一、线程池状态

　　 private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
    private static final int COUNT_BITS = Integer.SIZE - 3;
    private static final int CAPACITY   = (1 << COUNT_BITS) - 1;

    // runState is stored in the high-order bits
    private static final int RUNNING    = -1 << COUNT_BITS;
    private static final int SHUTDOWN   =  0 << COUNT_BITS;
    private static final int STOP       =  1 << COUNT_BITS;
    private static final int TIDYING    =  2 << COUNT_BITS;
    private static final int TERMINATED =  3 << COUNT_BITS;

    // Packing and unpacking ctl
    private static int runStateOf(int c)     { return c & ~CAPACITY; }
    private static int workerCountOf(int c)  { return c & CAPACITY; }
    private static int ctlOf(int rs, int wc) { return rs | wc; }

　　RUNNING :  该状态的线程池会接收新的任务，并处理阻塞队列中的任务。

　　SHUTDOWN : 该状态的线程池不会接收新的任务，但会处理阻塞队列中的任务。

　　STOP : 该状态的线程池不会接收新的任务，也不会处理阻塞队列中的任务，而且会中断正在执行的任务。

二、任务提交 方式

　　　1、execute

　　　　提交的任务必须实现Runnable接口，接口不带返回值

public void execute(Runnable command) {

　　　2、submit

　　  　　父类AbstractExecutorService提供有submit接口，可获取线程执行返回值。　　　　

 public Future<?> submit(Runnable task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<Void> ftask = newTaskFor(task, null);
        execute(ftask);
        return ftask;
    }

public <T> Future<T> submit(Callable<T> task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<T> ftask = newTaskFor(task);
        execute(ftask);
        return ftask;
    }

三、任务执行 — execute

　　execute  方法

public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        int c = ctl.get();
        if (workerCountOf(c) < corePoolSize) {
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        if (isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();
            if (! isRunning(recheck) && remove(command))
                reject(command);
            else if (workerCountOf(recheck) == 0)
                addWorker(null, false);
        }
        else if (!addWorker(command, false))
            reject(command);
    }

　　大致流程为：

　　　　1、通过workerCountOf方法得到线程池的当前线程数，如果当前线程数小于corePoolSize，则执行addWorker方法创建一个新的核心线程执行任务。

　　　　2、如果当前线程数大于等于corePoolSize时，检查线程池的运行状态，如果线程池运行状态为RUNNING，则尝试将任务加入阻塞队列。

　　　　3、再次检查线程池的运行状态，如果运行状态不为RUNNING，则从阻塞队列中删除任务并执行reject方法调用处理机制。

　　　　4、在2的基础上，如果加入阻塞队列失败，则会执行addWorker方法创建一个新的非核心线程执行任务。

　　　　5、在3的基础上，如果addWorker执行失败，则会调用reject调用处理机制。

　　addWorker方法

private boolean addWorker(Runnable firstTask, boolean core) {
        retry:
        for (;;) {
            int c = ctl.get();
            int rs = runStateOf(c);

            // Check if queue empty only if necessary.
            if (rs >= SHUTDOWN &&
                ! (rs == SHUTDOWN &&
                   firstTask == null &&
                   ! workQueue.isEmpty()))
                return false;

            for (;;) {
                int wc = workerCountOf(c);
                if (wc >= CAPACITY ||
                    wc >= (core ? corePoolSize : maximumPoolSize))
                    return false;
                if (compareAndIncrementWorkerCount(c))
                    break retry;
                c = ctl.get();  // Re-read ctl
                if (runStateOf(c) != rs)
                    continue retry;
                // else CAS failed due to workerCount change; retry inner loop
            }
        }

        boolean workerStarted = false;
        boolean workerAdded = false;
        Worker w = null;
        try {
            w = new Worker(firstTask);
            final Thread t = w.thread;
            if (t != null) {
                final ReentrantLock mainLock = this.mainLock;
                mainLock.lock();
                try {
                    // Recheck while holding lock.
                    // Back out on ThreadFactory failure or if
                    // shut down before lock acquired.
                    int rs = runStateOf(ctl.get());

