unsafe.compareAndSwapInt first argument menanig

Question

I am investigating java.util.concurrent.locks.AbstractQueuedSynchronizersource code.

From severals place invokes compareAndSetState method.

/**
 * Atomically sets synchronization state to the given updated
 * value if the current state value equals the expected value.
 * This operation has memory semantics of a {@code volatile} read
 * and write.
 *
 * @param expect the expected value
 * @param update the new value
 * @return {@code true} if successful. False return indicates that the actual
 *         value was not equal to the expected value.
 */
protected final boolean compareAndSetState(int expect, int update) {
    // See below for intrinsics setup to support this
    return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
}

Parameters expect and update are obvious and correspond the atomics parameters. But this is Object(rather than int).
How does this compares with expect?

glee8e · Answer 1 · 2017-03-16T01:18:01.183

1

It's the instance whose field state will be CAS-ed. The value is stored in that field. The instance-offset pair is a way to translate the field descriptor to a memory address, just like you need to supply the instance when using Field::set, Field::get or Method::invoke.

Btw, these sun classes' source is available online at openjdk's mercury repository.

edited Mar 16 '17 at 01:18

answered Mar 14 '17 at 11:52

glee8e

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How AbstractQueuedSynchronizer can have value? – gstackoverflow Mar 14 '17 at 11:58
It's the `AbstractQueuedSynchronizer::state` field, not the object itself. – glee8e Mar 14 '17 at 12:16
*The instance-offset pair* what is it? – gstackoverflow Mar 15 '17 at 08:50
It's the first and the second function parameter. – glee8e Mar 16 '17 at 01:16

unsafe.compareAndSwapInt first argument menanig

1 Answers1