FutureTask的使用与原理
FutureTask是java中一个用来实现可取消的同步计算的类。可取消是因为这个类可以调用cancel方法取消计算(其实也是有条件的取消)，同步是因为调用get方法获取计算结果的时候需要等待计算完成。
1.简单使用
Callable<Integer> callable = new Callable<Integer>()
{
@Override
public Integer call() throws Exception
{
int sum = 0;
int value = 10;
while (value-- >= 0)
{
sum += value;
try
{
Thread.sleep(500);
}
catch (InterruptedException e)
{
e.printStackTrace();
}
}
System.out.println("done");
return sum;
}
};
FutureTask<Integer> task = new FutureTask<Integer>(callable);
new Thread(task).start();
try
{
System.out.println(task.get());
}
catch (InterruptedException e)
{
e.printStackTrace();
}
catch (ExecutionException e)
{
e.printStackTrace();
}
FutureTask的使用比较简单，通过构造方法传入Callable对象或者是Runnable对象和返回值，新开一个线程执行这个Callable对象，最后通过get方法获取计算结果。
2.原理
从FutureTask的继承关系上来看，FutureTask实现了RunnableFuture接口，RunnbleFuture接口又继承了Runnable和Future接口，所以FutureTask可以看做是一个Runnable和Future的组合体。
public FutureTask(Callable<V> callable)
{
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW; // ensure visibility of callable
}
从构造方法上看，仅仅是设置了两个成员变量。当放到线程中去执行的时候，看看run方法。
public void run()
{
//1.CAS设置runner为当前的线程
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
{
//2.执行call的内容
result = c.call();
ran = true;
}
catch (Throwable ex)
{
result = null;
ran = false;
setException(ex);
}
//3.设置执行结果
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);
}
}
代码比较简单，首先通过CAS的方式设置runner变量为当前的线程，然后执行callable中的call方法，执行完成之后调用set方法，并且在fianlly中重置runner为null。再看下set方法。
protected void set(V v)
{
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING))
{
outcome = v;
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
finishCompletion();
}
}
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
}
set方法中，首先通过CAS的方式设置state为Completing，然后将结果设置给outcome，在此通过CAS将state设置为NORMAL.finishCompletion方法是将通过调用get方法而导致阻塞的线程唤醒。
调用get方法的时候会同步等待，看看get方法。
public V get() throws InterruptedException, ExecutionException
{
int s = state;
if (s <= COMPLETING)
s = awaitDone(false, 0L);
return report(s);
}
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);
}
}
调用get的时机，如果是在COMPLETING状态之后，直接调用report方法返回。如果是在任务完成之前，则会调用awaitDone方法。awaitDone方法采用一个自旋的方式来确定state的状态，调用步奏如下：
- 新建一个
WaitNode对象 - 通过CAS的方式将waitNode放到等待链表中
- 挂起当前的线程
当当前的任务执行完成之后，也就是调用set方法之后，state状态被重置，并且通过finishCompletion方法唤醒通过get方法而阻塞的线程，此时get方法继续执行，执行report方法。
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);
}
report方法直接将set方法中设置的outcome返回即可。
再来看下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;
}
调用cancel的时机决定了它的行为，如果调用的时候，任务还没有结束，首先通过CAS的方式设置state的状态为INTERRUPTING或者是CANCELLED，然后通过finishCompletion方法唤醒所有挂起的线程，那么在上面介绍的awaitDone方法将会跳出循环，进入report方法，抛出异常。如果任务已经结束，那么cancel方法直接返回false，表示cancel失败。所有cancel方法只是取消所有get方法的挂起，立刻返回当前的值或者抛出异常，而不是关闭当前的正在运行的线程。
