Sunday, September 26, 2010

AtomicReference + immutable = lockfree (code

import static java.lang.System.out;
import java.util.concurrent.atomic.AtomicReference;

public class AtomicAccount {
    public static void main(String[] a) {
        final AtomicAccount inst = new AtomicAccount(100);
        Runnable r = new Runnable() {
            public void run() {
                inst.addInterest();
            }
        };
        new Thread(r).start();
        new Thread(r).start();
    }

    public final AtomicReference<ImmutableAccount> AR = new AtomicReference<ImmutableAccount>();

    public AtomicAccount(float bal) {
        Account tmp = new Account();
        tmp.bal = bal;
        this.AR.set(new ImmutableAccount(tmp));
    }

    /**
     * add 1% to balance. load and store must be atomic.
     */
    void addInterest() {
        while (true) {
            ImmutableAccount b4 = this.AR.get();
            Account tmp = new Account();
            tmp.bal = b4.getBal() * 1.01f;
            ImmutableAccount update = new ImmutableAccount(tmp);
            if (this.AR.compareAndSet(b4, update)) {
                this.printBal();
                return;
            }
        }
    }

    public void printBal() {
        out.println("current balance = " + this.AR.get().getBal());
    }

    static private class ImmutableAccount {
        final private Account mutable = new Account();

        /**
         * if bal is a non-volatile double, we might read the 2 parts
         * un-atomically.
         *
         * @param a
         */
        public ImmutableAccount(Account a) {
            this.mutable.bal = a.bal;
            // copy other fields
        }

        public float getBal() {
            return this.mutable.bal;
        }
    }
}

class Account {
    float bal;
}

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