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。