Java Concurrency in Practice - Sample 14.12

Question

// Not really how java.util.concurrent.Semaphore is implemented
@ThreadSafe
public class SemaphoreOnLock {
    private final Lock lock = new ReentrantLock();
    // CONDITION PREDICATE: permitsAvailable (permits > 0)
    private final Condition permitsAvailable = lock.newCondition();
    @GuardedBy("lock") private int permits;

    SemaphoreOnLock(int initialPermits) {
        lock.lock();
        try {
            permits = initialPermits;
        } finally {
            lock.unlock();
        }
    }

/* other code omitted.... */

I have a question about the sample above which is extracted from Java Concurrency in Practice Listing 14.12 Counting Semaphore Implemented Using Lock.

I am wondering why we need to acquire the lock in the constructor (as shown lock.lock() is invoked). As far as i know, constructor is atomic (except the reference escaped) as no other thread can get the reference, hence, the half-constructed-object is not visible to other threads. Therefore, we do not need the synchronized modifier for constructors. Besides, we do not need to worry about the memory visibility as well, as long as the object is safely published.

So, why do we need to get the ReentrantLock object inside the constructor?

Answer 1

the half-constructed-object is not visible to other threads

It is not true. The object is visible to other threads at the time of construction if it has any non final/volatile fields. Therefore, other threads might see a default value for permits i.e 0 which might not be consistent with the current thread.

The Java memory model offers a special guarantee of initialization safety for immutable objects (object with only final fields). An object reference visible to another thread does not necessarily mean that the state of that object is visible to the consuming thread - JCP $3.5.2

From Listing 3.15 of Java Concurrency in Practice:

While it may seem that field values set in a constructor are the first values written to those fields and therefore that there are no "older" values to see as stale values, the Object constructor first writes the default values to all fields before subclass constructors run. It is therefore possible to see the default value for a field as a stale value.

Answer 2

(Just clarifying it for my own poor head - the other answers are correct).

Instances of this hypothetical SemaphoreOnLock class are intended to be shared. So thread T1 fully constructs an instance, and puts it somewhere where thread T2 can see it and invoke some method which requires reading the permits field. Some important things to note about the permits field:

1. it gets initialized, in the first case, to a default value of 0
2. it is then assigned a value (which may be other than the default of 0), by thread T1
3. it's not volatile
4. it's not final (which makes it kind of like a 'one shot volatile')

Therefore, if we want T2 to read the value that T1 last wrote, we need to synchronize. We have to do this in the constructor, just as we must in every other case. (That fact of it being an atomic assignment or not doesn't affect this visibility issue). The strategy of confining the constructed SemaphoreOnLock to a single thread doesn't work for us, because the whole idea of making it @Threadsafe is so that we can safely share it.

What this example illustrates is that "being threadsafe" applies to the construction of an object as well, when setting any non-static, non-final, non-volatile field to a value other than its default value.

Of course, we're not obliged to even think about this when we've got a @NotThreadsafe class. If the caller constructs us and decides to share us between two threads, then the caller must arrange for appropriate synchronization. In that scenario, we can do whatever we like in the constructor without worrying about visibility concerns - that's somebody else's problem.

Answer 3

I honestly don't see any valid use for the lock here, other than the fact that it introduces a memory fence. int assignments are atomic on 32/64 bit anyway.