public class AtomicLong {
public final long incrementAndGet() {
    return unsafe.getAndAddLong(this, valueOffset, 1L) + 1L;
  }
}

public final class Unsafe {
public final long getAndAddLong(Object var1, long var2, long var4) {
    long var6;
    do {
      var6 = this.getLongVolatile(var1, var2);
    } while(!this.compareAndSwapLong(var1, var2, var6, var6 + var4));
    return var6;
  }
}

function FetchAndAdd(address location, int inc) {
  int value := *location
  *location := value + inc
  return value
}

public class LongAdder extends Striped64 implements Serializable {
//...
}

abstract class Striped64 extends Number {
/** Number of CPUS, to place bound on table size */
  static final int NCPU = Runtime.getRuntime().availableProcessors();

  /**
   * Table of cells. When non-null, size is a power of 2.
   */
  transient volatile Cell[] cells;
   
@sun.misc.Contended static final class Cell {
    volatile long value;
    Cell(long x) { value = x; }
    final boolean cas(long cmp, long val) {
      return UNSAFE.compareAndSwapLong(this, valueOffset, cmp, val);
    }

    // Unsafe mechanics
    private static final sun.misc.Unsafe UNSAFE;
    private static final long valueOffset;
    static {
      try {
        UNSAFE = sun.misc.Unsafe.getUnsafe();
        Class<?> ak = Cell.class;
        valueOffset = UNSAFE.objectFieldOffset
          (ak.getDeclaredField("value"));
      } catch (Exception e) {
        throw new Error(e);
      }
    }
  }

}

static final class Cell {
    volatile long p0, p1, p2, p3, p4, p5, p6;
    volatile long value;
    volatile long q0, q1, q2, q3, q4, q5, q6;
    Cell(long x) { value = x; }

    final boolean cas(long cmp, long val) {
      return UNSAFE.compareAndSwapLong(this, valueOffset, cmp, val);
    }

    // Unsafe mechanics
    private static final sun.misc.Unsafe UNSAFE;
    private static final long valueOffset;
    static {
      try {
        UNSAFE = getUnsafe();
        Class<?> ak = Cell.class;
        valueOffset = UNSAFE.objectFieldOffset
          (ak.getDeclaredField("value"));
      } catch (Exception e) {
        throw new Error(e);
      }
    }
 }

/**
 * 底競(jìng)爭(zhēng)下直接更新base，類(lèi)似AtomicLong
 * 高并發(fā)下，會(huì)將每個(gè)線程的操作hash到不同的
 * cells數(shù)組中，從而將AtomicLong中更新
 * 一個(gè)value的行為優(yōu)化之后，分散到多個(gè)value中
 * 從而降低更新熱點(diǎn)，而需要得到當(dāng)前值的時(shí)候，直接
 * 將所有cell中的value與base相加即可，但是跟
 * AtomicLong(compare and change -> xadd)的CAS不同，
 * incrementAndGet操作及其變種
 * 可以返回更新后的值，而LongAdder返回的是void
 */
public class LongAdder {
  public void add(long x) {
    Cell[] as; long b, v; int m; Cell a;
    /**
     * 如果是第一次執(zhí)行，則直接case操作base
     */
    if ((as = cells) != null || !casBase(b = base, b + x)) {
      boolean uncontended = true;
      /**
       * as數(shù)組為空(null或者size為0)
       * 或者當(dāng)前線程取模as數(shù)組大小為空
       * 或者cas更新Cell失敗
       */
      if (as == null || (m = as.length - 1) < 0 ||
        (a = as[getProbe() & m]) == null ||
        !(uncontended = a.cas(v = a.value, v + x)))
        longAccumulate(x, null, uncontended);
    }
  }

  public long sum() {
    //通過(guò)累加base與cells數(shù)組中的value從而獲得sum
    Cell[] as = cells; Cell a;
    long sum = base;
    if (as != null) {
      for (int i = 0; i < as.length; ++i) {
        if ((a = as[i]) != null)
          sum += a.value;
      }
    }
    return sum;
  }
}

/**
 * openjdk.java.net/jeps/142
 */
@sun.misc.Contended static final class Cell {
  volatile long value;
  Cell(long x) { value = x; }
  final boolean cas(long cmp, long val) {
    return UNSAFE.compareAndSwapLong(this, valueOffset, cmp, val);
  }

