// 創(chuàng)建一個核心線程數(shù)為5，最大線程數(shù)為10，空閑線程存活時間為60s的線程池對象
val threadPoolExecutor = ThreadPoolExecutor(
    5, 10, 60,
    TimeUnit.MINUTES,
    ArrayBlockingQueue<Runnable>(100),
    RejectedExecutionHandler { _, _ -> println("reject submit thread to thread pool") }
)
 
// 測試
for (i in 1..10) {
    threadPoolExecutor.execute { println("execute thread is:${Thread.currentThread().name}") }
}
 
// 結(jié)果
// execute thread is:pool-1-thread-1
// execute thread is:pool-1-thread-1
// execute thread is:pool-1-thread-1
// execute thread is:pool-1-thread-1
// execute thread is:pool-1-thread-5
// execute thread is:pool-1-thread-5
// execute thread is:pool-1-thread-4
// execute thread is:pool-1-thread-3
// execute thread is:pool-1-thread-2
// execute thread is:pool-1-thread-1

/**
     * 創(chuàng)建一個ThreadPoolExecutor對象
     *
     * @param corePoolSize 核心線程數(shù)，這些線程會一直在線程池中，除非設(shè)置了 allowCoreThreadTimeOut
     * @param maximumPoolSize 最大線程數(shù)，運行線程創(chuàng)建的最大值
     * @param keepAliveTime 當線程數(shù)>核心線程數(shù)的時候，這個值就是空閑且非核心線程存活的時間
     * @param unit keepAliveTime的單位
     * @param workQueue 保存task的隊列，直到執(zhí)行execute()方法執(zhí)行
     * @param threadFactory ThreadFactory是一個接口，里面只有Thread newThread(Runnable r)方法，用來創(chuàng)建線程，
     *                      默認采用Executors.defaultThreadFactory()
     * @param handler 拒絕處理任務(wù)時的策略，如果線程池滿了且所有線程都不處于空閑狀態(tài)，
     *                通過RejectedExecutionHandler接口的rejectedExecution(Runnable r, ThreadPoolExecutor executor)來處理傳進來的Runnable
     *                系統(tǒng)提供了四種：CallerRunsPolicy(), AbortPolicy(), DiscardPolicy(), DiscardOldestPolicy()
     *                默認采用new AbortPolicy()
     */
    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue,
                              ThreadFactory threadFactory,
                              RejectedExecutionHandler handler){
        if (corePoolSize < 0 ||
                maximumPoolSize <= 0 ||
                maximumPoolSize < corePoolSize ||
                keepAliveTime < 0)
            throw new IllegalArgumentException();
        if (workQueue == null || threadFactory == null || handler == null)
            throw new NullPointerException();
        this.acc = System.getSecurityManager() == null ?
                null :
                AccessController.getContext();
        this.corePoolSize = corePoolSize;
        this.maximumPoolSize = maximumPoolSize;
        this.workQueue = workQueue;
        this.keepAliveTime = unit.toNanos(keepAliveTime);
        this.threadFactory = threadFactory;
        this.handler = handler;
    }

// 靜態(tài)內(nèi)部類
static class DefaultThreadFactory implements ThreadFactory {
    // 線程池的標識，從1開始沒創(chuàng)建一個線程池+1
    private static final AtomicInteger poolNumber = new AtomicInteger(1);
    // 線程組
    private final ThreadGroup group;
    // 線程名中的結(jié)尾標識，從1開始每創(chuàng)建一個線程+1
    private final AtomicInteger threadNumber = new AtomicInteger(1);
    // 線程名
    private final String namePrefix;
 
    DefaultThreadFactory() {
        SecurityManager s = System.getSecurityManager();
        group = (s != null) ? s.getThreadGroup() :
                              Thread.currentThread().getThreadGroup();
        namePrefix = "pool-" +
                      poolNumber.getAndIncrement() +
                     "-thread-";
    }
 
    public Thread newThread(Runnable r) {
        Thread t = new Thread(group, r,
                              namePrefix + threadNumber.getAndIncrement(),
                              0);
        if (t.isDaemon())
            t.setDaemon(false);
        if (t.getPriority() != Thread.NORM_PRIORITY)
            t.setPriority(Thread.NORM_PRIORITY);
        return t;
    }
}

public static class CallerRunsPolicy implements RejectedExecutionHandler {
    /**
     * Creates a {@code CallerRunsPolicy}.
     */
    public CallerRunsPolicy() {
    }
 
    /**
     * 如果線程池還沒關(guān)閉，那么就再次執(zhí)行這個Runnable
     */
    public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
        if (!e.isShutdown()) {
            r.run();
        }
    }
}

public static class AbortPolicy implements RejectedExecutionHandler {
    /**
     * Creates an {@code AbortPolicy}.
     */
    public AbortPolicy() {
    }
 
