2.4線程併發工具-Callable、Future和FutureTask原理+源碼解析

一、Callable、Future和FutureTask概述

1、Callable和Runnable的區別

Executor框架使用Runnable作爲其基本的任務表示形式。Runnable是一種侷限性很大的抽象，它沒有返回值並且不能拋出異常

@FunctionalInterface
public interface Runnable {
    /**
     * When an object implementing interface <code>Runnable</code> is used
     * to create a thread, starting the thread causes the object's
     * <code>run</code> method to be called in that separately executing
     * thread.
     * <p>
     * The general contract of the method <code>run</code> is that it may
     * take any action whatsoever.
     *
     * @see     java.lang.Thread#run()
     */
    public abstract void run();
}

許多任務實際上都是存在延遲的計算，對於這些任務，Callable是一種更好的抽象：它會返回一個值，並可能拋出一個異常

@FunctionalInterface
public interface Callable<V> {
    /**
     * Computes a result, or throws an exception if unable to do so.
     *
     * @return computed result
     * @throws Exception if unable to compute a result
     */
    V call() throws Exception;
}

2、Future

Future表示一個任務的生命週期，並提供了方法來判斷是否已經完成或取消，以及獲取任務的結果和取消任務等

public interface Future<V> {
    boolean cancel(boolean mayInterruptIfRunning);
 
    boolean isCancelled();
 
    boolean isDone();
 
    V get() throws InterruptedException, ExecutionException;
 
    V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException;
}

cancel方法用於取消任務，取消成功返回true，取消失敗返回false。mayInterruptIfRunning參數含義是是否允許取消正在運行的任務。

isCancelled方法用於判斷任務是否被取消成功，在任務正常結束之前被取消成功則會返回true。

isDone方法用於判斷任務是否已經完成

get用於獲取任務的返回值

get(long timeout, TimeUnit unit) 用來獲取執行結果，如果在指定時間內，還沒獲取到結果，就直接返回null。

綜上所述Future提供了三種功能
1、判斷任務是否完成
2、中斷任務
3、獲取結果

3、FutureTask

Future只是一個接口，無法直接創建對象，因此需要藉助FutureTask來使用

public class FutureTask<V> implements RunnableFuture<V> {}

FutureTask實現了RunnableFuture接口

public interface RunnableFuture<V> extends Runnable, Future<V> {
    /**
     * Sets this Future to the result of its computation
     * unless it has been cancelled.
     */
    void run();
}

RunnableFuture繼承自Runnable和Future
一個FutureTask 可以用來包裝一個 Callable 或是一個Runnable對象。因爲FurtureTask實現了Runnable方法，所以一個 FutureTask可以提交(submit)給一個Excutor執行(excution). 它同時實現了Callable, 所以也可以作爲Future得到Callable的返回值。

FutureTask有兩個很重要的屬性分別是state和runner

/**
     * The run state of this task, initially NEW.  The run state
     * transitions to a terminal state only in methods set,
     * setException, and cancel.  During completion, state may take on
     * transient values of COMPLETING (while outcome is being set) or
     * INTERRUPTING (only while interrupting the runner to satisfy a
     * cancel(true)). Transitions from these intermediate to final
     * states use cheaper ordered/lazy writes because values are unique
     * and cannot be further modified.
     *
     * Possible state transitions:
     * NEW -> COMPLETING -> NORMAL
     * NEW -> COMPLETING -> EXCEPTIONAL
     * NEW -> CANCELLED
     * NEW -> INTERRUPTING -> INTERRUPTED
     */
    private volatile int state;
    //線程初始狀態
    private static final int NEW          = 0;
    //運行中
    private static final int COMPLETING   = 1;
    //正常運行完成
    private static final int NORMAL       = 2;
    //異常
    private static final int EXCEPTIONAL  = 3;
    //被取消
    private static final int CANCELLED    = 4;
    //被中斷的中間態
    private static final int INTERRUPTING = 5;
    //被中斷的終態
    private static final int INTERRUPTED  = 6;

