线程安全之原子性--Atomic包-Part-3

AtomicReference与AtomicReferenceFiledUpdater比较分析

AtomicReference分析

@Slf4j
@ThreadSafe
public class AtomicExample4 {

    private static AtomicReference<Integer> count = new AtomicReference<>(0);

    public static void main(String[] args) {
        count.compareAndSet(0, 2); // 更新为2
        count.compareAndSet(0, 1); // 不
        count.compareAndSet(1, 3); // 不
        count.compareAndSet(2, 4); // 更新为4
        count.compareAndSet(3, 5); // 不
        log.info("count:{}", count.get());
      }
}

执行结果为：

根据结果，可以看出其运行顺序跟我们预期是完全吻合的。

看一下AtomicReference的compareAndSet(a, b)方法源码：

public final void lazySet(V newValue) {
    unsafe.putOrderedObject(this, valueOffset, newValue);
}

/**
 * Atomically sets the value to the given updated value
 * if the current value {@code ==} the expected value.
 * @param expect the expected value
 * @param update the new value
 * @return {@code true} if successful. False return indicates that
 * the actual value was not equal to the expected value.
 */
public final boolean compareAndSet(V expect, V update) {
    return unsafe.compareAndSwapObject(this, valueOffset, expect, update);
}

又是调用的unsafe.compareAndSwapObject(xxx)，再看一下：

1	public final native boolean compareAndSwapObject(Object var1, long var2, Object var4, Object var5);

看来这个类真的是个典型的AtomicXXX类！

AtomicReferenceFiledUpdater分析

写出一个小例子：

@Slf4j
@ThreadSafe
public class AtomicExample5 {

    //指明更新的类为AtomicExample5
    private static AtomicIntegerFieldUpdater<AtomicExample5> updater =
            //newUpdater()的第一个参数为该指明的类的class，第二参数为作用的字段区域的名字filedName（该字段必须被volatile标识声明）
            AtomicIntegerFieldUpdater.newUpdater(AtomicExample5.class, "count");

    //添加get方法
    @Getter
    public volatile int count = 100;

    public static void main(String[] args) {

        AtomicExample5 example5 = new AtomicExample5();

        if (updater.compareAndSet(example5, 100, 120)) {
            log.info("update success 1, {}", example5.getCount());
        }

        if (updater.compareAndSet(example5, 100, 120)) {
            log.info("update success 2, {}", example5.getCount());
        } else {
            log.info("update failed, {}", example5.getCount());
        }
    }
}