Future 模式

FutureTask

future模式：一種異步計算模式，並支持返回計算結果，在調用get()獲取到計算結果前能夠阻塞調用者線程

FutureTask設計原理

FutureTask是JDK針對與future模式的一種實現,它除了支持future特有的特色，還支持task的一些操做，好比取消，打斷。
一個FutureTask就是一個任務的計算單元，是調度的最小單位，它的調度藉助於JDK的Executor任務調度模型。須要開發人員建立好FutureTask對象後，並送入到Executor去等待調度

具體的執行過程，像下面是一段FutureTask的僞碼描述

建立一個futureTask對象task
提交task到調度器executor等待調度

等待調度中...

若是此時currentThread調取執行結果task.get(),會有幾種狀況
    
    if task 尚未被executor調度或正在執行中
        阻塞當前線程，並加入到一個阻塞鏈表中waitNode
    else if task被其它Thread取消，並取消成功 或task處於打斷狀態
        throw exception
    else if task執行完畢，返回執行結果，或執行存在異常，返回異常信息
        
            
若是此時有另一個線程調用task.get()
        
    執行過程同上

注意:executor在執行FutureTask前，會先判斷是否被取消，若是取消就不在執行，但執行後就不能夠在取消了

FutureTask 核心部分代碼

在futureTask定義task的轉態有：

private volatile int state;
private static final int NEW          = 0; // 建立
private static final int COMPLETING   = 1; // 完成
private static final int NORMAL       = 2; // 
private static final int EXCEPTIONAL  = 3; // invoke task 出現異常
private static final int CANCELLED    = 4; // cancel task 
private static final int INTERRUPTING = 5; // interrupting task 
private static final int INTERRUPTED  = 6;

建立一個FutureTask

建立futureTask只須要須要一個callable對象或runnable對象的參數，並在建立時設置狀態爲NEW

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

調用get()方法獲取執行結果方法

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 調度是執行的方法

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);
        }
    }