【源码阅读计划】浅析 Java 线程池工作原理及核心源码

private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
private static int ctlOf(int rs, int wc) {return rs | wc;} // rs: runState, wc: workerCount

    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;

    private static int runStateOf(int c)     {return c & ~CAPACITY;}
    private static int workerCountOf(int c)  {return c & CAPACITY;}

    public void execute(Runnable command) {if (command == null)
            throw new NullPointerException();
        /*
         * Proceed in 3 steps:
         *
         * 1. If fewer than corePoolSize threads are running, try to
         * start a new thread with the given command as its first
         * task.  The call to addWorker atomically checks runState and
         * workerCount, and so prevents false alarms that would add
         * threads when it shouldn't, by returning false.
         *
         * 2. If a task can be successfully queued, then we still need
         * to double-check whether we should have added a thread
         * (because existing ones died since last checking) or that
         * the pool shut down since entry into this method. So we
         * recheck state and if necessary roll back the enqueuing if
         * stopped, or start a new thread if there are none.
         *
         * 3. If we cannot queue task, then we try to add a new
         * thread.  If it fails, we know we are shut down or saturated
         * and so reject the task.
         */
        int c = ctl.get();                                  // 获取状态表示
        if (workerCountOf(c) < corePoolSize) {               // 1. 如果当前线程数小于核心线程数，直接新建线程执行任务
            if (addWorker(command, true))
                return;
            c = ctl.get();}
        if (isRunning(c) && workQueue.offer(command)) {     // 2. 如果核心线程数已满，且是运行状态并且队列未满，添加任务至队列
            int recheck = ctl.get();
            if (! isRunning(recheck) && remove(command))    // 再次检查运行状态，如果不是运行状态就从队列中删除任务，删除成功后执行拒绝策略，因为此时线程池状态不是 RUNNING
                reject(command);
            else if (workerCountOf(recheck) == 0)           // 如果当前线程数为 0，而我们又刚刚添加了一个任务，就新建一个空任务的线程，它会去轮询任务队列执行刚刚新增的任务
                addWorker(null, false);
        }
        else if (!addWorker(command, false))                // 添加失败，执行拒绝策略
            reject(command);
    }

/**
 * Performs blocking or timed wait for a task, depending on
 * current configuration settings, or returns null if this worker
 * must exit because of any of:
 * 1. There are more than maximumPoolSize workers (due to
 *    a call to setMaximumPoolSize).
 * 2. The pool is stopped.
 * 3. The pool is shutdown and the queue is empty.
 * 4. This worker timed out waiting for a task, and timed-out
 *    workers are subject to termination (that is,
 *    {@code allowCoreThreadTimeOut || workerCount > corePoolSize})
 *    both before and after the timed wait, and if the queue is
 *    non-empty, this worker is not the last thread in the pool.
 *
 * @return task, or null if the worker must exit, in which case
 *         workerCount is decremented
 */
private Runnable getTask() {
    boolean timedOut = false; // 最近一次从阻塞队列中获取任务是否超时？for (;;) {int c = ctl.get();
        int rs = runStateOf(c);

        // Check if queue empty only if necessary.
        // 为 true 的情况：// 1. 线程池为非 RUNNING 状态 且线程池正在停止
        // 2. 线程池状态为非 RUNNING 状态 且阻塞队列为空
        if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {decrementWorkerCount();     // 将 workCount 减 1
            return null;
        }

        int wc = workerCountOf(c);

        // Are workers subject to culling?
        // timed 变量用于判断是否需要进行超时控制。// allowCoreThreadTimeOut 默认是 false，也就是核心线程不允许进行超时；// wc > corePoolSize，表示当前线程池中的线程数量大于核心线程数量；// 对于超过核心线程数量的这些线程，需要进行超时控制
        boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;

