FutureTask是JDK中Future模式的标准实现，它同时实现了Runnable和Future两个接口，提供了可取消的异步计算，并且可以利用开始和取消计算的方法、查询计算是否完成的方法和获取计算结果的方法。
public class FutureTask<V> implements RunnableFuture<V> {
/** 所有的方法全部委托sync */
private final Sync sync;
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
sync = new Sync(callable);
}
public FutureTask(Runnable runnable, V result) {
sync = new Sync(Executors.callable(runnable, result));
}
public boolean isCancelled() {
return sync.innerIsCancelled();
}
public boolean isDone() {
return sync.innerIsDone();
}
public boolean cancel(boolean mayInterruptIfRunning) {
return sync.innerCancel(mayInterruptIfRunning);
}
public V get() throws InterruptedException, ExecutionException {
return sync.innerGet();
}
public V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException {
return sync.innerGet(unit.toNanos(timeout));
}
protected void done() { }
protected void set(V v) {
sync.innerSet(v);
}
protected void setException(Throwable t) {
sync.innerSetException(t);
}
public void run() {
sync.innerRun();
}
protected boolean runAndReset() {
return sync.innerRunAndReset();
}
private final class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = -7828117401763700385L;
/** State value representing that task is ready to run */
/** 代表起始状态 */
private static final int READY = 0;
/** State value representing that task is running */
/** 代表正在运行中状态 */
private static final int RUNNING = 1;
/** State value representing that task ran */
/** 代表运行完成的状态 */
private static final int RAN = 2;
/** State value representing that task was cancelled */
/** 代表被取消的状态 */
private static final int CANCELLED = 4;
/** The underlying callable */
private final Callable<V> callable;
/** The result to return from get() */
private V result;
/** The exception to throw from get() */
private Throwable exception;
/**
* The thread running task. When nulled after set/cancel, this
* indicates that the results are accessible. Must be
* volatile, to ensure visibility upon completion.
*/
private volatile Thread runner;
Sync(Callable<V> callable) {
this.callable = callable;
}
/**
* 判断是否完成或者是否取消
* 传入0或者1 都返回0 说明任务没有完成 也没有取消
*/
private boolean ranOrCancelled(int state) {
return (state & (RAN | CANCELLED)) != 0;
}
/**
* AbstractQueuedSynchronizer的模板方法
* 返回1可以获取锁 返回-1说明获取锁失败
* 调用innerIsDone 返回TRUE 说明任务已经执行完毕
* 返回FALSE 说明任务没有执行完毕
*/
protected int tryAcquireShared(int ignore) {
return innerIsDone() ? 1 : -1;
}
/**
* 释放锁 将执行当前任务的线程设置为null
*/
protected boolean tryReleaseShared(int ignore) {
runner = null;
return true;
}
//判断任务是否被取消
boolean innerIsCancelled() {
return getState() == CANCELLED;
}
//判断任务是否完成(取消也算完成)
boolean innerIsDone() {
return ranOrCancelled(getState()) && runner == null;
}
//获取结果
V innerGet() throws InterruptedException, ExecutionException {
//首先调用AbstractQueuedSynchronizer的方法，这个方法会调用子类方法tryAcquireShared 上面有讲
//如果当前任务已经完成，那么当前线程可以向下运行，否则把当前线程加入队列阻塞.
acquireSharedInterruptibly(0);
//判断状态 如果取消了就抛CancellationException异常.
if (getState() == CANCELLED)
throw new CancellationException();
//如果任务执行过程中出现异常，这里包装一下抛出ExecutionException.
if (exception != null)
throw new ExecutionException(exception);
return result;
}
//获取结果
V innerGet(long nanosTimeout) throws InterruptedException, ExecutionException, TimeoutException {
//调用AbstractQueuedSynchronizer里的方法
// return tryAcquireShared(arg) >= 0 ||doAcquireSharedNanos(arg, nanosTimeout);
// 首先tryAcquireShared调用它获取锁，也就是看任务完事没，如果任务完事了就返回TRUE，那么执行逻辑同上。
// 如果获取不到锁，那么就阻塞当前线程给定的时间，如果时间到了再次任务还没完成则抛出异常。
if (!tryAcquireSharedNanos(0, nanosTimeout))
throw new TimeoutException();
if (getState() == CANCELLED)
throw new CancellationException();
if (exception != null)
throw new ExecutionException(exception);
return result;
}
void innerSet(V v) {
for (;;) {
int s = getState();
if (s == RAN)
return;
if (s == CANCELLED) {
// aggressively release to set runner to null,
// in case we are racing with a cancel request
// that will try to interrupt runner
releaseShared(0);
return;
}
//正常完成 设置状态为RAN
if (compareAndSetState(s, RAN)) {
result = v;
releaseShared(0);
done(); //通知子类
return;
}
}
}
void innerSetException(Throwable t) {
for (;;) {
int s = getState();
if (s == RAN)
return;
if (s == CANCELLED) {
// aggressively release to set runner to null,
// in case we are racing with a cancel request
// that will try to interrupt runner
releaseShared(0);
return;
}
//设置异常
if (compareAndSetState(s, RAN)) {
exception = t;
releaseShared(0);
done();//通知子类
return;
}
}
}
//取消任务
boolean innerCancel(boolean mayInterruptIfRunning) {
for (;;) {
int s = getState();
//如果任务已经结束，则返回FALSE
if (ranOrCancelled(s))
return false;
//设置任务的状态为CANCELLED
if (compareAndSetState(s, CANCELLED))
break;
}
//如果参数mayInterruptIfRunning=TRUE,那么设置线程的终端状态
if (mayInterruptIfRunning) {
Thread r = runner;
if (r != null)
r.interrupt();
}
//释放锁
releaseShared(0);
//调用子类方法，通知状态改变
done();
return true;
}
void innerRun() {
//如果任务不是初始状态则直接结束
if (!compareAndSetState(READY, RUNNING))
return;
runner = Thread.currentThread();
if (getState() == RUNNING) { // recheck after setting thread
V result;
try {
result = callable.call();
} catch (Throwable ex) {
//我们写的任务方法里如果出现异常则调用setException
setException(ex);
return;
}
//设置结果
set(result);
} else {
//释放锁
releaseShared(0); // cancel
}
}
boolean innerRunAndReset() {
if (!compareAndSetState(READY, RUNNING))
return false;
try {
runner = Thread.currentThread();
if (getState() == RUNNING)
callable.call(); // don't set result
runner = null;
return compareAndSetState(RUNNING, READY);
} catch (Throwable ex) {
setException(ex);
return false;
}
}
}
}
http://m.blog.csdn.net/article/details?id=46293927