Java Synchronisation
Even tough monitors could rather trivially be mapped to mutexes and condition variables in Java, most of the work is handled internally and only a simple park/unpark interface (see PlatformEvent::park and PlatformEvent::unpark) is used to put threads into a sleeping state.
Note: PlatformEvent::park maps onto pthread_cond_wait on Linux (and Solaris if you don't set UseLWPSynchronization - otherwise it maps to lwp_cond_wait)
As might be expected, uncontended synchronisation is handled entirely in user-space.
Contended Synchronisation
When a monitor is contended, a simple synchronized block will be mapped to roughly (simplified) the following native code:
pthread_mutex_lock(&mutex);
while (locked) {
pthread_cond_wait(&cond, &mutex);
}
locked = 1;
pthread_mutex_unlock(&mutex);
// do something
pthread_mutex_lock(&mutex);
locked = 0;
pthread_cond_signal(&cond);
pthread_mutex_unlock(&mutex);
notify/notifyAll
To avoid contention when waking up threads via notify or notifyAll, Java queues the thread(s) for wakeup when releasing the monitor (one at a time).
Resources
- hotspot/src/share/vm/runtime/synchronizer.cpp (J2SE 6u2)
- hotspot/src/os/linux/vm/os_linux.cpp (J2SE 6u2)
- hotspot/src/os/solaris/vm/os_solaris.cpp (J2SE 6u2)