                    if (rs < SHUTDOWN ||
                        (rs == SHUTDOWN && firstTask == null)) {
                        if (t.isAlive()) // precheck that t is startable
                            throw new IllegalThreadStateException();
                        workers.add(w);
                        int s = workers.size();
                        if (s > largestPoolSize)
                            largestPoolSize = s;
                        workerAdded = true;
                    }
                } finally {
                    mainLock.unlock();
                }
                if (workerAdded) {
                    t.start();
                    workerStarted = true;
                }
            }
        } finally {
            if (! workerStarted)
                addWorkerFailed(w);
        }
        return workerStarted;
    }

　　　　大致流程为：

　　　　　　1、自旋检测线程池状态，如果状态大于SHUTDOWN，或者 firstTask为空 或队列为空 时，返回任务加入队列失败。

　　　　　　2、获取线程池当前线程数，通过core判断是否是创建核心线程，如果为true，并且当前线程数wc小于corePoolSize时，跳出循环创建新的线程。如果core为false,

　　　　　　　　则判断当前线程数wc是否小于maximumPoolSize，小于跳出循环。

　　　　　　3、线程池的工作线程时候通过Worker实现的，通过ReentrantLock加锁，再次通过线程池状态监测之后，将worker加入到HashSet<Worker> workers 里面

　　　　　　4、如果加入成功，则启动Worker中的线程。

　　Worker类

private final class Worker
        extends AbstractQueuedSynchronizer
        implements Runnable
    {
        /**
         * This class will never be serialized, but we provide a
         * serialVersionUID to suppress a javac warning.
         */
        private static final long serialVersionUID = 6138294804551838833L;

        /** Thread this worker is running in.  Null if factory fails. */
        final Thread thread;
        /** Initial task to run.  Possibly null. */
        Runnable firstTask;
        /** Per-thread task counter */
        volatile long completedTasks;

        /**
         * Creates with given first task and thread from ThreadFactory.
         * @param firstTask the first task (null if none)
         */
        Worker(Runnable firstTask) {
            setState(-1); // inhibit interrupts until runWorker
            this.firstTask = firstTask;
            this.thread = getThreadFactory().newThread(this);
        }

　　　　Worker类继承了AbstractQueuedSynchronizer（AQS）类，可以方便的实现工作线程的中止操作。

　　　　并且本身实现了Runnable接口，可单独作为任务在工作线程中执行。

　　runWorker 方法

final void runWorker(Worker w) {
        Thread wt = Thread.currentThread();
        Runnable task = w.firstTask;
        w.firstTask = null;
        w.unlock(); // allow interrupts
        boolean completedAbruptly = true;
        try {
            while (task != null || (task = getTask()) != null) {
                w.lock();
                // If pool is stopping, ensure thread is interrupted;
                // if not, ensure thread is not interrupted.  This
                // requires a recheck in second case to deal with
                // shutdownNow race while clearing interrupt
                if ((runStateAtLeast(ctl.get(), STOP) ||
                     (Thread.interrupted() &&
                      runStateAtLeast(ctl.get(), STOP))) &&
                    !wt.isInterrupted())
                    wt.interrupt();
                try {
                    beforeExecute(wt, task);
                    Throwable thrown = null;
                    try {
                        task.run();
                    } catch (RuntimeException x) {
                        thrown = x; throw x;
                    } catch (Error x) {
                        thrown = x; throw x;
                    } catch (Throwable x) {
                        thrown = x; throw new Error(x);
                    } finally {
                        afterExecute(task, thrown);
                    }
                } finally {
                    task = null;
                    w.completedTasks++;
                    w.unlock();
                }
            }
            completedAbruptly = false;
        } finally {
            processWorkerExit(w, completedAbruptly);
        }
    }

　　runWorker流程：

　　1、线程启动之后，通过unlock方法释放锁，设置AQS的state为0，表示运行中断；

　　2、获取第一个任务firstTask，并执行task的run方法，在执行run方法前，会对Worker加锁，任务执行完释放锁。

　　3、在任务执行前后，可根据业务自定义实现beforeExecute(wt, task); 和 afterExecute(task, thrown);。

　　4、任务执行完之后，调用getTask从阻塞队列中获取等待的任务，如果队列中没有任务，getTask方法会被阻塞并挂起，不会占用CPU资源。

　　getTask方法

private Runnable getTask() {
        boolean timedOut = false; // Did the last poll() time out?

