FutureTask的是实现类图如下：
Screenshot - 2016年12月01日 - 14时08分12秒.png
它的实现也比较简单，主要是用了UNSAFE类来实现大部分的功能。
它的属性主要有state(表示状态)，callable(FutureTask指向的要运行的callable对象)，outcome(Object对象,运行结果，可能是异常)， runner(Thread对象 线程)，waiters（WaitNode对象，它是链表元素，表示等待结果的线程）
它的许多操作都是以状态驱动的，通过CAS改变状态成功后进行后续操作：
下面两个方法在类中其他方法中会用到如run()方法中运行成功后设置结果值
/**
* 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();
}
}
/**
* Causes this future to report an {@link ExecutionException}
* with the given throwable as its cause, unless this future has
* already been set or has been cancelled.
*
* <p>This method is invoked internally by the {@link #run} method
* upon failure of the computation.
*
* @param t the cause of failure
*/
protected void setException(Throwable t) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = t;
UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
finishCompletion();
}
}
FutureTask的大部分方法比较简单，重点是一下几个：
public boolean cancel(boolean mayInterruptIfRunning) {//取消方法
if (!(state == NEW &&
UNSAFE.compareAndSwapInt(this, stateOffset, NEW,
mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))//向置状体为取消
return false;
try { // in case call to interrupt throws exception//置状态成功后
if (mayInterruptIfRunning) {//是否使用中断线程
try {
Thread t = runner;
if (t != null)
t.interrupt();
} finally { // final state
UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);
}
}
} finally {
finishCompletion();//取消线程运行后的后续操作
}
return true;
}
private void finishCompletion() {
// assert state > COMPLETING;
for (WaitNode q; (q = waiters) != null;) {//唤醒等待结果的线程
if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {//原子替换waiters变量为空
for (;;) {
Thread t = q.thread;
if (t != null) {
q.thread = null;
LockSupport.unpark(t);//唤醒线程
}
WaitNode next = q.next;
if (next == null)
break;
q.next = null; // unlink to help gc
q = next;
}
break;
}
}
done();//这个方法为空，子类可以实现它增加额外的的操作
callable = null; // to reduce footprint
}
第二个获取结果方法
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)//判断状态，然后等待，也会判断等待后返回的结果状态
throw new TimeoutException();
return report(s);//使用report方法发会结果
}
这个方法就是无限循环，每次循环只做满足条件的一件事
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 (Thread.interrupted()) {
removeWaiter(q);
throw new InterruptedException();
}
int s = state;
if (s > COMPLETING) {//状态已经表示结束了
if (q != null)
q.thread = null;//把q的线程之空，它是当前线程，表示当前线程已经不等待了，不必在其他的方法中唤醒了，q也没从队列移除，主要减少移除队列的次数，提高性能
return s;
}
else if (s == COMPLETING) // cannot time out yet
Thread.yield();
else if (q == null)
q = new WaitNode();//生成等待者，详见相应类，很简单
else if (!queued)
queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
q.next = waiters, q);//加入等待队列
else if (timed) {//等待一段时间
nanos = deadline - System.nanoTime();
if (nanos <= 0L) {
removeWaiter(q);
return state;
}
LockSupport.parkNanos(this, nanos);
}
else
LockSupport.park(this);//一直等待
}
}
private void removeWaiter(WaitNode node) {//移除等待者，
if (node != null) {
node.thread = null;//首先将移除者的线程置空，后面删除线程为空的等待者
retry:
for (;;) { // restart on removeWaiter race
for (WaitNode pred = null, q = waiters, s; q != null; q = s) {//一直到末尾才结束
s = q.next;
if (q.thread != null)//不是需要移除的节点，则增加个前节点
pred = q;
else if (pred != null) {//q现在是移除节点，但是前节点不为空，直接改链表结构
pred.next = s;
if (pred.thread == null) // check for race//前置节点的线程为空，则从头再来一次
continue retry;
}
else if (!UNSAFE.compareAndSwapObject(this, waitersOffset,
q, s))//并发修改了首节点，失败则从头再来
continue retry;
}
break;
}
}
}
//以当前线程来运行
public void run() {
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread()))//置runner为当前线程
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);
}
}
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