《Java并发编程实战》笔记-OneValueCache与原子引用技术

/**
 * NumberRange
 * <p/>
 * Number range class that does not sufficiently protect its invariants
 *
 * @author Brian Goetz and Tim Peierls
 */

public class NumberRange {
    // INVARIANT: lower <= upper
    private final AtomicInteger lower = new AtomicInteger(0);
    private final AtomicInteger upper = new AtomicInteger(0);

    public void setLower(int i) {
        // Warning -- unsafe check-then-act
        if (i > upper.get())
            throw new IllegalArgumentException("can't set lower to " + i + " > upper");
        lower.set(i);
    }

    public void setUpper(int i) {
        // Warning -- unsafe check-then-act
        if (i < lower.get())
            throw new IllegalArgumentException("can't set upper to " + i + " < lower");
        upper.set(i);
    }

    public boolean isInRange(int i) {
        return (i >= lower.get() && i <= upper.get());
    }
}

NumberRange既不能使用指向不可变对象的volatile引用来安全地实现上界和下界，也不能使用原子的整数来保存这两个边界。由于有一个不变性条件限制了两个数值，并且它们无法在同时更新时还维持该不变性条件，因此如果在数值范围类中使用volatile引用或者多个原子整数，那么将出现不安全的“先检查再运行”操作序列。

可以将OneValueCache中的技术与原子引用结合起来，它在更新上界或下界时能避免NumberRange的竞态条件。

/**
 * CasNumberRange
 * <p/>
 * Preserving multivariable invariants using CAS
 *
 * @author Brian Goetz and Tim Peierls
 */
@ThreadSafe
public class CasNumberRange {
    @Immutable
    private static class IntPair {
        // INVARIANT: lower <= upper
        final int lower;
        final int upper;

        public IntPair(int lower, int upper) {
            this.lower = lower;
            this.upper = upper;
        }
    }

    private final AtomicReference<IntPair> values = new AtomicReference<IntPair>(new IntPair(0, 0));

    public int getLower() {
        return values.get().lower;
    }

    public int getUpper() {
        return values.get().upper;
    }

    public void setLower(int i) {
        while (true) {
            IntPair oldv = values.get();
            if (i > oldv.upper)
                throw new IllegalArgumentException("Can't set lower to " + i + " > upper");
            IntPair newv = new IntPair(i, oldv.upper);
            if (values.compareAndSet(oldv, newv))
                return;
        }
    }

    public void setUpper(int i) {
        while (true) {
            IntPair oldv = values.get();
            if (i < oldv.lower)
                throw new IllegalArgumentException("Can't set upper to " + i + " < lower");
            IntPair newv = new IntPair(oldv.lower, i);
            if (values.compareAndSet(oldv, newv))
                return;
        }
    }
}