Variables inside Executor's Runnable

Question

Suppose I have a Runnable that does a simple file writing operation, and this Runnable is called with an executor.

With executor being a single thread Executor class..

public void doThis() {
    executor.execute(new Runnable() {
        @Override
        public void run() {
            file.write(_data);
        }
    });
}

does the immediate contents of _data get saved the moment execute() is called? Which means that once the runnable has been submitted to the queue, I can go ahead and make changes to _data, and the changes will not be written to the file?

_data = something
doThis();
_data = something else

is there a chance that I will end up doing file.write(something else)?

Answer 1

no to your first and second questions, and yes to your third question ... if the single thread that the executor is running on is a separate thread from the calling thread, then this is not a thread-safe operation. unless .execute() executes on the same exact thread as the caller, you need to avoid making changes to _data until .write is done.

A common way of working around this is to simply make a copy of _data before passing it off to the executor.