上周因为项目中的线程池参数设置的不合理，引发了一些问题，看了下代码，发现对JUC中的一些概念需要再清晰些。
Runnable @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(); }Runable是一个interface，定义了run()方法，The Runnable interface should be implemented by any class whose instances are intended to be executed by a thread。如果想在其他线程中执行你的task，需要实现这个接口。
Callable @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; }有了Runnable，为啥还需要Callable呢，可以看到Runnable和Callable的两个不同，第一，Runnable是没有返回值的，第二，Runnable是不会抛出checked exception的，而有时候我们需要知道任务执行之后的返回，同时也希望利用异常机制完成一些逻辑。所以有了Callable。
JUC中的Executors这个Factory类，提供了Runnable转Callable的方法。
Futurefuture 是一个inteface，提供了一系列方法，来帮助我们获取异步执行的task的执行状况和执行结果。
FutureTaskFutureTask实现了RunnableFuture接口，即既实现了Runnable接口，又实现了Future接口。所以他有两个功能，第一，作为一个task，提交到别的线程中异步执行，第二，通过future提供的一些接口，获取task的异步执行状态。
/** * 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; 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 */ private Callable<V> callable; /** The result to return or exception to throw from get() */ private Object outcome; // non-volatile, protected by state reads/writes /** The thread running the callable; CASed during run() */ private volatile Thread runner; /** Treiber stack of waiting threads */ private volatile WaitNode waiters;看下FutureTask的几个属性，首先state表示当前task的执行状态，其中，开始状态位NEW表示task还没开始执行。NORMAL，CANCELLED，INTERRUPTED为终态，COMPLETING和INTERRUPTING为临时状态，最终会通过上面的几个状态转移路径，转移到终态。
callable，表示具体执行的任务。
outcome， task 执行的返回结果
runner，执行这个task的线程
waiters，通过get方法获取此task执行结果被阻塞的线程。
看下几个核心的方法，我们知道，futuretask提交到别的线程里后，最终会调用task的run方法执行具体逻辑。
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); } }run方法执行时，首先检查当前的状态是否是NEW，如果不是NEW说明已经被执行过了。开始执行之前，标记执行当前task的线程到runner。
调用callable的run方法，执行。抛异常时，设置setException。正常结束时，set结果。看下这两步里都会调到的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 }这里主要是在通知所有阻塞在watch这个task结果的线程，通知他们当前task已经执行结束了。