一篇文章带你搞懂Java线程池实现原理

public class ThreadPoolDemo {

    public static void main(String[] args) {
        // 1. 创建线程池
        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(
                3,
                3,
                0L,
                TimeUnit.MILLISECONDS,
                new LinkedBlockingQueue<>(),
                Executors.defaultThreadFactory(),
                new ThreadPoolExecutor.AbortPolicy());
      
        // 2. 往线程池中提交3个任务
        for (int i = 0; i < 3; i++) {
            threadPoolExecutor.execute(() -> {
                System.out.println(Thread.currentThread().getName() + " 关注公众号：一灯架构");
            });
        }
      
        // 3. 关闭线程池
        threadPoolExecutor.shutdown();
    }
}

public class ThreadPoolExecutor extends AbstractExecutorService {

    // 线程池的控制状态，Integer长度是32位，前3位用来存储线程池状态，后29位用来存储线程数量
    private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
    // 线程个数所占的位数
    private static final int COUNT_BITS = Integer.SIZE - 3;
    // 线程池的最大容量，2^29-1，约5亿个线程
    private static final int CAPACITY = (1 << COUNT_BITS) - 1;

    // 独占锁，用来控制多线程下的并发操作
    private final ReentrantLock mainLock = new ReentrantLock();
    // 工作线程的集合
    private final HashSet<Worker> workers = new HashSet<>();
    // 等待条件，用来响应中断
    private final Condition termination = mainLock.newCondition();
    // 是否允许回收核心线程
    private volatile boolean allowCoreThreadTimeOut;
    // 线程数的历史峰值
    private int largestPoolSize;

    
    private volatile int corePoolSize;
    private volatile int maximumPoolSize;
    private volatile long keepAliveTime;
    private final BlockingQueue<Runnable> workQueue;
    private volatile ThreadFactory threadFactory;
    private volatile RejectedExecutionHandler handler;

}

// 往线程池中提交任务
public void execute(Runnable command) {
    // 1. 判断提交的任务是否为null
    if (command == null)
        throw new NullPointerException();

    int c = ctl.get();
    // 2. 判断线程数是否小于核心线程数
    if (workerCountOf(c) < corePoolSize) {
        // 3. 把任务包装成worker，添加到worker集合中
        if (addWorker(command, true))
            return;
        c = ctl.get();
    }
    // 4. 判断如果线程数不小于corePoolSize，并且可以添加到阻塞队列
    if (isRunning(c) && workQueue.offer(command)) {
        // 5. 重新检查线程池状态，如果线程池不是运行状态，就移除刚才添加的任务，并执行拒绝策略
        int recheck = ctl.get();
        if (!isRunning(recheck) && remove(command))
            reject(command);
        // 6. 判断如果线程数是0，就创建非核心线程（任务是null，会从阻塞队列中拉取任务）
        else if (workerCountOf(recheck) == 0)
            addWorker(null, false);
    }
    // 7. 如果添加阻塞队列失败，就创建一个Worker
    else if (!addWorker(command, false))
        // 8. 如果创建Worker失败说明已经达到最大线程数了，则执行拒绝策略
        reject(command);
}

