Future 模式

el09xccxy 發(fā)布于2019-08-14 12:44 / 2992人閱讀

摘要：模式一種異步計(jì)算模式，并支持返回計(jì)算結(jié)果，在調(diào)用獲取到計(jì)算結(jié)果前可以阻塞調(diào)用者線程設(shè)計(jì)原理是針對(duì)與模式的一種實(shí)現(xiàn)它除了支持特有的特點(diǎn)，還支持的一些操作，比如取消，打斷。

FutureTask

future模式：一種異步計(jì)算模式，并支持返回計(jì)算結(jié)果，在調(diào)用get()獲取到計(jì)算結(jié)果前可以阻塞調(diào)用者線程

FutureTask設(shè)計(jì)原理

FutureTask是JDK針對(duì)與future模式的一種實(shí)現(xiàn),它除了支持future特有的特點(diǎn)，還支持task的一些操作，比如取消，打斷。
一個(gè)FutureTask就是一個(gè)任務(wù)的計(jì)算單元，是調(diào)度的最小單位，它的調(diào)度借助于JDK的Executor任務(wù)調(diào)度模型。需要開(kāi)發(fā)人員創(chuàng)建好FutureTask對(duì)象后，并送入到Executor去等待調(diào)度

具體的執(zhí)行過(guò)程，像下面是一段FutureTask的偽碼描述

創(chuàng)建一個(gè)futureTask對(duì)象task
提交task到調(diào)度器executor等待調(diào)度

等待調(diào)度中...

如果此時(shí)currentThread調(diào)取執(zhí)行結(jié)果task.get(),會(huì)有幾種情況
    
    if task 還沒(méi)有被executor調(diào)度或正在執(zhí)行中
        阻塞當(dāng)前線程，并加入到一個(gè)阻塞鏈表中waitNode
    else if task被其它Thread取消，并取消成功 或task處于打斷狀態(tài)
        throw exception
    else if task執(zhí)行完畢，返回執(zhí)行結(jié)果，或執(zhí)行存在異常，返回異常信息
        
            
如果此時(shí)有另外一個(gè)線程調(diào)用task.get()
        
    執(zhí)行過(guò)程同上

注意:executor在執(zhí)行FutureTask前，會(huì)先判斷是否被取消，如果取消就不在執(zhí)行，但執(zhí)行后就不可以在取消了

FutureTask 核心部分代碼 在futureTask定義task的轉(zhuǎn)態(tài)有：

private volatile int state;
private static final int NEW          = 0; // 創(chuàng)建
private static final int COMPLETING   = 1; // 完成
private static final int NORMAL       = 2; // 
private static final int EXCEPTIONAL  = 3; // invoke task 出現(xiàn)異常
private static final int CANCELLED    = 4; // cancel task 
private static final int INTERRUPTING = 5; // interrupting task 
private static final int INTERRUPTED  = 6;

創(chuàng)建一個(gè)FutureTask

創(chuàng)建futureTask只需要需要一個(gè)callable對(duì)象或runnable對(duì)象的參數(shù)，并在創(chuàng)建時(shí)設(shè)置狀態(tài)為NEW

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

調(diào)用get()方法獲取執(zhí)行結(jié)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(diào)度是執(zhí)行的方法

public void run() {
        if (state != NEW ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return;
        try {
            Callable 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);
        }
    }