在前一篇ThreadPoolExecutor学习时，在AbstractExecutorService代码重看到submit一个任务时，利用FutureTask的来封装提交的任务。这也应该是FutureTask最正常的使用吧。
如果对Future不了解的，有本书《java多线程设计模式》，真心是本好书，只是翻译的比较呵呵，里面讲解了很多多线程模式，主从、pc、Future等。
结构
/**
* 存在的状态变换
* NEW -> COMPLETING -> NORMAL
* NEW -> COMPLETING -> EXCEPTIONAL
* NEW -> CANCELLED
* NEW -> INTERRUPTING -> INTERRUPTED
*/
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;
/** 构造赋值，要执行的任务 */
private Callable<V> callable;
/** get方法返回值，返回执行结果或者异常 */
private Object outcome; // non-volatile, protected by state reads/writes
/** 运行callable的线程 */
private volatile Thread runner;
/** outcome的等待栈（单向链表结构） */
private volatile WaitNode waiters;
/** 单向链表结构 */
static final class WaitNode {
volatile Thread thread;
volatile WaitNode next;
WaitNode() { thread = Thread.currentThread(); }
}
/** 2种构造，state写在后，后面使用的时候先读state，再读callable，volatile语义保证可见性，infoQ和并发网上各有一篇文章讲volatile语义的，可以看下 */
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW; // ensure visibility of callable
}
public FutureTask(Runnable runnable, V result) {
this.callable = Executors.callable(runnable, result);
this.state = NEW; // ensure visibility of callable
}
内部结构，没有太多的东西，一个volatile语义的state标识状态，一个waitNode链表维护所有等待节点，最重要的是volatile的语义。
在state之前还有段英文解释为什么放弃之前版本使用的AQS:
/*
* Revision notes: This differs from previous versions of this
* class that relied on AbstractQueuedSynchronizer, mainly to
* avoid surprising users about retaining interrupt status during
* cancellation races. Sync control in the current design relies
* on a "state" field updated via CAS to track completion, along
* with a simple Treiber stack to hold waiting threads.
*
* Style note: As usual, we bypass overhead of using
* AtomicXFieldUpdaters and instead directly use Unsafe intrinsics.
*/
之前版本使用AQS会出现cancel(true)问题，我看的代码是1.7的，后来找了之前版本的代码，最主要的原因是因为innerCancel()方法中的一段代码：
innerCancel:
----
Thread thread = runner;
if(thread != null)
thread.interrupt();
---
运行如下代码：
ExecutorService executor = Executors.newFixedThreadPool(1);
executor.submit(new Task()).cancel(true);
executor.submit(new Task());
假如第一个submit运行，然后cancel的时候运气不好，获取到runner后发生线程切换，executor执行第二个submit，然后又切换回去了，cancel代码继续运行，就会到thread.interrupt();这一句，这样再切换第二个task的任务的时候，就会发生中断。具体可以看参考里面的那个bug解释。
核心方法
先看下FutureTask的运行的基本流程：
1、 submit一个task，获的返回的Future：提交task后，就是ThreadPoolExecutor的那一套流程addworker-runwork-gettask-task.run，最后会到提交的task的run()方法；
2、 通过Future的get()获取执行的结果；
3、 也可以cancel()方法取消未执行的task；
run
public void run() {
/** 没有运行就设置runner */
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); //异常结束
}
if (ran)
set(result); //正常结束
}
} finally {
//在上面set或setException中会设置state状态，这里再设置runner为null，
//2个顺序颠倒，会出现state还未转换到下一个状态，结果runner为空，如果这时有调用run方法，就会有并发问题
runner = null;
// state must be re-read after nulling runner to prevent
// leaked interrupts
int s = state;//这里处理可能有cancel产生中断
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}
/** run正常结束时不推进状态，，不设置返回结果，这样就可以多次运行 */
protected boolean runAndReset() {
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread()))
return false;
boolean ran = false;
int s = state;
try {
Callable<V> c = callable;
if (c != null && s == NEW) {
try {
c.call(); // don't set result
ran = true;
} catch (Throwable ex) {
setException(ex);
}
}
} 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
s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
return ran && s == NEW;
}
run()方法涉及的状态变换。
1、 正常结束:推进状态NEW->COMPLETING->NORMAL，然后unpark所有等待线程：；
/** 状态NEW-COMPLETING-NORMAL */
protected void set(V v) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = v;