這幾種狀態的轉換可能爲

• NEW -> COMPLETING -> NORMAL（正常結束）
• NEW -> COMPLETING -> EXCEPTIONAL（異常結束）
• NEW -> CANCELLED（取消執行）
• NEW-INTERRUPTING-INTERRUPTED(被中斷)

通過幾種成員方法講述下狀態的轉換
1、構造函數

/**
     * Creates a {@code FutureTask} that will, upon running, execute the
     * given {@code Callable}.
     *
     * @param  callable the callable task
     * @throws NullPointerException if the callable is null
     */
    public FutureTask(Callable<V> callable) {
        if (callable == null)
            throw new NullPointerException();
        this.callable = callable;
        this.state = NEW;       // ensure visibility of callable
    }

    /**
     * Creates a {@code FutureTask} that will, upon running, execute the
     * given {@code Runnable}, and arrange that {@code get} will return the
     * given result on successful completion.
     *
     * @param runnable the runnable task
     * @param result the result to return on successful completion. If
     * you don't need a particular result, consider using
     * constructions of the form:
     * {@code Future<?> f = new FutureTask<Void>(runnable, null)}
     * @throws NullPointerException if the runnable is null
     */
    public FutureTask(Runnable runnable, V result) {
        this.callable = Executors.callable(runnable, result);
        this.state = NEW;       // ensure visibility of callable
    }

可以看到任務被創建時的初始狀態是NEW，構造函數有2種重載函數，一種接收Runnable，一種接收Callable，接收Runnable的時候會進行將Runnable轉爲Callable。如下

/**
     * Returns a {@link Callable} object that, when
     * called, runs the given task and returns the given result.  This
     * can be useful when applying methods requiring a
     * {@code Callable} to an otherwise resultless action.
     * @param task the task to run
     * @param result the result to return
     * @param <T> the type of the result
     * @return a callable object
     * @throws NullPointerException if task null
     */
    public static <T> Callable<T> callable(Runnable task, T result) {
        if (task == null)
            throw new NullPointerException();
        return new RunnableAdapter<T>(task, result);
    }

/**
     * A callable that runs given task and returns given result
     */
    static final class RunnableAdapter<T> implements Callable<T> {
        final Runnable task;
        final T result;
        RunnableAdapter(Runnable task, T result) {
            this.task = task;
            this.result = result;
        }
        public T call() {
            task.run();
            return result;
        }
    }

可以看到就是讓傳入的參數作爲call的返回值。使用call方法來調用run方法即可。把傳入的 T result 作爲callable的返回結果

啓動任務
當創建完一個Task通常會提交給Executors來執行，當然也可以使用Thread來執行，Thread的start()方法會調用Task的run()方法。看下FutureTask的run()方法的實現：

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);
        }
    }

首先判斷任務是否處於NEW的狀態，如果不是則直接返回。因爲任務在初始化完成之前可能就直接被cancel，這時任務已經不處於NEW狀態，可能處於Interrupting狀態，這也是任務沒完全啓動，取消時任務一直得不到啓動的原因。

如果是處於NEW狀態，則判斷task的線程是否爲null，如果不爲null，說明已經有線程在運行了，直接返回。如果爲null，則將當前線程賦值給task的線程。此處使用CAS是爲了保證對work Thread的賦值是原子性操作。保證多個線程對task進行提交的時候，run只被調用一次。

接下來任務不爲null，並且任務處於NEW狀態就啓動。正常執行就set結果，否則就setException。最後把work Thread設置爲null。

set方法

/**
     * Sets the result of this future to the given value unless
     * this future has already been set or has been cancelled.
     *
     * <p>This method is invoked internally by the {@link #run} method
     * upon successful completion of the computation.
     *
     * @param v the value
     */
    protected void set(V v) {
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            outcome = v;
            UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
            finishCompletion();
        }
    }