        for (;;) {
            int c = ctl.get();
            int rs = runStateOf(c);

            // Check if queue empty only if necessary.
            if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
                decrementWorkerCount();
                return null;
            }

            int wc = workerCountOf(c);

            // Are workers subject to culling?
            boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;

            if ((wc > maximumPoolSize || (timed && timedOut))
                && (wc > 1 || workQueue.isEmpty())) {
                if (compareAndDecrementWorkerCount(c))
                    return null;
                continue;
            }

            try {
                Runnable r = timed ?
                    workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                    workQueue.take();
                if (r != null)
                    return r;
                timedOut = true;
            } catch (InterruptedException retry) {
                timedOut = false;
            }
        }
    }

　　getTask流程：

Runnable r = timed ?
                    workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                    workQueue.take();

　　1、如果设定了超时机制，则通过 workQueue.poll（）方法来获取阻塞队列中的任务，如果队列中没有任务，则会在keepAliveTime时间后返回null。

　　2、如果未设置超时机制，并且当前线程数小于核心线程时，同时未设置允许核心线程超时的情况下，通过workQueue.take(); 方法来获取阻塞队列中的任务，如果没有任务，

　　　　则会一直等待并挂起，直到有新任务提交时，则会环信等待的队列并返回新的任务。

　　3、阻塞队列使用生产者与消费者模式，使用等待与唤醒使线程池线程挂起与唤起。

四、任务执行 -- submit

　　　　submit重载了多种实现方式

　　　　1、Callable　

public <T> Future<T> submit(Callable<T> task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<T> ftask = newTaskFor(task);
        execute(ftask);
        return ftask;
    }

　　　　2、Runnable

public <T> Future<T> submit(Runnable task, T result) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<T> ftask = newTaskFor(task, result);
        execute(ftask);
        return ftask;
    }

　　在实际业务中，Future和Callable是成双出现的，Callable负责产生结果，Future负责获取结果。

　　1、Callable类似于Runnable，只是Callable附带返回值。

　　2、Callable除了正常返回之外，如果线程出现异常，该异常也会返回，即Future的get方法可以获取到异常结果。

　　3、Future的get()方法会导致主线程阻塞，直到Callable执行完成。

　　FutureTask

　　futureTask内部状态

 * 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;

　　　　FutureTask 实现了Runnable接口，提交的任务可以交由工作线程处理，执行run方法。

　　get方法

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

　　调用get方法时，如果task的状态处于执行中或初始化，调用awaitDone方法对线程进行阻塞。

　　awaitDone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);
        }
    }

　　通过对Task的状态检测，如果Callable未执行完成，使用  LockSupport.park(this); 对当前线程进行阻塞。等待唤起，并将主线程封装成WaitNode 并存放在 waiters 链表中。

　　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 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方法流程：

　　　　　　通过对task的state判断，如果task为初始New状态，则执行call方法，获取call方法返回结果，并调用set方法

　　　　　　如果执行失败，则调用setException方法。

setException方法

　　　设置状态  EXCEPTIONAL

protected void setException(Throwable t) {
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            outcome = t;
            UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
            finishCompletion();
        }
    }

　　　set方法  

　　　　设置状态  NORMAL

protected void set(V v) {
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            outcome = v;
            UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
            finishCompletion();
        }
    }

　　finishCompletion();方法

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
    }

　　如果finishCompletion 检测到 通过get方法被阻塞的线程集 waiters 不为空时，获取的每一个节点，并使用   LockSupport.unpark(t); 对其唤醒。

　　最终使用report返回结果。