Read FutureTask source code

// a result that can be returned asynchronously
//It implements both Future and Runnable
// first look at the constructor
 public FutureTask(Callable<V> callable) {
        if (callable == null)
            throw new NullPointerException();
        this.callable = callable;
        this.state = NEW;       // ensure visibility of callable
    }

//Run the runnable and return the given result
public FutureTask(Runnable runnable, V result) {
        //The adapter mode converts the runnable interface
        this.callable = Executors.callable(runnable, result);
        this.state = NEW;       // ensure visibility of callable
    }

public static <T> Callable<T> callable(Runnable task, T result) {
        if (task == null)
            throw new NullPointerException();
        return new RunnableAdapter<T>(task, result);
    }

//The adapter mode converts the runnable interface
 static final class RunnableAdapter<T> implements Callable<T> {
        final Runnable task;
        final T result;
        RunnableAdapter(Runnable task, T result) {
            this.task = task;
            this.result = result;
        }
        public T call() {
            task.run();
            return result;
        }
    }



 public void run() {
        //If state is not equal to 0 or the setting is currently occupied by other threads, return directly.
        if (state != NEW ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return;
        try {
            Callable<V> c = callable;
            if (c != null && state == NEW) {
                V result;
                boolean ran;
                try {
                    result = c.call();
                    ran = true;
                } catch (Throwable ex) {
                    result = null;
                    ran = false;
                    setException(ex);
                }
		//If the set value has been executed
                if (ran)
                    set(result);
            }
        } finally {
           
            runner = null;
            int s = state;
	    //if cancelled
            if (s >= INTERRUPTING)
	    // Give up execution rights
                handlePossibleCancellationInterrupt(s);
        }
    }


//set the specified value
protected void set(V v) {
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            outcome = v;
            UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
            finishCompletion();
        }
    }


private void finishCompletion() {
        // release all waiting threads
        for (WaitNode q; (q = waiters) != null;) {
	    // Clear the current thread successfully
            if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
                for (;;) {
                    Thread t = q.thread;
                    if (t != null) {
                        q.thread = null;
			// release the thread
                        LockSupport.unpark(t);
                    }
                    WaitNode next = q.next;
                    if (next == null)
                        break;
                    q.next = null; // unlink to help gc
                    q = next;
                }
                break;
            }
        }
        // hook method
        done();

        callable = null;        // to reduce footprint
    }


private void handlePossibleCancellationInterrupt(int s) {
       
        if (s == INTERRUPTING)
            while (state == INTERRUPTING)
                Thread.yield(); // wait out pending interrupt

    }

protected void done() { }

// get the result
public V get() throws InterruptedException, ExecutionException {
        int s = state;
	//if still done
        if (s <= COMPLETING)
	    // block on this
            s = awaitDone(false, 0L);
        return report(s);
    }

// join the queue to block the current thread
 private int awaitDone(boolean timed, long nanos)
        throws InterruptedException {
        final long deadline = timed? System.nanoTime () + nanos: 0L;
        WaitNode q = null;
        boolean queued = false;
        for (;;) {
	    //if the current thread has been interrupted
            if (Thread.interrupted()) {
	        // clear him from the waiting queue
                removeWaiter(q);
                throw new InterruptedException();
            }

            int s = state;
	    // already executed
            if (s > COMPLETING) {
                if (q != null)
                    q.thread = null;
                return s;
            }
            else if (s == COMPLETING) // cannot time out yet
	        // Give up execution rights
                Thread.yield();
            else if (q == null)
                q = new WaitNode();
            else if (!queued)
	        // join the queue
                queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                     q.next = waiters, q);
            else if (timed) {
                nanos = deadline - System.nanoTime ();
		//timeout delete q
                if (nanos <= 0L) {
                    removeWaiter(q);
                    return state;
                }
		// Suspend the current thread
                LockSupport.parkNanos(this, nanos);
            }
            else
                LockSupport.park(this);
        }
    }

    private void removeWaiter(WaitNode node) {
        if (node != null) {
            node.thread = null;
            retry:
            for (;;) {          // restart on removeWaiter race
                for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
                    s = q.next;
                    if (q.thread != null)
                        pred = q;
	            //It is explained here that q.thread==null, q is the node that needs to be deleted.
                    else if (pred != null) {
		        //Modify the next node of the previous node
                        pred.next = s;
			//The previous node is deleted, and the cycle is repeated.
                        if (pred.thread == null)
                            continue retry;
                    }
		    //Go here to indicate that the first node is the deleted node
                    else if (!UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                          q, s))
			// set failure to recycle
                        continue retry;
                }
                break;
            }
        }
    }

//get value
private V report(int s) throws ExecutionException {
        Object x = outcome;
        if (s == NORMAL)
            return (V)x;
        if (s >= CANCELLED)
            throw new CancellationException();
        throw new ExecutionException((Throwable)x);
    }

//Waiting for a result for a certain period of time will timeout and throw an exception
public V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException {
        if (unit == null)
            throw new NullPointerException();
        int s = state;
        if (s <= COMPLETING &&
            (s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
            throw new TimeoutException();
        return report(s);
    }


//whether the task is canceled
public boolean isCancelled() {
        return state >= CANCELLED;
    }

//Is the task completed
 public boolean isDone() {
        return state != NEW;
    }


// Attempt to cancel the execution of the task
public boolean cancel(boolean mayInterruptIfRunning) {
        //The task has started and returns false directly
        if (state != NEW)
            return false;
	 // try to interrupt
        if (mayInterruptIfRunning) {
            if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, INTERRUPTING))
                return false;
            Thread t = runner;
            if (t != null)
                t.interrupt();
            UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED); // final state
        }
	// try to cancel
        else if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, CANCELLED))
            return false;
        finishCompletion();
        return true;
    }

// set result as exception
 protected void setException(Throwable t) {
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            outcome = t;
            UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
            finishCompletion();
        }
    }


//Execute the calculation without setting the result and reset it after execution
protected boolean runAndReset() {
        if (state != NEW ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return false;
        boolean ran = false;
        int s = state;
        try {
            Callable<V> c = callable;
            if (c != null && s == NEW) {
                try {
                    c.call(); // don't set result
                    ran = true;
                } catch (Throwable ex) {
                    setException(ex);
                }
            }
        } finally {
            runner = null;
            s = state;
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
        return ran && s == NEW;
    }