Future接口

public interface Future<V> {//取消任务执行 //当任务还没有启动，返回ture//当任务正在运行中，会根据mayInterruptIfRunning参数决定是否中断当前运行的线程来取消当前运行的任务//当任务已经完成或异常，其他一些原因不能中断时，返回falseboolean cancel(boolean mayInterruptIfRunning);//判断任务完成之前是否被取消//只用当任务时未启动状态或完成之前被取消 该方法才会返回true,其他情况都返回falseboolean isCancelled();//判断任务是否执行完成//异常退回，正常执行完成，被取消 都会返回trueboolean isDone();//获取任务执行结果会异常//会阻塞等到结果返回V get() throws InterruptedException, ExecutionException;//同上 只不过设置了超时时间，超时之后会返回出TimeoutException异常V get(long timeout, TimeUnit unit)throws InterruptedException, ExecutionException, TimeoutException;
}

RunnableFuture

public interface RunnableFuture<V> extends Runnable, Future<V> {//任务void run();
}

FutureTask

public class FutureTask<V> implements RunnableFuture<V> {/**线程可能变化的路线* 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;/** 需要执行的任务*/private Callable<V> callable;/**返回值 */private Object outcome; /**运行任务的线程*/private volatile Thread runner;/**表示等待线程的堆栈，在FutureTask的实现中，会通过CAS结合此堆栈交换任务的运行状态。*/private volatile WaitNode waiters;//状态大于CANCELLED 都返回true 表示成功取消public boolean isCancelled() {return state >= CANCELLED;}//只要状态不是NEW 都返回true 表示任务执行完成public boolean isDone() {return state != NEW;}public boolean cancel(boolean mayInterruptIfRunning) {//状态不是NEW 或者CAS返回值是false 则直接返回false,表示取消任务失败if (!(state == NEW &&UNSAFE.compareAndSwapInt(this, stateOffset, NEW,mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))return false;try {    //如果mayInterruptIfRunning参数是true,则将运行任务的线程中断if (mayInterruptIfRunning) {try {Thread t = runner;if (t != null)t.interrupt();} finally { // final state//将状态置为INTERRUPTEDUNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);}}} finally {finishCompletion();}return true;}private void finishCompletion() {// 定义一个循环 终止条件是waiters==nullfor (WaitNode q; (q = waiters) != null;) {//CAS成功之后，进入自旋循环if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {for (;;) {//取出线程Thread t = q.thread;if (t != null) {//将线程置为nullq.thread = null;//恢复该线程，让其运行完毕LockSupport.unpark(t);}WaitNode next = q.next;//判断如果next没有引用 则直接breakif (next == null)break;q.next = null; // unlink to help gcq = next;}break;}}//子类实现done();//将任务置为nullcallable = null;        // to reduce footprint}public V get() throws InterruptedException, ExecutionException {//获取当前状态大于COMPLETINGint s = state;if (s <= COMPLETING)//则判断下一步如何执行，并返回其状态s = awaitDone(false, 0L);//根据返回的状态 获取任务执行的返回值return report(s);}public V get(long timeout, TimeUnit unit)throws InterruptedException, ExecutionException, TimeoutException {//这里同上 ，只不过加了超时判断if (unit == null)throw new NullPointerException();int s = state;if (s <= COMPLETING &&(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)//如果超时 直接抛出TimeoutException异常throw new TimeoutException();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 (;;) {//自旋循环if (Thread.interrupted()) {//如果线程异常了，则直接抛出InterruptedException异常removeWaiter(q);throw new InterruptedException();}int s = state;if (s > COMPLETING) {//如果状态大于COMPLETING，不论是正常执行完成还是异常了，都是执行完成了if (q != null)q.thread = null;//执行完成之后将执行任务线程置空，gcreturn s;//将完成任务之后的状态返回}//如果状态是 COMPLETING 则将线程礼让出去else if (s == COMPLETING) // cannot time out yetThread.yield();else if (q == null)//如果WaitNode未空 则创建一个q = new WaitNode();else if (!queued)//CASqueued = UNSAFE.compareAndSwapObject(this, waitersOffset,q.next = waiters, q);else if (timed) {//如果设置了超时件nanos = deadline - System.nanoTime();if (nanos <= 0L) {//判断是否超时removeWaiter(q);return state;}LockSupport.parkNanos(this, nanos);}else//否则 暂停当前线程LockSupport.park(this);}}public void run() {//状态不是NEW 或者CAS为false 直接返回if (state != NEW ||!UNSAFE.compareAndSwapObject(this, runnerOffset,null, Thread.currentThread()))return;try {//要执行的任务Callable<V> c = callable;//当前状态是NEWif (c != null && state == NEW) {V result;boolean ran;try {//调用call方法执行任务 接受返回值result = c.call();//将ran置为trueran = true;} catch (Throwable ex) {result = null;ran = false;setException(ex);}//代表正常执行，并且有返回值if (ran)//将返回值设置给outcome，并将状态更新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 interruptsint s = state;if (s >= INTERRUPTING)//如果异常 则线程礼让handlePossibleCancellationInterrupt(s);}}
}