    /**
     * 這個策略就是拋出異常，不做其他處理
     */
    public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
        throw new RejectedExecutionException("Task " + r.toString() +
                " rejected from " +
                e.toString());
    }
}

public static class DiscardPolicy implements RejectedExecutionHandler {
    /**
     * Creates a {@code DiscardPolicy}.
     */
    public DiscardPolicy() {
    }
 
    /**
     * 什么也不做，也就是拋棄了這個Runnable
     */
    public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
    }
}

public static class DiscardOldestPolicy implements RejectedExecutionHandler {
    /**
     * Creates a {@code DiscardOldestPolicy} for the given executor.
     */
    public DiscardOldestPolicy() {
    }
 
    /**
     * 1. 線程池未關(guān)閉
     * 2. 獲取隊列中的下一個Runnable
     * 3. 獲取到了，但是不對它進行處理，也就是拋棄它
     * 4. 執(zhí)行我們傳過來的這個Runnable
     */
    public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
        if (!e.isShutdown()) {
            e.getQueue().poll();
            e.execute(r);
        }
    }
}

public class ThreadPoolExecutor extends AbstractExecutorService {
 
    // ctl 的低29位表示線程池中的線程數(shù)，高3位表示當前線程狀態(tài)
    private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
    // 29
    private static final int COUNT_BITS = Integer.SIZE - 3;
    // (2^29) -1
    private static final int CAPACITY   = (1 << COUNT_BITS) - 1;
 
    // 運行狀態(tài)：接受新任務(wù)并處理排隊的任務(wù)
    private static final int RUNNING    = -1 << COUNT_BITS;
    // 關(guān)閉狀態(tài)：不接受新任務(wù)，但處理排隊的任務(wù)
    private static final int SHUTDOWN   =  0 << COUNT_BITS;
    // 停止狀態(tài)：不接受新任務(wù)，不處理排隊的任務(wù)，中斷正在進行的任務(wù)
    private static final int STOP       =  1 << COUNT_BITS;
    // 整理狀態(tài)：整理狀態(tài)，所有任務(wù)已終止，workerCount為零，線程將運行terminate()方法
    private static final int TIDYING    =  2 << COUNT_BITS;
    // 終止狀態(tài)：terminate()方法執(zhí)行完成
    private static final int TERMINATED =  3 << COUNT_BITS;
 
    // 表示線程是否允許或停止
    private static int runStateOf(int c)     { return c & ~CAPACITY; }
    // 線程的有效數(shù)量
    private static int workerCountOf(int c)  { return c & CAPACITY; }
    private static int ctlOf(int rs, int wc) { return rs | wc; }
    
    ......后面的源碼暫時省略
}

public void execute(Runnable command) {
    if (command == null)
        throw new NullPointerException();
    int c = ctl.get();
    // 如果運行中的線程數(shù)小于核心線程數(shù)，執(zhí)行addWorker(command, true)創(chuàng)建新的核心Thread執(zhí)行任務(wù)
    if (workerCountOf(c) < corePoolSize) {
        if (addWorker(command, true))
            return;
        c = ctl.get();
    }
    // 1. 已經(jīng)滿足：運行中的線程數(shù)大于核心線程數(shù)，但是小于最大線程數(shù)
    // 2. 需要滿足：線程池在運行狀態(tài)
    // 3. 需要滿足：添加到工作隊列中成功
    if (isRunning(c) && workQueue.offer(command)) {
        int recheck = ctl.get();
        // 如果線程不在運行狀態(tài)，就從工作隊列中移除command
        // 并且執(zhí)行拒絕策略
        if (!isRunning(recheck) && remove(command))
            reject(command);
        // 線程池處于運行狀態(tài)，但是沒有線程，則addWorker(null, false)
        // 至于這里為什么要傳入一個null，因為在最外層的if條件中我們已經(jīng)將Runnable添加到工作隊列中了
        // 而且在runWorker()源碼中也可以得到答案，如果傳入的Runnable為空，就會去工作隊列中取task。
        else if (workerCountOf(recheck) == 0)
            addWorker(null, false);
    }
    // 執(zhí)行addWorker()創(chuàng)建新的非核心線程Thread執(zhí)行任務(wù)
    // addWorker() 失敗，執(zhí)行拒絕策略
    else if (!addWorker(command, false))
        reject(command);
}

private boolean addWorker(Runnable firstTask, boolean core) {
    // 標記外循環(huán)，比如在內(nèi)循環(huán)中break retry就直接跳出外循環(huán)
    retry:
    for (; ; ) {
        int c = ctl.get();
        int rs = runStateOf(c);
 
