AtomicLong

AtomicLong是作用是对长整形进行原子操作。

属于package java.util.concurrent.atomic;

AtomicLong使用了Unsafe类，Unsafe类提供了硬件级别的原子操作。（对其内容，将在下次说明）

public class AtomicLong extends Number implements java.io.Serializable {
    private static final long serialVersionUID = 1927816293512124184L;

// setup to use Unsafe.compareAndSwapLong for updates
    private static final Unsafe unsafe = Unsafe.getUnsafe();
    private static final long valueOffset;

/**
 * Records whether the underlying JVM supports lockless
 * compareAndSwap for longs. While the Unsafe.compareAndSwapLong
 * method works in either case, some constructions should be
 * handled at Java level to avoid locking user-visible locks.
 */
static final boolean VM_SUPPORTS_LONG_CAS = VMSupportsCS8();

/**
 * Returns whether underlying JVM supports lockless CompareAndSet
 * for longs. Called only once and cached in VM_SUPPORTS_LONG_CAS.
 */
private static native boolean VMSupportsCS8();

static {
  try {
    valueOffset = unsafe.objectFieldOffset
        (AtomicLong.class.getDeclaredField("value"));//获取一个内存地址的偏移量
  } catch (Exception ex) { throw new Error(ex); }
}

private volatile long value;

//构造值为 initialValue的AtomicLong对象
public AtomicLong(long initialValue) {
    value = initialValue;
}

//默认构造函数
public AtomicLong() {
}

//获取当前值
public final long get() {
    return value;
}
//设置为newValue
public final void set(long newValue) {
    value = newValue;
}

//延迟设为newValue。值期间会改变，最终设置为给定的值。
public final void lazySet(long newValue) {
    unsafe.putOrderedLong(this, valueOffset, newValue);//设置对象中valueOffset偏移地址对应的object型field的值为指定值
}

//原子方式设置为当前值，返回原先的值。
public final long getAndSet(long newValue) {
    while (true) {
        long current = get();
        if (compareAndSet(current, newValue))
            return current;
    }
}

// 如果当前值 == expect，则以原子方式将该值设置为update。更改成功返回true，否则返回false且不修改原值。
public final boolean compareAndSet(long expect, long update) {
    return unsafe.compareAndSwapLong(this, valueOffset, expect, update);
}

//其实和上面的方法一样。
public final boolean weakCompareAndSet(long expect, long update) {
    return unsafe.compareAndSwapLong(this, valueOffset, expect, update);
}

//原子自增，返回原值
public final long getAndIncrement() {
    while (true) {
        long current = get();
        long next = current + 1;
        if (compareAndSet(current, next))
            return current;
    }
}

 //原子自减，返回原值
public final long getAndDecrement() {
    while (true) {
        long current = get();
        long next = current - 1;
        if (compareAndSet(current, next))
            return current;
    }
}

//原子增delta，返回原值
public final long getAndAdd(long delta) {
    while (true) {
        long current = get();
        long next = current + delta;
        if (compareAndSet(current, next))
            return current;
    }
}

 //原子自增，返回新值
public final long incrementAndGet() {
    for (;;) {
        long current = get();
        long next = current + 1;
        if (compareAndSet(current, next))
            return next;
    }
}

 //原子自减，返回新值
public final long decrementAndGet() {
    for (;;) {
        long current = get();
        long next = current - 1;
        if (compareAndSet(current, next))
            return next;
    }
}

  //原子增delta，返回新值
public final long addAndGet(long delta) {
    for (;;) {
        long current = get();
        long next = current + delta;
        if (compareAndSet(current, next))
            return next;
    }
}

//返回String类型
public String toString() {
    return Long.toString(get());//对象转换
}

//返回int类型数值
public int intValue() {
    return (int)get();
}
//返回long类型数值
public long longValue() {
    return get();
}
//返回float类型数值
public float floatValue() {
    return (float)get();
}
//返回double类型数值
public double doubleValue() {
    return (double)get();
}

}

补充说明：
1.while（true），for(;;):乐观锁实现，volatile+CAS实现，更新不成功继续循环更新，效率比每次都加锁(悲观锁)高很多的。