FutureTask的源码分析java,多线程pangxiaoliang14的博客-

FutureTask 实现了 RunnableFuture 这个接口，在run（）方法中， 获取任务执行的结果，记录在outcome 字段中。它定义了一个state变量，记录了线程池异步任务执行的状态。通过判断任务状态和比较任务状态，来设置任务运行的结果和异常信息。它把state设置为volatile变量，可以保证它的可见性。

 public class FutureTask<V> implements RunnableFuture<V> 

     /** The underlying callable; nulled out after running */     private Callable<V> callable;     /** The result to return or exception to throw from get() */     private Object outcome; // non-volatile, protected by state reads/writes     /** The thread running the callable; CASed during run() */     private volatile Thread runner;     /** Treiber stack of waiting threads */     private volatile WaitNode waiters;

在构造方法中，初始化Callable对象， 并且把状态设置为NEW.

    /**      * Creates a {@code FutureTask} that will, upon running, execute the      * given {@code Callable}.      *      * @param  callable the callable task      * @throws NullPointerException if the callable is null      */     public FutureTask(Callable<V> callable) {         if (callable == null)             throw new NullPointerException();         this.callable = callable;         this.state = NEW;       // ensure visibility of callable     }

它的任务状态一共有七种。

     private volatile int state;     private static final int NEW          = 0;     private static final int COMPLETING   = 1;     private static final int NORMAL       = 2;     private static final int EXCEPTIONAL  = 3;     private static final int CANCELLED    = 4;     private static final int INTERRUPTING = 5;     private static final int INTERRUPTED  = 6;

那么，它的状态变化又有哪些可能性呢？主要包含四种情况

 * NEW -> COMPLETING -> NORMAL * NEW -> COMPLETING -> EXCEPTIONAL * NEW -> CANCELLED * NEW -> INTERRUPTING -> INTERRUPTED

下面来看下它的几个重要的方法逻辑

1. run()

 public void run() {         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 (ran)                     set(result);             }         } finally {             // runner must be non-null until state is settled to             // prevent concurrent calls to run()             runner = null;             // state must be re-read after nulling runner to prevent             // leaked interrupts             int s = state;             if (s >= INTERRUPTING)                 handlePossibleCancellationInterrupt(s);         }  }

 在这个方法里面， 先去判断当前任务的状态是否为空和是否为new， 如果是空或者不是处于新建的状态，则直接退出run方法。

接下来，通过Callable的call()方法，获取到异步任务的执行结果。如果正常，则设置结果。否则设置异常信息。在finally中，要把正在运行callable的线程设置为空，避免有多个线程同时执行call()方法。然后重新判断状态是否处于中断，如果是，则处理中断异常。

它的 set(result) 的逻辑是怎样的呢？我们来看下。

2. set(V v) 

    /**      * Sets the result of this future to the given value unless      * this future has already been set or has been cancelled.      *      * <p>This method is invoked internally by the {@link #run} method      * upon successful completion of the computation.      *      * @param v the value      */     protected void set(V v) {         if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {             outcome = v;             UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state             finishCompletion();         }     }

其中stateOffset 是定义当前的状态，利用UNSAFE的方法来进行比较并交换。这样做可以保证线程安全，并且不用加锁。

    // Unsafe mechanics     private static final sun.misc.Unsafe UNSAFE;     private static final long stateOffset;     private static final long runnerOffset;     private static final long waitersOffset;     static {         try {             UNSAFE = sun.misc.Unsafe.getUnsafe();             Class<?> k = FutureTask.class;             stateOffset = UNSAFE.objectFieldOffset                 (k.getDeclaredField("state"));             runnerOffset = UNSAFE.objectFieldOffset                 (k.getDeclaredField("runner"));             waitersOffset = UNSAFE.objectFieldOffset                 (k.getDeclaredField("waiters"));         } catch (Exception e) {             throw new Error(e);         }     }

把任务状态设置为COMPLETING， 然后把 结果 赋值给 Object 对象 outcome，然后再把 任务状态设置为NORMAL， 表示该线程异步任务已经正常完成。在finishCompletion方法中，唤醒其他等待执行该任务的线程，最后把callable对象设置为null。

    /**      * Removes and signals all waiting threads, invokes done(), and      * nulls out callable.      */     private void finishCompletion() {         // assert state > COMPLETING;         for (WaitNode q; (q = waiters) != null;) {             if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {                 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     }

3.  get()

     /**      * @throws CancellationException {@inheritDoc}      */     public V get() throws InterruptedException, ExecutionException {         int s = state;         if (s <= COMPLETING)             s = awaitDone(false, 0L);         return report(s);     }

 在这个get() 方法中，会判断状态是否完成，如果任务还没完成， 就等待它完成。然后执行report()方法获取任务结果。那任务结果保存在哪里？ 就是在我们刚才执行run（）方法的时候，已经把结果保存在Object类型的outcome变量中。所以就是要获取outcome变量的值，然后返回。

    /**      * Returns result or throws exception for completed task.      *      * @param s completed state value      */     @SuppressWarnings("unchecked")     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);     }

如果任务状态是正常，就返回任务结果。 如果是取消状态，就抛出取消异常。如果是任务在执行过程中出现了异常呢？在刚才的run()方法中， 我们已经把任务执行的异常捕获保存在outcome变量中了，因此这里把 outcome变量的异常信息转为ExecutionException，然后抛出即可。 我们可以在获取线程池异步任务的结果中，捕获到异常信息，从而进行处理。

4. cancel 方法

 public boolean cancel(boolean mayInterruptIfRunning) {         if (!(state == NEW &&               UNSAFE.compareAndSwapInt(this, stateOffset, NEW,                   mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))             return false;         try {    // in case call to interrupt throws exception             if (mayInterruptIfRunning) {                 try {                     Thread t = runner;                     if (t != null)                         t.interrupt();                 } finally { // final state                     UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);                 }             }         } finally {             finishCompletion();         }         return true; }

如果要取消正在执行的任务，首先要把当前任务状态从NEW设置为INTERRUPTING，如果设置失败，则不能中断该任务。然后调用interrupt（）方法把当前正在执行callable的线程中断， 再把任务状态改为INTERRUPTED,  最后唤醒其他线程，返回true。所以整个过程任务状态变化是：

 NEW -> INTERRUPTING -> INTERRUPTED

5. 最后总结下一些注意点。  

  Future接口调用get()方法取得处理的结果值时是阻塞性的，如果调用Future对象的get()方法时，任务尚未执行完成，则调用get()方法时一直阻塞到此任务完成时为止。

Callable接口的call()方法可以有返回值，而Runnable接口的run()方法没有返回值。

Callable接口的call()方法可以声明抛出异常，而Runnable接口的run()方法不可以声明抛出异常。执行完Callable接口中的任务后，返回值是通过Future接口进行获得的。