How is the runner used to run the Callable in FutureTask<V>

Question

In the source code of FutureTask<V>, there is a volatile instance variable Thread runner, where the comment told that it is the thread running the callable. However, the runner is never initialized in the source code. Moreover, I could not find any clue how this runner is used to run the callable.

Question: Since the runner is never initialized, how it is used to run the Callable?

Answer 1

In the code you can see

if (state != NEW ||
        !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                     null, Thread.currentThread()))

at the start of the run method. Ignoring the state != NEW part; this attempts to set the runner variable to the result of Thread.currentThread() (and will succeed only if it is currently null). Only if this succeeds (returns true) will the run method be able to go through the rest of the code in this block. And since the result of Thread.currentThread() will be the Thread which called the run method, the documentation is accurate (at least, after this initial if part is successfully evaluated).