在原有的多线程（Thread与Runable）上线程执行方法run()的返回值为void而在在并发包先使用Callable的Call方法的时候是有一个V范型返回。
Callable接口源码：
@FunctionalInterface
public interface Callable<V> {
V call() throws Exception;
}这个接口是不同与Runable，在Runable中run方法是没有返回值的，而在Callable中要求返回一个传入的范型V，这里我们可以设计多线程的执行单元，同时返回一定的信息。
Future接口源码：
public interface Future<V> {
boolean cancel(boolean mayInterruptIfRunning);
boolean isCancelled();
boolean isDone();
V get() throws InterruptedException, ExecutionException;
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}Futrue为线程提供五个能力，而这些能力为我们对线程的执行状态作出判断。
--cancel 还未执行完，可以取消线程执行
--get 获得执行的返回结果
--isDone 判断是否执行完
--isCancelled 判断计算是否别取消
实现类FutrueTask实现RunableFutrue接口，而RunableFutrue接口继承啦Runable与Futrue接口，提供了五种Task的状态能够让我们时刻获取到线程的状态
private volatile int state; // 注意volatile关键字
/**
* 在构建FutureTask时设置，同时也表示内部成员callable已成功赋值，
* 一直到worker thread完成FutureTask中的run();
*/
private static final int NEW = 0;
/**
* woker thread在处理task时设定的中间状态，处于该状态时，
* 说明worker thread正准备设置result.
*/
private static final int COMPLETING = 1;
/**
* 当设置result结果完成后，FutureTask处于该状态，代表过程结果，
* 该状态为最终状态final state,(正确完成的最终状态)
*/
private static final int NORMAL = 2;
/**
* 同上，只不过task执行过程出现异常，此时结果设值为exception,
* 也是final state
*/
private static final int EXCEPTIONAL = 3;
/**
* final state, 表明task被cancel（task还没有执行就被cancel的状态）.
*/
private static final int CANCELLED = 4;
/**
* 中间状态，task运行过程中被interrupt时，设置的中间状态
*/
private static final int INTERRUPTING = 5;
/**
* final state, 中断完毕的最终状态，几种情况，下面具体分析
*/
private static final int INTERRUPTED = 6;state初始化为NEW。只有在set, setException和cancel方法中state才可以转变为终态。在任务完成期间，state的值可能为COMPLETING或INTERRUPTING。
state有四种可能的状态转换：
* NEW -> COMPLETING -> NORMAL
* NEW -> COMPLETING -> EXCEPTIONAL
* NEW -> CANCELLED
* NEW -> INTERRUPTING -> INTERRUPTED
其他成员变量的含义：
/** The underlying callable; nulled out after running */
private Callable<V> callable; // 具体run运行时会调用其方法call()，并获得结果，结果时置为null.
/** The result to return or exception to throw from get() */
private Object outcome; // non-volatile, protected by state reads/writes 没必要为votaile,因为其是伴随state 进行读写，而state是FutureTask的主导因素。
/** The thread running the callable; CASed during run() */
private volatile Thread runner; //具体的worker thread.
/** Treiber stack of waiting threads */
private volatile WaitNode waiters; //Treiber stack 并发stack数据结构，用于存放阻塞在该futuretask#get方法的线程。Task的状态变化（Task的一个生命周期）
在FutureTask被创建的的时候首先构造函数确保Task的状态为NEW
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW; // ensure visibility of callable
}注意：Runable转换成Callable的方法，其实就是通过Callable的call方法调用Runable的run方法，然后将传入的V返回。
public FutureTask(Runnable runnable, V result) {
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);
}
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;
}
}在创建Task成功后，此时的Task的状态为NEW，然后调用FutureTask的run方法。
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);//将状态设置为EXCEPTION
}
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);
}
}在执行期间如果发生异常就会调用setException方法将状态设置为EXCEPTIONAL。
protected void setException(Throwable t) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = t;
UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
finishCompletion();
}
}顺利完成就会调用set(result)方法将状态设置为NORMAL。
protected void set(V v) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = v;
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
finishCompletion();
}
}最后执行finishCompletion方法，会解除所有阻塞的worker thread， 调用done()方法，将成员变量callable设为null。这里使用了LockSupport类来解除线程阻塞。
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
}FutureTask的get方法，首先会判断是否为完成状态，如果是说明执行过set或setException方法啦，直接返回report，如果是未完成状态则会调用awaitDone去阻塞线程。
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
s = awaitDone(false, 0L);
return report(s);
}如果是NORMAL说明执行没有任何错误，所以通过get方法获得的就是执行返回的结果，如果是取消状态，说明调用过setException，则抛出CancellationException异常，否则抛出ExcecutionException异常。
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);
}
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) {
if (q != null)
q.thread = null;
return s;
}
else if (s == COMPLETING) // cannot time out yet
Thread.yield();
else if (q == null)
q = new WaitNode();
else if (!queued)
queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
q.next = waiters, q);
else if (timed) {
nanos = deadline - System.nanoTime();
if (nanos <= 0L) {
removeWaiter(q);
return state;
}
LockSupport.parkNanos(this, nanos);
}
else
LockSupport.park(this);
}
}awaiteDone简单的可以看做轮询查看FutureTask的状态。在执行awaiteDone期间（get阻塞期间）：
1、如果执行get的线程被中断，则移除FutureTask的所有阻塞队列中的线程（waiters),并抛出中断异常;
2、如果FutureTask的状态转换为完成状态（正常完成或取消），则返回完成状态;
3、如果FutureTask的状态变为COMPLETING，则说明正在set结果，此时让线程等一等;
4、如果FutureTask的状态为初始态NEW，则当前线程加入到FutureTask的阻塞线程中去;
5、如果get方法没有设置超时时间，则阻塞当前调用get线程；如果设置了超时时间，则判断是否达到超时时间，如果达到，则移除FutureTask 的所有阻塞列队中的线程，并返回此时FutureTask的状态，如果未到达时间，则在剩下的时间内继续阻塞当前线程。