Java8 原子弹类之LongAdder源码分析add使用场景 LongAdder是否能够替换AtomicLong

/**
  * 处理涉及初始化，调整大小，创建新Cell，和/或争用的更新案例
  *
  * @param x 值
  * @param fn 更新方法
  * @param wasUncontended 调用
  */
 final void longAccumulate(long x, LongBinaryOperator fn, boolean wasUncontended) {
     int h;
     // 获取线程probe的值
     if ((h = getProbe()) == 0) {
         // 值为0则初始化
         ThreadLocalRandom.current(); //强制初始化
         h = getProbe();
         wasUncontended = true;
     }
     boolean collide = false;                // True if last slot nonempt
     for (;;) {
         Cell[] as; Cell a; int n; long v;
         // 这个if分支处理上述四个条件中的前两个相似，此时cells数组已经初始化了并且长度大于0
         if ((as = cells) != null && (n = as.length) > 0) {
             // 线程对应的cell为null
             if ((a = as[(n - 1) & h]) == null) {
                 // 如果busy锁未被占有
                 if (cellsBusy == 0) {       // Try to attach new Cell
                     // 新建一个cell
                     Cell r = new Cell(x);   // Optimistically create
                     // 检测busy是否为0,并且尝试锁busy
                     if (cellsBusy == 0 && casCellsBusy()) {
                         boolean created = false;
                         try {               // Recheck under lock
                             Cell[] rs; int m, j;
                             //再次确认线程probe所对应的cell为null，将新建的cell赋值
                             if ((rs = cells) != null &&
                                 (m = rs.length) > 0 &&
                                 rs[j = (m - 1) & h] == null) {
                                 rs[j] = r;
                                 created = true;
                             }
                         } finally {
                             // 解锁
                             cellsBusy = 0;
                         }
                         if (created)
                             break;
                         //如果失败，再次尝试
                         continue;           // Slot is now non-empty
                     }
                 }
                 collide = false;
             }
             //置为true后交给循环重试
             else if (!wasUncontended)       // CAS already known to fail
                 wasUncontended = true;      // Continue after rehash
             //尝试给线程对应的cell update
             else if (a.cas(v = a.value, ((fn == null) ? v + x :
                                          fn.applyAsLong(v, x))))
                 break;
             else if (n >= NCPU || cells != as)
                 collide = false;            // At max size or stale
             else if (!collide)
                 collide = true;
             //在以上条件都无法解决的情况下尝试扩展cell
             else if (cellsBusy == 0 && casCellsBusy()) {
                 try {
                     if (cells == as) {      // Expand table unless stale
                         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
             }
             h = advanceProbe(h);
         }
         //此时cells还未进行第一次初始化，进行初始化
         else if (cellsBusy == 0 && cells == as && casCellsBusy()) {
             boolean init = false;
             try {                           // Initialize table
                 if (cells == as) {
                     Cell[] rs = new Cell[2];
                     rs[h & 1] = new Cell(x);
                     cells = rs;
                     init = true;
                 }
             } finally {
                 cellsBusy = 0;
             }
             if (init)
                 break;
         }
         //busy锁不成功或者忙，则再重试一次casBase对value直接累加
         else if (casBase(v = base, ((fn == null) ? v + x :
                                     fn.applyAsLong(v, x))))
             break;                          // Fall back on using base
     }
 }
  /**
   * Spinlock (locked via CAS) used when resizing and/or creating Cells.
   * 通过cas实现的自旋锁，用于扩大或者初始化cells
   */
  transient volatile int cellsBusy;