Java FutureTask源碼分析及使用詳解

了解一下什麼是FutureTask？

FutureTask 是一個可取消的非同步計算。

FutureTask提供了對Future的基本實現，可以呼叫方法去開始和取消一個計算，可以查詢計算是否完成，並且取得計算結果。

FutureTask只能在計算完成後取得到計算結果，一旦計算完成，將無法重新啟動或取消，除非呼叫runAndReset方法。

FutureTask除了實作了Future介面以外，也實作了Runnable接口，因此FutureTask是可以交由執行緒池的Executor執行，也可以直接使用一個非同步執行緒呼叫執行（futureTask.run()）。

FutureTask 是如何實現的呢？

首先，我們看一下FutureTask類別的繼承結構，如下圖，它實現的是RunnableFuture接口，而RunnableFuture#繼承自Future和函數式介面Runnable，所以說FutureTask本質就是一個可運行的Future。

Future 介面約定了一些非同步運算類別必須實現的功能，原始碼如下：

package java.util.concurrent;
public interface Future<V> {
    /**
     * 尝试取消任务的执行，并返回取消结果。
     * 参数mayInterruptIfRunning：是否中断线程。
     */
    boolean cancel(boolean mayInterruptIfRunning);
    /**
     * 判断任务是否被取消（正常结束之前被被取消返回true）
     */
    boolean isCancelled();
    /**
     * 判断当前任务是否执行完毕，包括正常执行完毕、执行异常或者任务取消。
     */
    boolean isDone();
    /**
     * 获取任务执行结果，任务结束之前会阻塞。
     */
    V get() throws InterruptedException, ExecutionException;
    /**
     * 在指定时间内尝试获取执行结果。若超时则抛出超时异常TimeoutException
     */
    V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException;
}

Runnable 接口我們都很熟悉，他就是一個函數式接口，我們常用其創建一個線程。

package java.lang;
?
@FunctionalInterface
public interface Runnable {
 ? ?
 ? ?public abstract void run();
}

FutureTask就是一個將要被執行的任務，它包含了上述介面具體的實現，FutureTask內部定義了任務的狀態state和一些狀態的常數，它的內部核心是一個Callable callable，我們透過建構函式可以傳入callable或是runnable，最後都會內部轉為callable，因為我們需要取得非同步任務的執行結果，而只有透過Callable所建立的執行緒才會回傳結果。