UNSAFE.putOrderedInt(this, stateOffset, NORMAL);
finishCompletion(); //unpark所有等待节点
}
}
/** unpark所有等待节点 */
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); //unpark所有等待
}
WaitNode next = q.next;
if (next == null)
break;
q.next = null; // unlink to help gc
q = next;
}
break;
}
}
done(); //空，子类实现
callable = null; // to reduce footprint
}
1、 task.run有异常产生，推进状态NEW->COMPLETING->EXCEPTIONAL，唤醒等待线程：；
protected void setException(Throwable t) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = t; //这里将异常返回，get时获取到异常
UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
finishCompletion();
}
}
1、 处理可能由cancel(true)产生的中断，保证这个中断一定会在run或runAndReset方法中会被处理：；
/** 确保由cancel(true)产生的中断一定会在run或runandreset中处理 */
private void handlePossibleCancellationInterrupt(int s) {
// 如果状态为INTERRUPTING，那肯定是有cancel(true)在处理了，那就yield
if (s == INTERRUPTING)
while (state == INTERRUPTING)
Thread.yield(); // wait out pending interrupt
// assert state == INTERRUPTED;
// 这里没办法区分中断来自cancel还是外部产生，没有清除中断状态
//
// Thread.interrupted();
}
cancel
先看cancel()再看get()方法：
public boolean cancel(boolean mayInterruptIfRunning) {
if (state != NEW) //如果已经运行，不能取消
return false;
if (mayInterruptIfRunning) { //容许中断，那就中断
if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, INTERRUPTING))
return false;
Thread t = runner;
if (t != null)
t.interrupt(); //中断
UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED); // 推荐状态到最终状态
}
else if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, CANCELLED)) //取消
return false;
finishCompletion();
return true;
}
2种状态变换：
1、 容许中断：NEW->INTERRUPTING->INTERRUPTED；
2、 不容许中断：NEW->CANCELLED；
所有的最终状态时都会finishCompletion()unpark等待线程。
get
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING) //如果任务还未运行或者运行中，那就等
s = awaitDone(false, 0L);
return report(s); //根据状态返回
}
/** get超时等待 */
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(); //超时后还未到最终状态那就timeout异常
return report(s);
}
/** 根据状态返回 */
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);
}
get()思路还是挺简单的，如果还没到最终状态，那就加入链表等待(之前说过到最终状态时会调用finishCompletion),否则就根据状态返回。
awaitDone
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()) {
//如果线程中断，那就remove节点，抛出中断异常
removeWaiter(q);
throw new InterruptedException();
}
int s = state;
if (s > COMPLETING) { //有结果了，那就返回状态
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(); //一般会new一个节点
else if (!queued)
queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
q.next = waiters, q); //加入链表
else if (timed) { //看是否需要等待超时控制
nanos = deadline - System.nanoTime();
if (nanos <= 0L) {
removeWaiter(q);
return state;
}
LockSupport.parkNanos(this, nanos);
}
else
LockSupport.park(this);
}
}
/** 超时或线程中断时移除节点 */
private void removeWaiter(WaitNode node) {
if (node != null) {
node.thread = null;
retry:
for (;;) { // restart on removeWaiter race
for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
s = q.next;
if (q.thread != null)
pred = q;
else if (pred != null) {
pred.next = s;
if (pred.thread == null) // check for race
continue retry;
}
else if (!UNSAFE.compareAndSwapObject(this, waitersOffset,
q, s))
continue retry;
}
break;
}
}
}
其他方法
public boolean isCancelled() {
return state >= CANCELLED;
}
public boolean isDone() {
return state != NEW;
}
/** finishCompletion方法调用，可以用来处理收尾 */
protected void done() { }
总结
最主要的应该还是通过state标识各种运行状态吧，其他的都是Future这种模式本身的特性实现。
参考
1、 http://www.infoq.com/cn/articles/java-memory-model-4/深入理解Java内存模型（四）——volatile；
2、 http://bugs.java.com/bugdatabase/view_bug.do?bug_id=8016247以前版本使用aqs时的并发bug；