/**
  * Attempts to cancel execution of this task. This attempt will
  * fail if the task has already completed or could not be
  * cancelled for some other reason. If successful, and this task
  * has not started when {@code cancel} is called, execution of
  * this task is suppressed. After this method returns
  * successfully, unless there is an intervening call to {@link
  * #reinitialize}, subsequent calls to {@link #isCancelled},
  * {@link #isDone}, and {@code cancel} will return {@code true}
  * and calls to {@link #join} and related methods will result in
  * {@code CancellationException}.
  *
  * <p>This method may be overridden in subclasses, but if so, must
  * still ensure that these properties hold. In particular, the
  * {@code cancel} method itself must not throw exceptions.
  *
  * <p>This method is designed to be invoked by <em>other</em>
  * tasks. To terminate the current task, you can just return or
  * throw an unchecked exception from its computation method, or
  * invoke {@link #completeExceptionally}.
  *
  * @param mayInterruptIfRunning this value has no effect in the
  * default implementation because interrupts are not used to
  * control cancellation.
  *
  * @return {@code true} if this task is now cancelled
  */
 public boolean cancel(boolean mayInterruptIfRunning) {
     return setCompletion(CANCELLED) == CANCELLED;
 }

/**
  * Marks completion and wakes up threads waiting to join this task,
  * also clearing signal request bits.
  *
  * @param completion one of NORMAL, CANCELLED, EXCEPTIONAL
  * @return completion status on exit
  */
 private int setCompletion(int completion) {
     for (int s;;) {
         if ((s = status) < 0)
             return s;
         if (UNSAFE.compareAndSwapInt(this, statusOffset, s, completion)) {
             if (s != 0)
                 synchronized (this) { notifyAll(); }
             return completion;
         }
     }
 }

/**
     * Main pool control -- a long packed with:
     * AC: Number of active running workers minus target parallelism (16 bits)
     * TC: Number of total workers minus target parallelism (16bits)
     * ST: true if pool is terminating (1 bit)
     * EC: the wait count of top waiting thread (15 bits)
     * ID: ~poolIndex of top of Treiber stack of waiting threads (16 bits)
     *
     * When convenient, we can extract the upper 32 bits of counts and
     * the lower 32 bits of queue state, u = (int)(ctl >>> 32) and e =
     * (int)ctl.  The ec field is never accessed alone, but always
     * together with id and st. The offsets of counts by the target
     * parallelism and the positionings of fields makes it possible to
     * perform the most common checks via sign tests of fields: When
     * ac is negative, there are not enough active workers, when tc is
     * negative, there are not enough total workers, when id is
     * negative, there is at least one waiting worker, and when e is
     * negative, the pool is terminating.  To deal with these possibly
     * negative fields, we use casts in and out of "short" and/or
     * signed shifts to maintain signedness.
     */
    volatile long ctl;

    // bit positions/shifts for fields
    private static final int  AC_SHIFT   = 48;
    private static final int  TC_SHIFT   = 32;
    private static final int  ST_SHIFT   = 31;
    private static final int  EC_SHIFT   = 16;

    // bounds
    private static final int  MAX_ID     = 0x7fff;  // max poolIndex
    private static final int  SMASK      = 0xffff;  // mask short bits
    private static final int  SHORT_SIGN = 1 << 15;
    private static final int  INT_SIGN   = 1 << 31;

    // masks
    private static final long STOP_BIT   = 0x0001L << ST_SHIFT;
    private static final long AC_MASK    = ((long)SMASK) << AC_SHIFT;
    private static final long TC_MASK    = ((long)SMASK) << TC_SHIFT;

    // units for incrementing and decrementing
    private static final long TC_UNIT    = 1L << TC_SHIFT;
    private static final long AC_UNIT    = 1L << AC_SHIFT;

    // masks and units for dealing with u = (int)(ctl >>> 32)
    private static final int  UAC_SHIFT  = AC_SHIFT - 32;
    private static final int  UTC_SHIFT  = TC_SHIFT - 32;
    private static final int  UAC_MASK   = SMASK << UAC_SHIFT;
    private static final int  UTC_MASK   = SMASK << UTC_SHIFT;
    private static final int  UAC_UNIT   = 1 << UAC_SHIFT;
    private static final int  UTC_UNIT   = 1 << UTC_SHIFT;

