FutureTask 从源码分析为什么会线程阻塞

怼烎@ 2022-05-29 06:07 205阅读 0赞

### 实例demo ###

FutureTask task = new FutureTask(new Callable(){
    
        public Object call(){
            //....
            return new Object();
        }
    });
    task.run();
    //这步会阻塞等待
    task.get();

> 问题：为什么线程会在task.get()阻塞等待?

### FutureTask 的构造函数 ###

> 2个构造函数，分别接受Callable和Runnable ，  
> 注意有个int state，在构造的时候设成NEW

public FutureTask<V> implements RunnableFuture<V> {
         private volatile int state;
    
         public FutureTask(Callable<V> callable) {
            if (callable == null)
                throw new NullPointerException();
            this.callable = callable;
            this.state = NEW;     //在构造的时候设成NEW  
        }
        
          public FutureTask(Runnable runnable, V result) {
            this.callable = Executors.callable(runnable, result);
            this.state = NEW;      //在构造的时候设成NEW
        }
        
    }

FutureTask的继承了Runnable 和 Future

public interface RunnableFuture<V> extends Runnable, Future<V>

public interface Future<V> {
        boolean cancel(boolean mayInterruptIfRunning);
        boolean isCancelled();
        boolean isDone();
        //这里面比较关键就是get方法，也是这个blog的重点
        V get() throws InterruptedException, ExecutionException;
        V get(long timeout, TimeUnit unit)
            throws InterruptedException, ExecutionException, TimeoutException;
    }

### FutureTask里的run() ###

> 由上面继承关系可知道FutureTask 继承Runnable ，那么必然会有run方法的实现。

public void run() {
            //第一个条件：FutureTask  构造方法里可知道state=NEW ，如果不满足就return 
            //第二个条件 : 多线程的情况下，cas去争抢runnerOffset,也就是private volatile Thread runner;
            //runnerOffset是private volatile Thread runner的一个内存地址，将当前线程设入到runner这个变量里，没抢到的就return了
            //这个保证了多线程下FutureTask的线程安全
            if (state != NEW ||
                !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                             null, Thread.currentThread()))
                return;
            try {
            //进入到这里代表已经抢到了
                Callable<V> c = callable;
                if (c != null && state == NEW) {
                    V result;
                    boolean ran;
                    try {
                        //调用上面实例中的call方法
                        result = c.call();
                        ran = true;
                    } catch (Throwable ex) {
                        result = null;
                        ran = false;
                        setException(ex);
                    }
                    //如果完成了设置result,并且会更新state 这个字段的状态
                    if (ran)
                        set(result);
                }
            } finally {
                ...
            }
        }
     protected void set(V v) {
            //sate 从NEW变成COMPLETING
            if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
               //设置完成结果
                outcome = v;
                //sate 从COMPLETING变成NORMAL 
                UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
                finishCompletion();
            }
       }

run()方法里都写了注释解释，最后还有个 finishCompletion()，留待后面分析。接下来分析get()方法。

### FutureTask里的get() ###

> 为什么调用get()会等待就在awaitDone的分析里。

public class FutureTask<V> implements RunnableFuture<V> {
       private volatile int state;
        public V get() throws InterruptedException, ExecutionException {
            int s = state;
            if (s <= COMPLETING)
                //!!!关键的awaitDone，会阻塞等待
                s = awaitDone(false, 0L);
            return report(s);
        }
      //从名字看就是等待完成,这是为什么线程会阻塞的方法
      //这是个for(;;) 死循环，这是为什么会阻塞的原因之一不是全部
      //线程是不会一直在这里不断的执行，会封装成一个waitnode成为一个等待链表的节点，最后还有个LockSupport.park。等待LockSupport.unPark
         private int awaitDone(boolean timed, long nanos)
            throws InterruptedException {
            //如果timed为true,就是等待多长时间还没完成的话就直接返回了
            final long deadline = timed ? System.nanoTime() + nanos : 0L;
            WaitNode q = null;
            boolean queued = false;       
            for (;;) {
                if (Thread.interrupted()) {
                    removeWaiter(q);
                    throw new InterruptedException();
                }           
                int s = state;
                //NEW=0 , COMPLETING   = 1,NORMAL       = 2，EXCEPTIONAL  = 3
                //CANCELLED    = 4,INTERRUPTING = 5,INTERRUPTED  = 6
                //s > COMPLETING表示完成或其它异常状态可以 return ,这是for循环的跳出点
                if (s > COMPLETING) {
                    if (q != null)
                        q.thread = null;
                    return s;
                }
                else if (s == COMPLETING) //完成中，线程也没必要那么积极了
                    Thread.yield();
                else if (q == null)
                //将线程封装成一个WaitNode，WaitNode是个单链表的节点,多个WaitNode组成了一个等待的链
                    q = new WaitNode();
                else if (!queued)
                //调用cas将WaitNode替换到waitersOffset（waiters的内存地址）里，成功替换后 ，WaitNode会在等待链头，并返回queued 为true,表示已经排队成功。
                    queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                         q.next = waiters, q);
                else if (timed) {
                 // 如果设置timed=true和最长等待时间，超过这个等待时间直接就返回
                    nanos = deadline - System.nanoTime();
                    if (nanos <= 0L) {
                        removeWaiter(q);
                        return state;
                    }
                    LockSupport.parkNanos(this, nanos);
                }
                else
                   //阻塞，LockSupport.unpark(Thread)会唤醒
                   LockSupport.park(this);
            }
        }
     }
        static final class WaitNode {
            volatile Thread thread;
            volatile WaitNode next;
            WaitNode() { thread = Thread.currentThread(); }
        }

上面的get()方法会使得线程等待，如果超时就抛出超时异常。什么候线程会唤醒呢？run()里最后还留了个`finishCompletion()`

private void finishCompletion() {
          //waiters就是前面的set里利用cas设入的waitNode
            for (WaitNode q; (q = waiters) != null;) {
                //先将waiters设成null
                if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
                //遍历单链表，依次去unpark,也就是唤醒线程，直到尾部
                    for (;;) {
                        Thread t = q.thread;
                        if (t != null) {
                            q.thread = null;
                            LockSupport.unpark(t);
                        }
                        WaitNode next = q.next;
                        if (next == null)
                            break;
                        q.next = null; // unlink to help gc
                        q = next;
                    }
                    break;
                }
            }
            done();
    
            callable = null;        // to reduce footprint
        }