Java concurrency - improving a copy-on-read collection

Question

I have a multithreaded application, where a shared list has write-often, read-occasionally behaviour.

Specifically, many threads will dump data into the list, and th

Answer 1

You can use a ReadWriteLock to allow multiple threads to perform add operations on the backing list in parallel, but only one thread to make the snapshot. While the snapshot is being prepared all other add and snapshot request are put on hold.

A ReadWriteLock maintains a pair of associated locks, one for read-only operations and one for writing. The read lock may be held simultaneously by multiple reader threads, so long as there are no writers. The write lock is exclusive.

class CopyOnReadList {

    // free to use any concurrent data structure, ConcurrentLinkedQueue used as an example
    private final ConcurrentLinkedQueue items = new ConcurrentLinkedQueue();
    private final ReadWriteLock rwLock = new ReentrantReadWriteLock();
    private final Lock shared = rwLock.readLock();
    private final Lock exclusive = rwLock.writeLock(); 

    public void add(T item) {
        shared.lock(); // multiple threads can attain the read lock
        // try-finally is overkill if items.add() never throws exceptions
        try {
          // Add item while holding the lock.
          items.add(item);
        } finally {
          shared.unlock();
        }
    }

    public List makeSnapshot() {
        List copy = new ArrayList(); // probably better idea to use a LinkedList or the ArrayList constructor with initial size
        exclusive.lock(); // only one thread can attain write lock, all read locks are also blocked
        // try-finally is overkill if for loop never throws exceptions
        try {
          // Make a copy while holding the lock.
          for (T t : items) {
            copy.add(t);
          }
        } finally {
          exclusive.unlock();
        }
        return copy;
    }

}

Edit:

The read-write lock is so named because it is based on the readers-writers problem not on how it is used. Using the read-write lock we can have multiple threads achieve read locks but only one thread achieve the write lock exclusively. In this case the problem is reversed - we want multiple threads to write (add) and only thread to read (make the snapshot). So, we want multiple threads to use the read lock even though they are actually mutating. Only thread is exclusively making the snapshot using the write lock even though snapshot only reads. Exclusive means that during making the snapshot no other add or snapshot requests can be serviced by other threads at the same time.

As @PeterLawrey pointed out, the Concurrent queue will serialize the writes aqlthough the locks will be used for as minimal a duration as possible. We are free to use any other concurrent data structure, e.g. ConcurrentDoublyLinkedList. The queue is used only as an example. The main idea is the use of read-write locks.