// 添加worker
private boolean addWorker(Runnable firstTask, boolean core) {
    retry:
    for (; ; ) {
        int c = ctl.get();
        int rs = runStateOf(c);
        // 1. 检查是否允许提交任务
        if (rs >= SHUTDOWN &&
                !(rs == SHUTDOWN &&
                        firstTask == null &&
                        !workQueue.isEmpty()))
            return false;
        // 2. 使用死循环保证添加线程成功
        for (; ; ) {
            int wc = workerCountOf(c);
            // 3. 校验线程数是否超过容量限制
            if (wc >= CAPACITY ||
                    wc >= (core ? corePoolSize : maximumPoolSize))
                return false;
            // 4. 使用CAS修改线程数
            if (compareAndIncrementWorkerCount(c))
                break retry;
            c = ctl.get();
            // 5. 如果线程池状态变了，则从头再来
            if (runStateOf(c) != rs)
                continue retry;
        }
    }
    boolean workerStarted = false;
    boolean workerAdded = false;
    Worker w = null;
    try {
        // 6. 把任务和新线程包装成一个worker
        w = new Worker(firstTask);
        final Thread t = w.thread;
        if (t != null) {
            // 7. 加锁，控制并发
            final ReentrantLock mainLock = this.mainLock;
            mainLock.lock();
            try {
                // 8. 再次校验线程池状态是否异常
                int rs = runStateOf(ctl.get());
                if (rs < SHUTDOWN ||
                        (rs == SHUTDOWN && firstTask == null)) {
                    // 9. 如果线程已经启动，就抛出异常
                    if (t.isAlive())
                        throw new IllegalThreadStateException();
                    // 10. 添加到worker集合中
                    workers.add(w);
                    int s = workers.size();
                    // 11. 记录线程数历史峰值
                    if (s > largestPoolSize)
                        largestPoolSize = s;
                    workerAdded = true;
                }
            } finally {
                mainLock.unlock();
            }
            if (workerAdded) {
                // 12. 启动线程
                t.start();
                workerStarted = true;
            }
        }
    } finally {
        if (!workerStarted)
            addWorkerFailed(w);
    }
    return workerStarted;
}

private final class Worker
        extends AbstractQueuedSynchronizer
        implements Runnable {
    // 工作线程
    final Thread thread;
    // 任务
    Runnable firstTask;

    // 创建worker，并创建一个新线程（用来执行任务）
    Worker(Runnable firstTask) {
        setState(-1);
        this.firstTask = firstTask;
        this.thread = getThreadFactory().newThread(this);
    }
}

// 线程执行入口
public void run() {
    runWorker(this);
}

// 线程运行核心方法
final void runWorker(Worker w) {
    Thread wt = Thread.currentThread();
    Runnable task = w.firstTask;
    w.firstTask = null;
    w.unlock();
    boolean completedAbruptly = true;
    try {
        // 1. 如果当前worker中任务是null，就从阻塞队列中获取任务
        while (task != null || (task = getTask()) != null) {
            // 加锁，保证thread不被其他线程中断（除非线程池被中断）
            w.lock();
            // 2. 校验线程池状态，是否需要中断当前线程
            if ((runStateAtLeast(ctl.get(), STOP) ||
                    (Thread.interrupted() &&
                            runStateAtLeast(ctl.get(), STOP))) &&
                    !wt.isInterrupted())
                wt.interrupt();
            try {
                beforeExecute(wt, task);
                Throwable thrown = null;
                try {
                    // 3. 执行run方法
                    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++;
                // 解锁
                w.unlock();
            }
        }
        completedAbruptly = false;
    } finally {
        // 4. 从worker集合删除当前worker
        processWorkerExit(w, completedAbruptly);
    }
}

// 从阻塞队列中拉取任务
private Runnable getTask() {
    boolean timedOut = false;
    for (; ; ) {
        int c = ctl.get();
        int rs = runStateOf(c);
        // 1. 如果线程池已经停了，或者阻塞队列是空，就回收当前线程
        if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
            decrementWorkerCount();
            return null;
        }
        int wc = workerCountOf(c);
        // 2. 再次判断是否需要回收线程
        boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
        if ((wc > maximumPoolSize || (timed && timedOut))
                && (wc > 1 || workQueue.isEmpty())) {
            if (compareAndDecrementWorkerCount(c))
                return null;
            continue;
        }
        try {
            // 3. 从阻塞队列中拉取任务
            Runnable r = timed ?
                    workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                    workQueue.take();
            if (r != null)
                return r;
            timedOut = true;
        } catch (InterruptedException retry) {
            timedOut = false;
        }
    }
}