        // 直接返回false有以下3種情況：
        // 1. 線程池狀態(tài)為STOP、TIDYING、TERMINATED
        // 2. 線程池狀態(tài)不是running狀態(tài)，并且firstTask不為空
        // 3. 線程池狀態(tài)不是running狀態(tài)，并且工作隊列為空
        if (rs >= SHUTDOWN &&
                !(rs == SHUTDOWN && firstTask == null && !workQueue.isEmpty()))
            return false;
 
        for (; ; ) {
            int wc = workerCountOf(c);
            // 如果添加的是核心線程，但是運行的線程數(shù)大于等于核心線程數(shù)，那么就不添加了，直接返回
            // 如果添加的是非核心線程，但是運行的線程數(shù)大于等于最大線程數(shù)，那么也不添加，直接返回
            if (wc >= CAPACITY ||
                    wc >= (core ? corePoolSize : maximumPoolSize))
                return false;
            // 增加workerCount的值 +1
            if (compareAndIncrementWorkerCount(c))
                // 跳出外循環(huán)
                break retry;
            c = ctl.get();  // 重新檢查線程池狀態(tài)
            if (runStateOf(c) != rs)
                continue retry;
            // 重新檢查的狀態(tài)和之前不合，再次從外循環(huán)進入
        }
    }
 
    boolean workerStarted = false;
    boolean workerAdded = false;
    Worker w = null;
    try {
        w = new Worker(firstTask);
        final Thread t = w.thread;
        if (t != null) {
            // 線程池重入鎖
            final ReentrantLock mainLock = this.mainLock;
            // 獲得鎖
            mainLock.lock();
            try {
                // Recheck while holding lock.
                // Back out on ThreadFactory failure or if
                // shut down before lock acquired.
                int rs = runStateOf(ctl.get());
                // 線程池在運行狀態(tài)或者是線程池關(guān)閉同時Runnable也為空
                if (rs < SHUTDOWN ||
                        (rs == SHUTDOWN && firstTask == null)) {
                    if (t.isAlive()) // precheck that t is startable
                        throw new IllegalThreadStateException();
                    // 想Worker中添加新的Worker
                    workers.add(w);
                    int s = workers.size();
                    if (s > largestPoolSize)
                        largestPoolSize = s;
                    workerAdded = true;
                }
            } finally {
                // 釋放鎖
                mainLock.unlock();
            }
            // 如果添加成功，啟動線程
            if (workerAdded) {
                t.start();
                workerStarted = true;
            }
        }
    } finally {
        if (!workerStarted)
            addWorkerFailed(w);
    }
    return workerStarted;
}

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;
        // 這邊是把Runnable傳給了Thread，也就是說Thread.run()就是執(zhí)行了下面的run()方法
        this.thread = getThreadFactory().newThread(this);
    }
 
    /**
     * Delegates main run loop to outer runWorker
     */
    public void run() {
        runWorker(this);
    }
}

final void runWorker(Worker w) {
        Thread wt = Thread.currentThread();
        Runnable task = w.firstTask;
        w.firstTask = null;
        w.unlock(); // 釋放鎖，允許中斷
        boolean completedAbruptly = true;
        try {
            // 1. worker中的task不為空
            // 2. 如果worker的task為空，那么取WorkerQueue的task
            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 {
                    // 這是一個空方法，可由子類實現(xiàn)
                    beforeExecute(wt, task);
                    Throwable thrown = null;
                    try {
                        // 執(zhí)行task
                        task.run();
                    } 
                    .... 省略
                    // 這是一個空方法，可由子類實現(xiàn)
                    finally {
                        afterExecute(task, thrown);
                    }
                } finally {
                    task = null;
                    w.completedTasks++;
                    w.unlock();
                }
            }
            completedAbruptly = false;
        } finally {
            processWorkerExit(w, completedAbruptly);
        }
    }

```java
private Runnable getTask() {
    // 進入死循環(huán)
    for (; ; ) {
        try {
            // 為true的條件：
            // allowCoreThreadTimeOut=true: 核心線程需根據(jù)keepAliveTime超時等待
            // 核心線程數(shù)已滿
            boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
            // 如果timed為true，執(zhí)行BlockQueue.poll()，這個操作在取不到task的時候會等待keepAliveTime，然后返回null
            // 如果timed為false，執(zhí)行BlockQueue.take()，這個操作在隊列為空的時候一直阻塞
            Runnable r = timed ?
                    workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                    workQueue.take();
            if (r != null)
                return r;
        }
    }
}
```