Monday, September 27, 2010

AtomicBoolean is irreplaceable

People question the justification for seemingly trivial classes like AtomicSomething. Let's take the simplest and show AtomicBoolean is irreplaceable.

compareAndSet(true, false) means "if value in main memory before this clock cycle is positive, then toggle it, otherwise do nothing. No locking please."

Read it over and over, and you would realize it's impossible without Atomics. Volatiles can't do it, because this involves more than a single load or store. AtomicBoolean does the load and the store in one clock cycle, using special hardware features -- "cmpxchg" in intel instruction.

java's getAndSet() is written using compareAndSet()

Q: Now, since a volatile done/stop flag is frequently used to stop a thread, shall we replace it with an AtomicBoolean?
A: no harm, but I don't see the need.

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