  // Unsafe mechanics
  private static final sun.misc.Unsafe UNSAFE;
  private static final long valueOffset;
  static {
    try {
      UNSAFE = sun.misc.Unsafe.getUnsafe();
      Class<?> ak = Cell.class;
      valueOffset = UNSAFE.objectFieldOffset
        (ak.getDeclaredField("value"));
    } catch (Exception e) {
      throw new Error(e);
    }
  }
}

abstract class Striped64 extends Number {

  final void longAccumulate(long x, LongBinaryOperator fn,
               boolean wasUncontended) {
    int h;
    if ((h = getProbe()) == 0) {
      /**
       * 若getProbe為0，說(shuō)明需要初始化
       */
      ThreadLocalRandom.current(); // force initialization
      h = getProbe();
      wasUncontended = true;
    }
    boolean collide = false;        // True if last slot nonempty
    /**
     * 失敗重試
     */
    for (;;) {
      Cell[] as; Cell a; int n; long v;
      if ((as = cells) != null && (n = as.length) > 0) {
        /**
         * 若as數(shù)組已經(jīng)初始化,(n-1) & h 即為取模操作，相對(duì) % 效率要更高
         */
        if ((a = as[(n - 1) & h]) == null) {
          if (cellsBusy == 0) {    // Try to attach new Cell
            Cell r = new Cell(x);  // Optimistically create
            if (cellsBusy == 0 && casCellsBusy()) {//這里casCellsBusy的作用其實(shí)就是一個(gè)spin lock
              //可能會(huì)有多個(gè)線程執(zhí)行了`Cell r = new Cell(x);`,
              //因此這里進(jìn)行cas操作，避免線程安全的問(wèn)題，同時(shí)前面在判斷一次
              //避免正在初始化的時(shí)其他線程再進(jìn)行額外的cas操作
              boolean created = false;
              try {        // Recheck under lock
                Cell[] rs; int m, j;
                //重新檢查一下是否已經(jīng)創(chuàng)建成功了
                if ((rs = cells) != null &&
                  (m = rs.length) > 0 &&
                  rs[j = (m - 1) & h] == null) {
                  rs[j] = r;
                  created = true;
                }
              } finally {
                cellsBusy = 0;
              }
              if (created)
                break;
              continue;      // Slot 現(xiàn)在是非空了，continue到下次循環(huán)重試
            }
          }
          collide = false;
        }
        else if (!wasUncontended)    // CAS already known to fail
          wasUncontended = true;   // Continue after rehash
        else if (a.cas(v = a.value, ((fn == null) ? v + x :
                       fn.applyAsLong(v, x))))
          break;//若cas更新成功則跳出循環(huán)，否則繼續(xù)重試
        else if (n >= NCPU || cells != as) // 最大只能擴(kuò)容到CPU數(shù)目， 或者是已經(jīng)擴(kuò)容成功，這里只有的本地引用as已經(jīng)過(guò)期了
          collide = false;      // At max size or stale
        else if (!collide)
          collide = true;
        else if (cellsBusy == 0 && casCellsBusy()) {
          try {
            if (cells == as) {   // 擴(kuò)容
              Cell[] rs = new Cell[n << 1];
              for (int i = 0; i < n; ++i)
                rs[i] = as[i];
              cells = rs;
            }
          } finally {
            cellsBusy = 0;
          }
          collide = false;
          continue;          // Retry with expanded table
        }
        //重新計(jì)算hash(異或)從而嘗試找到下一個(gè)空的slot
        h = advanceProbe(h);
      }
      else if (cellsBusy == 0 && cells == as && casCellsBusy()) {
        boolean init = false;
        try {              // Initialize table
          if (cells == as) {
            /**
             * 默認(rèn)size為2
             */
            Cell[] rs = new Cell[2];
            rs[h & 1] = new Cell(x);
            cells = rs;
            init = true;
          }
        } finally {
          cellsBusy = 0;
        }
        if (init)
          break;
      }
      else if (casBase(v = base, ((fn == null) ? v + x : // 若已經(jīng)有另一個(gè)線程在初始化，那么嘗試直接更新base
                    fn.applyAsLong(v, x))))
        break;             // Fall back on using base
    }
  }

  final boolean casCellsBusy() {
    return UNSAFE.compareAndSwapInt(this, CELLSBUSY, 0, 1);
  }

  static final int getProbe() {
    /**
     * 通過(guò)Unsafe獲取Thread中threadLocalRandomProbe的值
     */
    return UNSAFE.getInt(Thread.currentThread(), PROBE);
  }

    // Unsafe mechanics
    private static final sun.misc.Unsafe UNSAFE;
    private static final long BASE;
    private static final long CELLSBUSY;
    private static final long PROBE;
    static {
      try {
        UNSAFE = sun.misc.Unsafe.getUnsafe();
        Class<?> sk = Striped64.class;
        BASE = UNSAFE.objectFieldOffset
          (sk.getDeclaredField("base"));
        CELLSBUSY = UNSAFE.objectFieldOffset
          (sk.getDeclaredField("cellsBusy"));
        Class<?> tk = Thread.class;
        //返回Field在內(nèi)存中相對(duì)于對(duì)象內(nèi)存地址的偏移量
        PROBE = UNSAFE.objectFieldOffset
          (tk.getDeclaredField("threadLocalRandomProbe"));
      } catch (Exception e) {
        throw new Error(e);
      }
    }
}