简谈java并发FutureTask的实现

//以下五个方法实现接口Future中方法
public boolean isCancelled(); 
public boolean isDone(); 
public boolean cancel();
public V get() throws InterruptedException, ExecutionException;
public V get(long timeout, TimeUnit unit);
//实现接口Runnable中方法
public void run();

/**
* Created by yuanqiongqiong on 2019/4/9.
*/
public class FutureTaskTest {
private static ExecutorService executorService = Executors.newFixedThreadPool(1);
public static void main(String []args) {
Callable callable = new AccCallable(1, 2);
FutureTask futureTask = new FutureTask(callable);
executorService.execute(futureTask);
System.out.println("go to do other things in main thread");
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("go back in main thread");
try {
int result = (int) futureTask.get();
System.out.println("result is " + result);
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
}
static class AccCallable implements Callable<Integer> {
private int a;
private int b;
public AccCallable(int a, int b) {
this.a = a;
this.b = b;
}
@Override
public Integer call() throws Exception {
System.out.println("acc a and b in threadId = " + Thread.currentThread().getName());
return a + b;
}
}
}

go to do other things in main thread
acc a and b in threadId = pool-1-thread-1
go back in main thread
result is 3

//futureTask执行状态
private volatile int state;
//具体的执行任务，会在run方法中抵用callable.call()
private Callable<V> callable;
//执行结果
private Object outcome; 
//获取结果的等待线程节点
private volatile WaitNode waiters;

/**
* Possible state transitions:
* NEW -> COMPLETING -> NORMAL
* NEW -> COMPLETING -> EXCEPTIONAL
* NEW -> CANCELLED
* NEW -> INTERRUPTING -> INTERRUPTED
*/
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;

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) {
//构造函数传入runnable对象时调用静态工具类Executors的方法转换为一个callable对象
this.callable = Executors.callable(runnable, result);
this.state = NEW; // ensure visibility of callable
}

public void run() {
//1.如果执行状态不是NEW或者有其他线程执行该任务，直接返回
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread()))
return;
try {
Callable<V> c = callable;
//2.如果执行状态是NEW,即任务还没执行，直接调用callable.call()方法获取执行结果
if (c != null && state == NEW) {
V result;
boolean ran;
try {
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
//3.发生异常，更新status为EXCEPTIONAL，唤醒挂起线程
setException(ex);
}
//4.如果结果成功返回，调用set方法将设置outcome，更改status执行状态，唤醒挂起线程
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);
}
}

protected void set(V v) {
//将执行状态变更为COMPLETING
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
//设置执行结果
outcome = v;
//设置执行状态为NORMAL
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
//执行完成后处理操作，具体就是遍历阻塞链表，删除链表节点，并唤醒每个节点关联的线程
finishCompletion();
}
}

public V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException {
if (unit == null)
throw new NullPointerException();
int s = state;
//执行状态是NEW或者COMPLETING时执行awaitDone将线程加入等待队列中并挂起线程
if (s <= COMPLETING &&
(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
throw new TimeoutException();
//根据执行状态status进行结果封装
return report(s);
}
//我理解这块是get的核心逻辑
private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
//如果设置了超时时间，计算还有多长时间超时
final long deadline = timed ? System.nanoTime() + nanos : 0L;
WaitNode q = null;
boolean queued = false;
for (;;) {
//如果当前线程被中断，删除等待队列中的节点，并抛出异常
if (Thread.interrupted()) {
removeWaiter(q);
throw new InterruptedException();
}
int s = state;
//如果执行状态已经完成或者发生异常，直接跳出自旋返回
if (s > COMPLETING) {
if (q != null)
q.thread = null;
return s;
}
//如果执行状态是正在执行，说明线程已经被加入到等待队列中，放弃cpu进入下次循环(真正的自旋)
else if (s == COMPLETING) // cannot time out yet
Thread.yield();
//第一次进入循环，创建节点
else if (q == null)
q = new WaitNode();
//将节点加入到等待队列中，waiters相当于头阶段，不断将头结点更新为新节点
else if (!queued)
queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
q.next = waiters, q);
else if (timed) {
//如果设置了超时时间，在进行下次循环前查看是否已经超时，如果超时删除该节点进行返回
nanos = deadline - System.nanoTime();
if (nanos <= 0L) {
removeWaiter(q);
return state;
}
//挂起当前节点
LockSupport.parkNanos(this, nanos);
}
else
LockSupport.park(this);
}
}