        /*
         * wc > maximumPoolSize 的情况是因为可能在此方法执行阶段同时执行了 setMaximumPoolSize 方法；* timed && timedOut 如果为 true，表示当前操作需要进行超时控制，并且上次从阻塞队列中获取任务发生了超时
         * 超时说明队列中获取不到任务，即不需要这么多线程，因此可以适当减少非核心线程
         * 接下来判断，如果有效线程数量大于 1，或者阻塞队列是空的，那么尝试将 workerCount 减 1；* 如果减 1 失败，则返回重试。* 如果 wc == 1 时，也就说明当前线程是线程池中唯一的一个线程了。*/
        if ((wc > maximumPoolSize || (timed && timedOut))
            && (wc > 1 || workQueue.isEmpty())) {if (compareAndDecrementWorkerCount(c)) // 减少非核心线程数量
                return null;
            continue;   // 重试
        }

        try {
            // 从阻塞队列获取任务
            Runnable r = timed ?
                workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) : // poll：等待 keepAliveTime, 若队列为空，返回 null
                workQueue.take(); // take: 若队列为空，直接阻塞 
            if (r != null)
                return r;
            timedOut = true;  // r 为空，表示超时了，返回循环重试
        } catch (InterruptedException retry) {timedOut = false; // 如果获取任务时当前线程发生了中断，则设置 timedOut 为 false 并返回循环重试}
    }
}

/**
 * Class Worker mainly maintains interrupt control state for
 * threads running tasks, along with other minor bookkeeping.
 * This class opportunistically extends AbstractQueuedSynchronizer
 * to simplify acquiring and releasing a lock surrounding each
 * task execution.  This protects against interrupts that are
 * intended to wake up a worker thread waiting for a task from
 * instead interrupting a task being run.  We implement a simple
 * non-reentrant mutual exclusion lock rather than use
 * ReentrantLock because we do not want worker tasks to be able to
 * reacquire the lock when they invoke pool control methods like
 * setCorePoolSize.  Additionally, to suppress interrupts until
 * the thread actually starts running tasks, we initialize lock
 * state to a negative value, and clear it upon start (in
 * runWorker).
 */
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;                                                // 持有的线程
    /** Initial task to run.  Possibly null. */
    Runnable firstTask;                                                 // 第一次执行的任务
    /** Per-thread task counter */
    volatile long completedTasks;                                       // 完成的任务数量

    /**
     * Creates with given first task and thread from ThreadFactory.
     * @param firstTask the first task (null if none)
     */
    Worker(Runnable firstTask) {setState(-1); // state 默认值设为 -1，控制未执行的新建线程不该被中断
        this.firstTask = firstTask;
        this.thread = getThreadFactory().newThread(this);
    }

    /** Delegates main run loop to outer runWorker  */
    public void run() {runWorker(this);
    }

    // Lock methods
    //
    // The value 0 represents the unlocked state.
    // 值为 0 表示未加锁状态(线程空闲)
    // The value 1 represents the locked state.
    // 值为 1 表示锁定状态（线程忙）protected boolean isHeldExclusively() {     // 判断是否被锁定（线程正在执行任务）, 返回 true 表示加锁（排他的）return getState() != 0;}

    protected boolean tryAcquire(int unused) {  // 尝试获取独占锁锁
        if (compareAndSetState(0, 1)) {         // state 为 0 才会成功，不允许重入
            setExclusiveOwnerThread(Thread.currentThread());    // 设置当前线程占有锁
            return true;
        }
        return false;
    }

    protected boolean tryRelease(int unused) {  // 尝试释放锁
        setExclusiveOwnerThread(null);
        setState(0);
        return true;
    }

    public void lock()        { acquire(1); }                   // 获取独占锁，acuire 会调用 tryAcquire，tryAcquire 失败会中断线程
    public boolean tryLock()  { return tryAcquire(1); }
    public void unlock()      { release(1); }                   // 释放独占锁，runWorker 中用来设置允许中断（state+1=0）public boolean isLocked() { return isHeldExclusively(); }   // 检查是否被加锁

    void interruptIfStarted() { // 中断线程
        Thread t;
        // 判断是否可以中断线程：// 线程状态不是 -1（新建状态）且不为空且未被中断，就可以中断线程
        if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
            try {t.interrupt();                      // 中断线程
            } catch (SecurityException ignore) {}}
    }
}