可以看到，如果task處於NEW，則付值爲COMPLETING，再將結果設置到outcome中，再將任務設置成NORMAL狀態。
整個狀態的流轉就是上面提到的：- NEW -> COMPLETING -> NORMAL（正常結束）

最後調用finishCompletion（）方法

/**
     * Removes and signals all waiting threads, invokes done(), and
     * nulls out 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
    }

```java
可以看到主要任務是喚醒等待在task上的所有線程，WaitNode存放的是所有調用get方法阻塞在該task上的線程。
調用done方法；將task的任務設置成null。至此結束整個run的流程。

接下來看下get方法

```java
/**
     * @throws CancellationException {@inheritDoc}
     */
    public V get() throws InterruptedException, ExecutionException {
        int s = state;
        if (s <= COMPLETING)
            s = awaitDone(false, 0L);
        return report(s);
    }

可以看到，第一步是判斷任務的狀態是否是完成狀態，如果是完成狀態，則調用report方法

/**
     * Returns result or throws exception for completed task.
     *
     * @param s completed state value
     */
    @SuppressWarnings("unchecked")
    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);
    }

如果任務狀態NORMAL，則表示任務正常結束，已經設置了返回值，直接返回。如果任務是取消或者中斷則表示任務異常，則拋出異常。

如果get時,FutureTask的狀態爲未完成狀態，則調用awaitDone方法進行阻塞。awaitDone():

/**
     * Awaits completion or aborts on interrupt or timeout.
     *
     * @param timed true if use timed waits
     * @param nanos time to wait, if timed
     * @return state upon completion
     */
    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);
        }
    }

awaitDone可以看作是輪詢查詢status的變化，在get阻塞的期間
1、如果調用get的線程被中斷，則移除所有阻塞在等待隊列上的線程，並拋出InterruptedException異常。
2、如果status表示任務已經完成（正常完成、被取消）直接返回任務狀態
3、如果任務正在執行，證明此時正在set結果，讓線程等待下。
4、如果線程是NEW狀態，將該線程加入到等待隊列當中
5、如果get未設置超時時間，則讓線程繼續阻塞
6、設置了超時時間檢查時間是否已到，到了的話移除所有的等待線程，並返回當前的任務status，沒到的話繼續等待。

四、舉例說明Callable、Future和FutureTask框架的使用

package cn.enjoy.controller.thread;

import cn.enjoy.controller.thread.DBPOLL.SleepTools;

import java.util.Random;
import java.util.concurrent.Callable;
import java.util.concurrent.FutureTask;

/**
 * @author：wangle
 * @description：
 * @version:V1.0
 * @date：2020-03-23 21:55
 **/
public class CallableFutureFutureTask {

    private static class UseCallable implements Callable<Integer> {
        @Override
        public Integer call()throws Exception{
            Integer i =0;
            System.out.println("Callable開始計算");
            for( i=0;i<100000;i++){
                System.out.println("run");
                System.out.println(Thread.currentThread().isInterrupted());
                if(Thread.currentThread().isInterrupted()){
                    System.out.println("線程被中斷");
                    break;
                }
            }
            System.out.println("線程done");
            return i;
        }
    }

    public static void main(String[] args)throws Exception{

        UseCallable useCallable = new UseCallable();
        FutureTask<Integer> futureTask = new FutureTask<Integer>(useCallable);
        Thread testThread = new Thread(futureTask);
        testThread.start();
        Thread.sleep(1000);
        futureTask.cancel(true);
        System.out.println(futureTask.get());
        SleepTools.second(2);
    }
}

1、cancel後，任務直接被終止結束，因爲我在線程中進行捕獲了中斷進行了處理，如果不進行處理則不會被中斷，感興趣的可以試試。（因爲java的線程是協作的，並非是搶佔的）
2、調用get方法後會一直阻塞到任務執行完成。

注
本文源碼來自JDK1.8，1.6是使用AQS實現