Future 模式

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 interrupts
            int s = state;
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
    }