Question

I have a multi-threaded application that has a centrlaised list that is updated (written to) only by the main thread. I then have several other threads that need to periodically retrieve the list in its current state. Is there a method that can allow me to do this?

Answer 1

That depends on how you want to restrict the concurrency. The easiest way is probably using CopyOnWriteArrayList. When you grab an iterator from it, that iterator will mirror how the list looked at the point in time when the iterator was created – subsequent modifications will not be visible to the iterator. The upside is that it can cope with quite a lot of contention, the drawback is that adding new items is rather expensive.

The other way of doing is locking, the simplest way is probably wrapping the list with Collections.synchronizedList and synchronizing on the list when iterating.

A third way is using some kind of BlockingQueue and feed the new elements to the workers.

Edit: As the OP stated only a snapshot is needed, CopyOnWriteArrayList is probably the best out-of-the-box alternative. An alternative (for cheaper adding, but costlier reading) is just creating a copy of a synchronizedList when traversion is needed (copy-on-read rather than copy-on-write):

List<Foo> originalList = Collections.synchronizedList(new ArrayList());

public void mainThread() {
    while(true)
        originalList.add(getSomething());
}

public void workerThread() {
    while(true) {
        List<Foo> copiedList;
        synchronized (originalList) {
             copiedList = originalList.add(something);
        }
        for (Foo f : copiedList) process(f);
    }
}

Edit: Come to think of it, the copy-on-read version could simplified a bit to avoid all synchronized blocks:

List<Foo> originalList = Collections.synchronizedList(new ArrayList());

public void mainThread() {
    while(true)
        originalList.add(getSomething());
}

public void workerThread() {
    while(true) {
        for (Foo f : originalList.toArray(new Foo[0])) 
            process(f);
    }
}

Edit 2: Here’s a simple wrapper for a copy-on-read list which doesn’t use any helpers, and which tries to be as fine-grained in the locking as possible (I’ve deliberately made it somewhat excessive, bordering on suboptimal, to demonstrate where locking is needed):

class CopyOnReadList<T> {

    private final List<T> items = new ArrayList<T>();

    public void add(T item) {
        synchronized (items) {
            // Add item while holding the lock.
            items.add(item);
        }
    }

    public List<T> makeSnapshot() {
        List<T> copy = new ArrayList<T>();
        synchronized (items) {
            // Make a copy while holding the lock.
            for (T t : items) copy.add(t);
        }
        return copy;
    }

}

// Usage:
CopyOnReadList<String> stuff = new CopyOnReadList<String>();
stuff.add("hello");
for (String s : stuff.makeSnapshot())
    System.out.println(s);

Basically, when you to lock when you:

1. … add an item to the list.
2. … iterate over the list to make a copy of it.