Christof Meerwald@java_sync.notes

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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:

while (locked) {
    pthread_cond_wait(&cond, &mutex);
locked = 1;

// do something

locked = 0;


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).



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Christof Meerwald <cmeerw@cmeerw.org>
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