聊聊原子类AtomicInteger

AtomicInteger

自增1并获取之前值

/**
 * Atomically increments by one the current value.
 *
 * @return the previous value
 */
public final int getAndIncrement() {
    return unsafe.getAndAddInt(this, valueOffset, 1);
}

自增1并获取更新后值

/**
 * Atomically increments by one the current value.
 *
 * @return the updated value
 */
public final int incrementAndGet() {
    return unsafe.getAndAddInt(this, valueOffset, 1) + 1;
}

valueOffset 是 AtomicInteger 保存值的变量，在class中的Offset，Unsafe类中会用到

private static final long valueOffset;

static {
    try {
        valueOffset = unsafe.objectFieldOffset
            (AtomicInteger.class.getDeclaredField("value"));
    } catch (Exception ex) { throw new Error(ex); }
}
##保存操作值的变量
private volatile int value;

Unsafe::getAndAddInt 源码

/**
 * Atomically adds the given value to the current value of a field
 * or array element within the given object <code>o</code>
 * at the given <code>offset</code>.
 *
 * @param o object/array to update the field/element in
 * @param offset field/element offset
 * @param delta the value to add
 * @return the previous value
 * @since 1.8
 */
public final int getAndAddInt(Object o, long offset, int delta) {
    int v;
    do {
        //获取对象中对应属性的值
        v = getIntVolatile(o, offset);
        //这行是关键，调用处理器硬件指令，这个操作是原子的
        //v是期望值，v + delta是准备设置的新值
        //如果此时未有其实线程更改v则返回成功，否则失败
    } while (!compareAndSwapInt(o, offset, v, v + delta));
    return v;
}

Unsafe::compareAndSwapInt

/**
 * Atomically update Java variable to <tt>x</tt> if it is currently
 * holding <tt>expected</tt>.
 * @return <tt>true</tt> if successful
 */
public final native boolean compareAndSwapInt(Object o, long offset,
                                              int expected,
                                              int x);