[JAVA] Is drainTo of LinkedBlockingQueue safe? I followed the source

Overview

I decided to use drainTo because I had to get the status of the temporary point from LinkedBlockingQueue in my business.

drainTo is

Removes all available elements from this queue and adds them to the specified collection.

It is a method called.

However, if you look at the help,

The behavior of this operation is undefined if the specified collection is modified while the operation is in progress.

Publisher

There was an uneasy sentence, and I investigated whether there was a problem.

2017/11/30 postscript

• In the comment, you pointed out that ** the specified collection ** refers to the collection passed as an argument. Therefore, if the collection passed as an argument is thread-safe, there seems to be no problem. However, the following survey contents will be left as they are.

Source

LinkedBlockingQueue.java

/**
* @throws UnsupportedOperationException {@inheritDoc}
* @throws ClassCastException            {@inheritDoc}
* @throws NullPointerException          {@inheritDoc}
* @throws IllegalArgumentException      {@inheritDoc}
*/
public int drainTo(Collection<? super E> c, int maxElements) {
    if (c == null)
        throw new NullPointerException();
    if (c == this)
        throw new IllegalArgumentException();
    if (maxElements <= 0)
        return 0;
    boolean signalNotFull = false;
    final ReentrantLock takeLock = this.takeLock;
    takeLock.lock();
    try {
        int n = Math.min(maxElements, count.get());
        // count.get provides visibility to first n Nodes
        Node<E> h = head;
        int i = 0;
        try {
            while (i < n) {
                Node<E> p = h.next;
                c.add(p.item);
                p.item = null;
                h.next = h;
                h = p;
                ++i;
            }
            return n;
        } finally {
            // Restore invariants even if c.add() threw
            if (i > 0) {
                // assert h.item == null;
                head = h;
                signalNotFull = (count.getAndAdd(-i) == capacity);
            }
        }
    } finally {
        takeLock.unlock();
        if (signalNotFull)
            signalNotFull();
    }
}

In addition, what is actually used is drainTo with one argument,

LinkedBlockingQueue.java

return drainTo(c, Integer.MAX_VALUE);

And, since it is just calling the same method that is overloaded, there is no problem if you check here.

variable

takeLock A lock object for locking the ** retrieval ** of data.

putLock Although it does not appear in the above source, it is a lock object for locking the ** addition ** of data.

count ʻAtomicIntegertype field. Every time I change the data, it seems that I change it manually in the class. (It seems that it is not just returningQueue.size`)

Commentary

We will look at only the necessary parts in order from the top.

• Rock acquisition

final ReentrantLock takeLock = this.takeLock;
takeLock.lock();

Acquire the acquisition lock before starting the data acquisition.

• Do you need an additional lock? * I thought, but it is no longer needed due to the following processing.

• Get the number

int n = Math.min(maxElements, count.get());

Before starting data acquisition, ** decide how many items to acquire from the beginning **. This allows you to ** ignore data added during the process of drainTo **.

Conclusion

• Data retrieval is safe because the lock is acquired in drainTo.
• Data addition is done safely regardless of drainTo because the number of acquired data is calculated first.
• Therefore, LinkedBlockingQueue.drainTo can be safely executed even in a multithreaded environment.

It seems that it can be said.

Digression (impression)

personally,

** If you want to get data all at once, you should lock the additions in the meantime! ** **

I will think.

I was wondering if there was an approach of "deciding the number of data first". It was a very good study.