先介绍几个常见类 Runnable、Callable、Future、RunnableFuture、Thread 等。

参考 [Thread 类源码阅读] (https://www.jianshu.com/p/543d2bc5f54a) 中的构造函数，可知 Java 创建线程的方法有两种。
第一种： 继承 Thread 类，重写 run 方法；
第二种：继承 Runable 接口， 重写 run 方法，并作为 Thread 类的构造参数。
代码分别如下：

// 继承 Thread 类，重写 run 方法
new Thread() {
    @Override
    public void run() {
        System.out.println("继承 thread 类子线程运行");
    }
}.start();

// 继承 Runable 接口， 重写 run 方法，并作为 Thread 类的构造参数；
new Thread(new Runnable() {
    @Override
    public void run() {
        System.out.println("runable 子线程运行");
    }
}).start();

但是在学习 Java 线程相关支持时，还经常看到另外一种方法，使用 Callable 接口， 并且这种方式可以有返回值。
先看代码

FutureTask<Integer> futureTaskCallable = new FutureTask<>(new Callable<Integer>() {
    @Override
    public Integer call() throws Exception {
        return 1;
    }
});
new Thread(futureTaskCallable).start();
System.out.println("futureTaskCallable 子线程运行结果：" + futureTaskCallable.get());

明明 Thread 类能接收的参数类型就是 Runable 接口对象，怎么就冒出来一个 FutureTask。
现在开始看看 FutureTask 代码结构。

/**
 * A cancellable asynchronous computation.  This class provides a base
 * implementation of {@link Future}, with methods to start and cancel
 * a computation, query to see if the computation is complete, and
 * retrieve the result of the computation.  The result can only be
 * retrieved when the computation has completed; the {@code get}
 * methods will block if the computation has not yet completed.  Once
 * the computation has completed, the computation cannot be restarted
 * or cancelled (unless the computation is invoked using
 * {@link #runAndReset}).
 * 可取消的异步计算。
 * 这个类提供了 Future 的基本实现，包括启动和取消计算，查查看计算是否完成以及取回计算结果的方法。
 * 结果仅当计算完成的情况下才可返回，如果计算未完成 get 方法会被阻塞。
 *一旦计算完成，计算就不可以被重启或者取消，除非计算被 runAndReset 方法执行
 *
 * <p>A {@code FutureTask} can be used to wrap a {@link Callable} or
 * {@link Runnable} object.  Because {@code FutureTask} implements
 * {@code Runnable}, a {@code FutureTask} can be submitted to an
 * {@link Executor} for execution.
 * FutureTask 可以用来包装 Callable 或 Runnable 对象， 因为 FutureTask 实现了 Runnable 接口， 所以 FutureTask 可以被submitted 给 Executor 类执行。
 *
 * <p>In addition to serving as a standalone class, this class provides
 * {@code protected} functionality that may be useful when creating
 * customized task classes.
 *
 * @since 1.5
 * @author Doug Lea
 * @param <V> The result type returned by this FutureTask's {@code get} methods
 */
public class FutureTask<V> implements RunnableFuture<V> {
}

/**
 * A {@link Future} that is {@link Runnable}. Successful execution of
 * the {@code run} method causes completion of the {@code Future}
 * and allows access to its results.
 * 是 Runnable 的 Future。成功执行 run 方法会导致 Future完成并允许访问其结果。
 * @see FutureTask
 * @see Executor
 * @since 1.6
 * @author Doug Lea
 * @param <V> The result type returned by this Future's {@code get} method
 */
public interface RunnableFuture<V> extends Runnable, Future<V> {
    /**
     * Sets this Future to the result of its computation
     * unless it has been cancelled.
     */
    void run();
}

由代码可已看到如下类结构图。

image.png

虽然还不知道 FutureTask 是如何工作的，但是由类图可知，FutureTask 实现了接口 Runnable。所以第三种创建线程的方式，实际上还是第二种。
下面开始逐行死磕 FutureTask 代码。

成员变量

/**
 * 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.
 * 此任务的运行状态，最初为NEW。
 * 运行状态仅在set，setException和cancel方法中转换为终端状态。
 * 在完成期间，状态可能会采用COMPLETING（正在设置结果时）或
 * INTERRUPTING（仅在中断满足cancel（true）的运行者）的瞬态值。从这些中间状态到最终状态的转换使用成本更低的有序惰性写入，因为值是唯一的，无法进一步修改
 *
 * Possible state transitions:
 * 可能的状态变化路线
 * NEW -> COMPLETING -> NORMAL
 * NEW -> COMPLETING -> EXCEPTIONAL
 * NEW -> CANCELLED
 * NEW -> INTERRUPTING -> INTERRUPTED
 */
private volatile int state;
private static final int NEW          = 0;
private static final int COMPLETING   = 1;
private static final int NORMAL       = 2;
private static final int EXCEPTIONAL  = 3;
private static final int CANCELLED    = 4;
private static final int INTERRUPTING = 5;
private static final int INTERRUPTED  = 6;

/** The underlying callable; nulled out after running */
// 包装的 callable， 运行结束后置 null
private Callable<V> callable;
/** The result to return or exception to throw from get() */
// 返回值 或者 get 方法抛出的异常
private Object outcome; // non-volatile, protected by state reads/writes
/** The thread running the callable; CASed during run() */
// 执行 callable 的线程， run 方法执行时，会使用cas更新
private volatile Thread runner;
/** Treiber stack of waiting threads */
// 等待线程堆栈
private volatile WaitNode waiters;

构造函数

/**
 * Creates a {@code FutureTask} that will, upon running, execute the
 * given {@code Callable}.
 * 创建一个 FutureTask，它将在运行时执行给定的 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.
 * 创建一个 FutureTask，它将在运行时执行给定的 Runnable，在成功完成后执行 get 返回给定的结果。
 * @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
}

