FutureTask源码阅读与理解

//以下是state的几种状态
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;

//具体任务
private Callable<V> callable;

//任务执行完毕结果存放属性
private Object outcome;

//记录执行任务的线程
private volatile Thread runner;

//链表结构，表示等待结果的线程
private volatile WaitNode waiters;

//run方法就是执行任务的方法
public void run() {
//判断state并试着去更新线程变量
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread()))
return;
try {
Callable<V> c = callable;
if (c != null && state == NEW) {
//返回数据
V result;
//是否执行成功标识
boolean ran;
try {
//执行方法
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
//如果执行成功则执行set方法
if (ran)
set(result);
}
} finally {
runner = null;
int s = state;
//判断state如果被中断则让出资源并等待被杀死
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}

protected void set(V v) {
//更新state状态为完成
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
//结果赋值给outcome
outcome = v;
//最终状态
UNSAFE.putOrderedInt(this, stateOffset, NORMAL);
//唤醒一些等待结果的线程
finishCompletion();
}
}

private void finishCompletion() {
for (WaitNode q; (q = waiters) != null;) {
//将等待者置空
if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
//遍历所有等待者
for (;;) {
Thread t = q.thread;
if (t != null) {
q.thread = null;
//唤醒线程
LockSupport.unpark(t);
}
WaitNode next = q.next;
if (next == null)
break;
q.next = null; // unlink to help gc
q = next;
}
break;
}
}

//完成任务并唤醒所有等待结果线程再执行done方法
done();

callable = null;        // to reduce footprint
}

//获取数据的方法，可限时：
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
s = awaitDone(false, 0L);
return report(s);
}

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)
throw new TimeoutException();
return report(s);
}
//具体的取值-等待方法
private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
//可限时挂起
final long deadline = timed ? System.nanoTime() + nanos : 0L;
WaitNode q = null;
boolean queued = false;
for (;;) {
//如果线程被中断，则删除等待
if (Thread.interrupted()) {
removeWaiter(q);
throw new InterruptedException();
}

//如果大于已完成则返回
int s = state;
if (s > COMPLETING) {
if (q != null)
q.thread = null;
return s;
}
//说明执行任务的线程正在给outcome赋值，让掉cpu
else if (s == COMPLETING) // cannot time out yet
Thread.yield();
//如果q==null那就创建一个等待节点
else if (q == null)
q = new WaitNode();
//试着加入队列
else if (!queued)
queued = UNSAFE.compareAndSwapObject(this, waitersOffset,q.next = waiters, q);
//是否有设置超时时间
else if (timed) {
nanos = deadline - System.nanoTime();
if (nanos <= 0L) {
removeWaiter(q);
return state;
}
LockSupport.parkNanos(this, nanos);
}
//挂起线程
else
LockSupport.park(this);
}
}