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

        // Check if queue empty only if necessary.
        /*
         * 这个 if 判断
         * 如果 rs >= SHUTDOWN，则表示此时不再接收新任务；* 接着判断以下 3 个条件，只要有 1 个不满足，则返回 false：*  1. rs == SHUTDOWN，这时表示关闭状态，不再接受新提交的任务，但却可以继续处理阻塞队列中已保存的任务
         *  2. firsTask 为空
         *  3. 阻塞队列不为空
         * 
         * 首先考虑 rs == SHUTDOWN 的情况；* 这种情况下不会接受新提交的任务，所以在 firstTask 不为空的时候会返回 false；* 然后，如果 firstTask 为空，并且 workQueue 也为空，则返回 false，* 因为队列中已经没有任务了，不需要再添加线程了
         */
        if (rs >= SHUTDOWN &&
            ! (rs == SHUTDOWN &&
               firstTask == null &&
               ! workQueue.isEmpty()))
            return false;                                                   // 添加失败

        for (;;) {int wc = workerCountOf(c); 
            // 如果 wc 超过 CAPACITY，也就是 ctl 的低 29 位的最大值（二进制是 29 个 1），返回 false；// 这里的 core 是 addWorker 方法的第二个参数，如果为 true 表示根据 corePoolSize 来比较，若为 false 则根据 maximumPoolSize 来比较。if (wc >= CAPACITY ||
                wc >= (core ? corePoolSize : maximumPoolSize))               // 参数 core 在此作用
                return false;
            if (compareAndIncrementWorkerCount(c))                          // CAS 尝试修改 workerCount
                break retry;                                                // 修改成功，退出 retry 代码块
            c = ctl.get();  // Re-read ctl                                  // 修改失败，重新获取 ctl
            if (runStateOf(c) != rs)                                        // 线程池运行状态发生改变，重新执行外层 for 循环
                continue retry;
            // else CAS failed due to workerCount change; retry inner loop
        }
    }
    // 修改成功，执行新建线程操作
    boolean workerStarted = false;
    boolean workerAdded = false;
    Worker w = null;
    try {w = new Worker(firstTask);                                      // 新建 Worker 对象
        final Thread t = w.thread;                                      // 每个 Worker 对象都持有一个线程, 由线程工厂创建
        if (t != null) {                                                // 线程不为空, 互斥添加 Worker 对象
            final ReentrantLock mainLock = this.mainLock;
            mainLock.lock();                                            // 加锁
            try {
                // Recheck while holding lock.
                // Back out on ThreadFactory failure or if
                // shut down before lock acquired.
                int rs = runStateOf(ctl.get());
                // rs < SHUTDOWN 表示是 RUNNING 状态；// 如果 rs 是 RUNNING 状态或者 rs 是 SHUTDOWN 状态并且 firstTask 为 null，向线程池中添加线程
                // 因为在 SHUTDOWN 时不会在添加新的任务，但还是会执行 workQueue 中的任务，所以新增一个无任务的线程可以让其从队列中获取任务
                if (rs < SHUTDOWN ||
                    (rs == SHUTDOWN && firstTask == null)) {if (t.isAlive()) // precheck that t is startable
                        throw new IllegalThreadStateException();
                    workers.add(w);                                     // workers 是一个 HashSet 负责管理 Worker 对象
                    int s = workers.size();
                    if (s > largestPoolSize)                         // 记录线程池中出现的最大的线程数量             
                        largestPoolSize = s;
                    workerAdded = true;
                }
            } finally {mainLock.unlock();                                      // 解锁
            }
            if (workerAdded) {                                          // Worker 对象添加成功，立即执行线程
                t.start();                                              // 启动时会调用 Worker 类中的 run 方法，Worker 本身实现了 Runnable 接口，所以一个 Worker 类型的对象也是一个线程。workerStarted = true;
            }
        }
    } finally {if (! workerStarted)
            addWorkerFailed(w);
    }
    return workerStarted;
}

