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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:

> java_sync_contended.c:

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)