it seems that they are legacy code. eg.
  enum {
    jump_ring_buffer_size = 16
  }
and this.
  enum {
    is_definitely_current_thread = true
  };

Another trivial thing is that some member functions should declare const. eg.
  jlong allocated_bytes() { return _allocated_bytes; }
  bool is_suspendible_thread() { return _suspendible_thread; }