    // masks and units for dealing with e = (int)ctl
    private static final int  E_MASK     = 0x7fffffff; // no STOP_BIT
    private static final int  EC_UNIT    = 1 << EC_SHIFT;

public class SumTask extends RecursiveTask<Long>{  
    private static final int THRESHOLD = 10;  
      
    private long start;  
    private long end;  
      
    public SumTask(long n) {  
        this(1,n);  
    }  
      
    private SumTask(long start, long end) {  
        this.start = start;  
        this.end = end;  
    }  
    @Override  
    protected Long compute() {  
        long sum = 0;  
        if((end - start) <= THRESHOLD){  
            for(long l = start; l <= end; l++){  
                sum += l;  
            }  
        }else{  
            long mid = (start + end) >>> 1;  
            SumTask left = new SumTask(start, mid);  
            SumTask right = new SumTask(mid + 1, end);  
            left.fork();  
            right.fork();  
            sum = left.join() + right.join();  
        }  
        return sum;  
    }  
    private static final long serialVersionUID = 1L;  
}

/**
    * Arranges to asynchronously execute this task.  While it is not
    * necessarily enforced, it is a usage error to fork a task more
    * than once unless it has completed and been reinitialized.
    * Subsequent modifications to the state of this task or any data
    * it operates on are not necessarily consistently observable by
    * any thread other than the one executing it unless preceded by a
    * call to {@link #join} or related methods, or a call to {@link
    * #isDone} returning {@code true}.
    *
    * <p>This method may be invoked only from within {@code
    * ForkJoinPool} computations (as may be determined using method
    * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
    * result in exceptions or errors, possibly including {@code
    * ClassCastException}.
    *
    * @return {@code this}, to simplify usage
    */
   public final ForkJoinTask<V> fork() {
       ((ForkJoinWorkerThread) Thread.currentThread())
           .pushTask(this);
       return this;
   }

/**
  * Pushes a task. Call only from this thread.
  *
  * @param t the task. Caller must ensure non-null.
  */
 final void pushTask(ForkJoinTask<?> t) {
     ForkJoinTask<?>[] q; int s, m;
     if ((q = queue) != null) {    // ignore if queue removed
         // 1
         long u = (((s = queueTop) & (m = q.length - 1)) << ASHIFT) + ABASE;
         // 2
         UNSAFE.putOrderedObject(q, u, t);
         // 3
         queueTop = s + 1;         // or use putOrderedInt
         if ((s -= queueBase) <= 2)
             pool.signalWork();
         else if (s == m)
             // 4
             growQueue();
     }
 }

public class FinalExample {
    int i;                            //普通变量
    final int j;                      //final变量
    static FinalExample obj;

    public void FinalExample () {     //构造函数
        i = 1;                        //写普通域
        j = 2;                        //写final域
    }

    public static void writer () {    //写线程A执行
        obj = new FinalExample ();
    }

    public static void reader () {       //读线程B执行
        FinalExample object = obj;       //读对象引用
        int a = object.i;                //读普通域
        int b = object.j;                //读final域
    }
}

class MonitorExample {
    int a = 0;

    public synchronized void writer() {  //1
        a++;                             //2
    }                                    //3

    public synchronized void reader() {  //4
        int i = a;                       //5
        ……
    }                                    //6
}

class VolatileFeaturesExample {
    volatile long vl = 0L;  //使用volatile声明64位的long型变量

    public void set(long l) {
        vl = l;   //单个volatile变量的写
    }

    public void getAndIncrement () {
        vl++;    //复合（多个）volatile变量的读/写
    }


    public long get() {
        return vl;   //单个volatile变量的读
    }
}

class VolatileFeaturesExample {
    long vl = 0L;               // 64位的long型普通变量

    public synchronized void set(long l) {     //对单个的普通 变量的写用同一个监视器同步
        vl = l;
    }

    public void getAndIncrement () { //普通方法调用
        long temp = get();           //调用已同步的读方法
        temp += 1L;                  //普通写操作
        set(temp);                   //调用已同步的写方法
    }

    public synchronized long get() { 
    //对单个的普通变量的读用同一个监视器同步
        return vl;
    }
}