！boolean是Java中的原子操作吗？

Question

Can the code below can work in multithreading? Is the !boolean a atomic operation in java?

volatile boolean flag = true;
if (!flag){
    do something
}

Answer 1

Because it's marked volatile and the value is a primitive, you are guaranteed that the read will always be consistent (no possibility of reading a partially-initialized value).

Even if it weren't marked volatile , the JVM will always atomically read any primitive value that is stored in 32 bits or smaller. This is a requirement of the language in JLS 17.6 . (JLS 17.7, which allows "word tearing" for 64-bit primitive values, namely long and double , says that this behavior is implementation-specific. In practice, on all production 64-bit JVM implementations I know of, reads of even 64-bit values are always atomic.)

The operation of inverting the boolean may not be atomic with the read, but if so it will work by copying the boolean value to the JVM instruction stack, so there's still no possibility of data corruption (since pushing the value onto the operand stack is an atomic operation). More likely, the JIT will just invert your if condition and jump to the other branch rather than actually inverting the boolean value.

Of course, you still don't have any guarantee you won't read the value immediately before it is changed by another thread, so literally the very next instruction may begin executing concurrently with the boolean being set to true by another thread.

Answer 2

Is the !boolean a[n] atomic operation in java?

No. There are three operations here:

1. Load the variable
2. Compare and branch.
3. 'do something'.

A thread-switch can occur between 1 and 2, or 2 and 3, and the value of the underlying boolean can change at either time, or during 3, or after it.

So if you want to be sure that 'do something' only happens if flag is simultaneously false, you will have to synchronize, or use a semaphore.