FutureTask

future模式：一种异步计算模式，并支持返回计算结果，在调用get()获取到计算结果前可以阻塞调用者线程

FutureTask设计原理

FutureTask是JDK针对与future模式的一种实现,它除了支持future特有的特点，还支持task的一些操作，比如取消，打断。
一个FutureTask就是一个任务的计算单元，是调度的最小单位，它的调度借助于JDK的Executor任务调度模型。需要开发人员创建好FutureTask对象后，并送入到Executor去等待调度

具体的执行过程，像下面是一段FutureTask的伪码描述

创建一个futureTask对象task提交task到调度器executor等待调度等待调度中...如果此时currentThread调取执行结果task.get(),会有几种情况        if task 还没有被executor调度或正在执行中        阻塞当前线程，并加入到一个阻塞链表中waitNode    else if task被其它Thread取消，并取消成功 或task处于打断状态        throw exception    else if task执行完毕，返回执行结果，或执行存在异常，返回异常信息        如果此时有另外一个线程调用task.get()执行过程同上       

注意:executor在执行FutureTask前，会先判断是否被取消，如果取消就不在执行，但执行后就不可以在取消了

FutureTask 核心部分代码

在futureTask定义task的转态有：

private volatile int state;private static final int NEW          = 0; // 创建private static final int COMPLETING   = 1; // 完成private static final int NORMAL       = 2; // private static final int EXCEPTIONAL  = 3; // invoke task 出现异常private static final int CANCELLED    = 4; // cancel task private static final int INTERRUPTING = 5; // interrupting task private static final int INTERRUPTED  = 6;

创建一个FutureTask

创建futureTask只需要需要一个callable对象或runnable对象的参数，并在创建时设置状态为NEW

public FutureTask(Callable<V> callable) {        if (callable == null)throw new NullPointerException();        this.callable = callable;        this.state = NEW;       // ensure visibility of callable}

调用get()方法获取执行结果方法

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()) {    removeWaiter(q);    throw new InterruptedException();}int s = state;if (s > COMPLETING) {    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();else if (!queued)    queued = UNSAFE.compareAndSwapObject(this, waitersOffset, q.next = waiters, q);else if (timed) {    nanos = deadline - System.nanoTime();    if (nanos <= 0L) {        removeWaiter(q);        return state;    }    LockSupport.parkNanos(this, nanos);}else    LockSupport.park(this);        }}

executor 调度是执行的方法

public void run() {        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);    }    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 interruptsint s = state;if (s >= INTERRUPTING)    handlePossibleCancellationInterrupt(s);        }    }