until a corresponding number of unlock() calls have been made. The default is QMutex::NonRecursive .

See also:
lock(), and unlock().


QMutex

public QMutex(QMutex.RecursionMode mode)
Constructs a new mutex. The mutex is created in an unlocked state.

If mode is QMutex::Recursive , a thread can lock the same mutex multiple times and the mutex won't be unlocked until a corresponding number of unlock() calls have been made. The default is QMutex::NonRecursive .

See also:
lock(), and unlock().

Method Detail

lock

public final void lock()
Locks the mutex. If another thread has locked the mutex then this call will block until that thread has unlocked it.

Calling this function multiple times on the same mutex from the same thread is allowed if this mutex is a recursive mutex . If this mutex is a non-recursive mutex , this function will dead-lock when the mutex is locked recursively.

See also:
unlock().


tryLock

public final boolean tryLock()
Attempts to lock the mutex. If the lock was obtained, this function returns true. If another thread has locked the mutex, this function returns false immediately.

If the lock was obtained, the mutex must be unlocked with unlock() before another thread can successfully lock it.

Calling this function multiple times on the same mutex from the same thread is allowed if this mutex is a recursive mutex . If this mutex is a non-recursive mutex , this function will always return false when attempting to lock the mutex recursively.

See also:
lock(), and unlock().


tryLock

public final boolean tryLock(int timeout)
This is an overloaded member function, provided for convenience.

Attempts to lock the mutex. This function returns true if the lock was obtained; otherwise it returns false. If another thread has locked the mutex, this function will wait for at most timeout milliseconds for the mutex to become available.

Note: Passing a negative number as the timeout is equivalent to calling lock(), i.e. this function will wait forever until mutex can be locked if timeout is negative.

If the lock was obtained, the mutex must be unlocked with unlock() before another thread can successfully lock it.

Calling this function multiple times on the same mutex from the same thread is allowed if this mutex is a recursive mutex . If this mutex is a non-recursive mutex , this function will always return false when attempting to lock the mutex recursively.

See also:
lock(), and unlock().


unlock

public final void unlock()
Unlocks the mutex. Attempting to unlock a mutex in a different thread to the one that locked it results in an error. Unlocking a mutex that is not locked results in undefined behavior.

See also:
lock().