Serialized Form javadoc for java.util.concurrent.ForkJoinTask says:
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  Serialized Fields
  ...
  int status

      Run control status bits packed into a single int to minimize footprint and
      to ensure atomicity (via CAS). Status is initially zero, and takes on
      nonnegative values until completed, upon which status holds COMPLETED.
      CANCELLED, or EXCEPTIONAL, which use the top 3 bits. Tasks undergoing
      blocking waits by other threads have SIGNAL_MASK bits set -- bit 15 for
      external (nonFJ) waits, and the rest a count of waiting FJ threads. (This
      representation relies on ForkJoinPool max thread limits). Completion of a
      stolen task with SIGNAL_MASK bits set awakens waiter via notifyAll. Even
      though suboptimal for some purposes, we use basic builtin wait/notify to
      take advantage of "monitor inflation" in JVMs that we would otherwise need
      to emulate to avoid adding further per-task bookkeeping overhead. Note
      that bits 16-28 are currently unused. Also value 0x80000000 is available
      as spare completion value.
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There are two issues here:
  - the constants CANCELLED, EXCEPTIONAL and SIGNAL_MASK have package-private
    access so their exact values are not available at javadoc;
  - the ForkJoinTask class does not contain a constant with name COMPLETED.