内部类 WaitNode

    /**
     * Simple linked list nodes to record waiting threads in a Treiber
     * stack.  See other classes such as Phaser and SynchronousQueue
     * for more detailed explanation.
     * 简单栈实现
     */
    static final class WaitNode {
        volatile Thread thread;
        volatile WaitNode next;
        WaitNode() { thread = Thread.currentThread(); }
    }

FutureTask 自有逻辑

返回结果 或者 抛出异常方法 report

/**
 * Returns result or throws exception for completed task.
 * 返回结果 或者 抛出异常
 *
 * @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);
}

protected void done() { }

/**
 * Sets the result of this future to the given value unless
 * this future has already been set or has been cancelled.
 * 除非已经设置或取消了否则将此 future 的结果设置为给定值。
 * <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();
    }
}

/**
 * Causes this future to report an {@link ExecutionException}
 * with the given throwable as its cause, unless this future has
 * already been set or has been cancelled.
 * 除非已经设置或取消了该未来，否则将将结果设置为给定 Throwable 
 *
 * <p>This method is invoked internally by the {@link #run} method
 * upon failure of the computation.
 *
 * @param t the cause of failure
 */
protected void setException(Throwable t) {
    if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
        outcome = t;
        UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
        finishCompletion();
    }
}

/**
 * Ensures that any interrupt from a possible cancel(true) is only
 * delivered to a task while in run or runAndReset.
 * 确保来自可能的cancel（true）的任何中断仅在运行或runAndReset时才传递给任务。
 */
private void handlePossibleCancellationInterrupt(int s) {
    // It is possible for our interrupter to stall before getting a
    // chance to interrupt us.  Let's spin-wait patiently.
    if (s == INTERRUPTING)
        while (state == INTERRUPTING)
            Thread.yield(); // wait out pending interrupt

    // assert state == INTERRUPTED;

    // We want to clear any interrupt we may have received from
    // cancel(true).  However, it is permissible to use interrupts
    // as an independent mechanism for a task to communicate with
    // its caller, and there is no way to clear only the
    // cancellation interrupt.
    //
    // Thread.interrupted();
}

/**
 * Removes and signals all waiting threads, invokes done(), and
 * nulls out callable.
 * 删除并通知所有等待线程，调用  done 方法，并使 callable 无效。
 */
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
}

/**
 * Awaits completion or aborts on interrupt or timeout.
 * 等待完成，或者在中断或超时时中止。
 * @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);
    }
}

/**
 * Tries to unlink a timed-out or interrupted wait node to avoid
 * accumulating garbage.  Internal nodes are simply unspliced
 * without CAS since it is harmless if they are traversed anyway
 * by releasers.  To avoid effects of unsplicing from already
 * removed nodes, the list is retraversed in case of an apparent
 * race.  This is slow when there are a lot of nodes, but we don't
 * expect lists to be long enough to outweigh higher-overhead
 * schemes.
 * 尝试取消链接超时或中断的等待节点，以避免积累垃圾。
 * 内部节点在没有CAS的情况下根本不会被拼接，因为如果释放者无论如何都要遍历它们，这是无害的
 * 为了避免从已删除的节点取消拆分的影响，在出现明显竞争的情况下会重新遍历该列表。
 * 当节点很多时，这很慢，但是我们认为列表足够长。
 */
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;
        }
    }
}

继承自 Future 的方法

// 返回 FutureTask 是否取消，判断条件 state >= CANCELLED
public boolean isCancelled() {
    return state >= CANCELLED;
}

// 返回 FutureTask 是否结束，判断条件 state != NEW
public boolean isDone() {
    return state != NEW;
}

// 取消任务
// 只能取消状态为 NEW 的 FutureTask
public boolean cancel(boolean mayInterruptIfRunning) {
    // 
    if (!(state == NEW &&
          UNSAFE.compareAndSwapInt(this, stateOffset, NEW,
              mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))
        return false;
    // 中断执行线程
    // finally 执行 finishCompletion
    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;
}

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

/**
 * @throws CancellationException {@inheritDoc}
 */
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);
}

继承 Runable 的方法

run

public void run() {
    // 如果 状态不为 NEW 或者 cas更新执行 callable 的线程为当前线程失败，返回
    if (state != NEW ||
        !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                     null, Thread.currentThread()))
        return;
    try {
        Callable<V> c = callable;
        // callable 非空 且 状态为NEW,执行callable的 call 方法。
        // 执行成功， 更新返回值到结果
        // 执行异常，更新异常到结果
        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()
        // 执行结束后， runner 置 null
        runner = null;
        // state must be re-read after nulling runner to prevent
        // leaked interrupts
        // 执行结束后，如果状态为 INTERRUPTING 或 INTERRUPTED， 处理可能的取消动作和中断
        int s = state;
        if (s >= INTERRUPTING)
            handlePossibleCancellationInterrupt(s);
    }
}

/**
 * Executes the computation without setting its result, and then
 * resets this future to initial state, failing to do so if the
 * computation encounters an exception or is cancelled.  This is
 * designed for use with tasks that intrinsically execute more
 * than once.
 * 在不设置计算结果的情况下执行计算，然后将此将来状态重置为初始状态，如果计算遇到异常或被取消，则无法执行此操作。
 * 它设计用于与内部执行多次的任务。
 *
 * @return {@code true} if successfully run and reset
 */
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't set result
                ran = true;
            } catch (Throwable ex) {
                setException(ex);
            }
        }
    } 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
        s = state;
        if (s >= INTERRUPTING)
            handlePossibleCancellationInterrupt(s);
    }
    return ran && s == NEW;
}