我們可以透過此時的狀態來判斷Future中isCancelled()，isDone()的回傳結果。

以下為FutureTask原始碼，內含核心原始碼分析註解

package java.util.concurrent;
import java.util.concurrent.locks.LockSupport;
public class FutureTask<V> implements RunnableFuture<V> {
    /**
     * 任务的运行状态
     */
    private volatile int state;
    private static final int NEW          = 0; // 新建
    private static final int COMPLETING   = 1; // 完成
    private static final int NORMAL       = 2; // 正常
    private static final int EXCEPTIONAL  = 3; // 异常
    private static final int CANCELLED    = 4; // 取消
    private static final int INTERRUPTING = 5; // 中断中
    private static final int INTERRUPTED  = 6; // 中断的
    private Callable<V> callable;
    /**
 	 * 返回结果
 	 */
    private Object outcome; 
    private volatile Thread runner;
    private volatile WaitNode waiters;
    ...
    public FutureTask(Callable<V> callable) {
        if (callable == null)
            throw new NullPointerException();
        this.callable = callable;
        this.state = NEW;       
    }
    public FutureTask(Runnable runnable, V result) {
        this.callable = Executors.callable(runnable, result);
        this.state = NEW;       
    }
    public boolean isCancelled() {
        return state >= CANCELLED;
    }
    public boolean isDone() {
        return state != NEW;
    }
    /*
     * 取消任务实现
     *   如果任务还没有启动就调用了cancel(true)，任务将永远不会被执行。
     *   如果任务已经启动，参数mayInterruptIfRunning将决定任务是否应该中断执行该任务的线程，以尝试中断该任务。
     *   如果任务任务已经取消、已经完成或者其他原因不能取消，尝试将失败。
     */
    public boolean cancel(boolean mayInterruptIfRunning) {
        if (!(state == NEW &&
              UNSAFE.compareAndSwapInt(this, stateOffset, NEW,
                  mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))
            return false;
        try {    // in case call to interrupt throws exception
            if (mayInterruptIfRunning) {
                try {
                    Thread t = runner;
                    if (t != null)
                        t.interrupt();
                } finally { // final state
                    UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);
                }
            }
        } finally {
            finishCompletion();
        }
        return true;
    }
        /*
     * 等待获取结果
     *   获取当前状态，判断是否执行完成。并且判断时间是否超时
     *   如果任务没有执行完成，就阻塞等待完成，若超时抛出超时等待异常。
     */
    public V get() throws InterruptedException, ExecutionException {
        int s = state;
        if (s <= COMPLETING)
            s = awaitDone(false, 0L);
        return report(s);
    }
  /*
     * 等待获取结果
     *   获取当前状态，判断是否执行完成。
     *   如果任务没有执行完成，就阻塞等待完成。
     */
    public V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException {
        if (unit == null)
            throw new NullPointerException();
        int s = state;
        if (s <= COMPLETING &&
            (s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
            throw new TimeoutException();
        return report(s);
    }
    /**
     * 根据状态判断返回结果还是异常
     */
    private V report(int s) throws ExecutionException {
        Object x = outcome;
        if (s == NORMAL)
            return (V)x;
        if (s >= CANCELLED)
            throw new CancellationException();
        throw new ExecutionException((Throwable)x);
    }
    protected void done() { }
    /**
     * 设置结果借助CAS确认状态是否完成状态
     */
    protected void set(V v) {
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            outcome = v;
            UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
            finishCompletion();
        }
    }
    /**
     * 设置异常，当运行完成出现异常，设置异常状态
     */
    protected void setException(Throwable t) {
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            outcome = t;
            UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
            finishCompletion();
        }
    }
    /*
     * 执行callable获取结果，或者异常
     *   判断状态是不是启动过的，如果是新建才可以执行run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 = null;
            int s = state;
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
    }
    /**
     * 重新执行
     */
    protected boolean runAndReset() {
        if (state != NEW ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return false;
        boolean ran = false;
        int s = state;
        try {
            Callable<V> c = callable;
            if (c != null && s == NEW) {
                try {
                    c.call(); // don&#39;t set result
                    ran = true;
                } catch (Throwable ex) {
                    setException(ex);
                }
            }
        } finally {
            runner = null;
            s = state;
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
        return ran && s == NEW;
    }
    /*
     * 处理可能取消的中断
     */
    private void handlePossibleCancellationInterrupt(int s) {
        if (s == INTERRUPTING)
            while (state == INTERRUPTING)
                Thread.yield(); 
    }
    static final class WaitNode {
        volatile Thread thread;
        volatile WaitNode next;
        WaitNode() { thread = Thread.currentThread(); }
    }
    /**
     * 移除并唤醒所有等待线程,执行done,置空callable
     */
    private void finishCompletion() {
        // assert state > COMPLETING;
        for (WaitNode q; (q = waiters) != null;) {
            if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
                for (;;) {
                    Thread t = q.thread;
                    if (t != null) {
                        q.thread = null;
                        LockSupport.unpark(t);
                    }
                    WaitNode next = q.next;
                    if (next == null)
                        break;
                    q.next = null; // unlink to help gc
                    q = next;
                }
                break;
            }
        }
        done();
        callable = null;        // to reduce footprint
    }
    /**
     * 等待完成
     * 首先判断是否超时
     * 处理中断的，然后处理异常状态的，处理完成的...
     */
    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);
        }
    }
    /**
     * 去除等待
     */
    private void removeWaiter(WaitNode node) {
        if (node != null) {
            node.thread = null;
            retry:
            for (;;) {          // restart on removeWaiter race
                for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
                    s = q.next;
                    if (q.thread != null)
                        pred = q;
                    else if (pred != null) {
                        pred.next = s;
                        if (pred.thread == null) // check for race
                            continue retry;
                    }
                    else if (!UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                          q, s))
                        continue retry;
                }
                break;
            }
        }
    }
    // Unsafe mechanics
    private static final sun.misc.Unsafe UNSAFE;
    private static final long stateOffset;
    private static final long runnerOffset;
    private static final long waitersOffset;
    static {
        try {
            UNSAFE = sun.misc.Unsafe.getUnsafe();
            Class<?> k = FutureTask.class;
            stateOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("state"));
            runnerOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("runner"));
            waitersOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("waiters"));
        } catch (Exception e) {
            throw new Error(e);
        }
    }
}

FutureTask 執行流程

一般來說，我們可以認為FutureTask具有以下三種狀態：

未啟動：新建的FutureTask，在run()沒執行之前，FutureTask處於未啟動狀態。

private static final int NEW          = 0; // 新建

已啟動：FutureTask物件的run方法啟動並執行的過程中，FutureTask處於已啟動狀態。

已完成：FutureTask正常執行結束，或FutureTask執行被取消(FutureTask物件cancel方法)，或是FutureTask物件run方法執行拋出例外而導致中斷而結束，FutureTask都處於已完成狀態。

private static final int COMPLETING   = 1; // 完成
private static final int NORMAL       = 2; // 完成后正常设置结果
private static final int EXCEPTIONAL  = 3; // 完成后异常设置异常
private static final int CANCELLED    = 4; // 执行取消
private static final int INTERRUPTING = 5; // 中断中
private static final int INTERRUPTED  = 6; // 中断的

FutureTask 的使用

使用一（直接新建一個執行緒呼叫）：

FutureTask<Integer> task = new FutureTask<>(new Callable() {
	@Override
    public Integer call() throws Exception {
    	return sum();
    }
});
new Thread(task).stat();
Integer result = task.get();

使用二（結合執行緒池使用）

FutureTask<Integer> task = new FutureTask<>(new Callable() {
	@Override
    public Integer call() throws Exception {
    	return sum();
    }
});
Executors.newCachedThreadPool().submit(task);
Integer result = task.get();

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