Need Fool Proof Synchronization of ArrayList in A Multi-threaded Environment

Question

I\'ve been at this for a week now doing my research on how to properly synchronize an ArrayList.

My main issue in a nutshell is I have a \"master\" ArrayList of objects.

Answer 1

Use CopyOnWriteArrayList, and synchronize on write operations only

CopyOnWriteArrayList list = ...

final Object writeLock = new Object();

void writeOpA()
{
    synchronized(writeLock)
    {
        read/write list
    }
}
void writeOpB()
{
    synchronized(writeLock)
    {
        read/write list
    }
}

Therefore no two write sessions will overlap with each other.

Reads require no lock. But a read session may see a changing list. If we want a read session to see a snapshot of the list, either use iterator(), or take a snapshot by toArray().

---

It's probably even better if you do the copy-on-write yourselves

volatile Foo data = new Foo(); // ArrayList in your case

final Object writeLock = new Object();

void writeOpA()
{
    synchronized(writeLock)
    {
        Foo clone = data.clone();
        // read/write clone
        data = clone;
    }
}
void writeOpB()
{
    // similar...
}

void readSession()
{
    Foo snapshot = data;
    // read snapshot
}