/**
 * Main worker run loop.  Repeatedly gets tasks from queue and
 * executes them, while coping with a number of issues:
 *
 * 1. We may start out with an initial task, in which case we
 * don't need to get the first one. Otherwise, as long as pool is
 * running, we get tasks from getTask. If it returns null then the
 * worker exits due to changed pool state or configuration
 * parameters.  Other exits result from exception throws in
 * external code, in which case completedAbruptly holds, which
 * usually leads processWorkerExit to replace this thread.
 *
 * 2. Before running any task, the lock is acquired to prevent
 * other pool interrupts while the task is executing, and then we
 * ensure that unless pool is stopping, this thread does not have
 * its interrupt set.
 *
 * 3. Each task run is preceded by a call to beforeExecute, which
 * might throw an exception, in which case we cause thread to die
 * (breaking loop with completedAbruptly true) without processing
 * the task.
 *
 * 4. Assuming beforeExecute completes normally, we run the task,
 * gathering any of its thrown exceptions to send to afterExecute.
 * We separately handle RuntimeException, Error (both of which the
 * specs guarantee that we trap) and arbitrary Throwables.
 * Because we cannot rethrow Throwables within Runnable.run, we
 * wrap them within Errors on the way out (to the thread's
 * UncaughtExceptionHandler).  Any thrown exception also
 * conservatively causes thread to die.
 *
 * 5. After task.run completes, we call afterExecute, which may
 * also throw an exception, which will also cause thread to
 * die. According to JLS Sec 14.20, this exception is the one that
 * will be in effect even if task.run throws.
 *
 * The net effect of the exception mechanics is that afterExecute
 * and the thread's UncaughtExceptionHandler have as accurate
 * information as we can provide about any problems encountered by
 * user code.
 *
 * @param w the worker
 */
final void runWorker(Worker w) {Thread wt = Thread.currentThread();
    Runnable task = w.firstTask;
    w.firstTask = null;
    w.unlock(); // allow interrupts 设置为允许中断
    boolean completedAbruptly = true;   // 异常退出标志
    try {while (task != null || (task = getTask()) != null) {        // getTask 轮询阻塞队列
            w.lock();       // 加锁
            /*
            * 3 个判断:
                * 1、runStateAtLeast(ctl.get(), STOP)为真说明当前状态大于等于 STOP 此时需要给他一个中断信号
                * 2、wt.isInterrupted()查看当前是否设置中断状态如果为 false 则说明为设置中断状态
                * 3、Thread.interrupted() && runStateAtLeast(ctl.get(), STOP) 获取当前中断状态且清除中断状态 
                *    这个判断为真的话说明当前被设置了中断状态 (有可能是线程池执行的业务代码设置的，然后重置了) 且当前状态变成了大于等于 STOP 的状态了
                * 
             * 判断为真的两种情况:
                * 1、如果当前线程大于等于 STOP 且未设置中断状态 整个判断为 true 第一个 runStateAtLeast(ctl.get(), STOP)为 true !wt.isInterrupted()为 true
                * 2、第一次判断的时候不大于 STOP 且当前设置了中断状态 (Thread.interrupted() 把中断状态又刷新了) 且设置完了之后线程池状态大于等于 STOP 了
                *    Thread.interrupted() && runStateAtLeast(ctl.get(), STOP) 为 true !wt.isInterrupted()为 true
                *
            */
            if ((runStateAtLeast(ctl.get(), STOP) || (Thread.interrupted() && runStateAtLeast(ctl.get(), STOP))) 
                && !wt.isInterrupted())
                wt.interrupt();         // 设置中断
            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();         // 释放独占锁}
        }
        completedAbruptly = false;  // 若 while 循环中抛出异常这句就不会被执行，表示为异常退出循环
    } finally {processWorkerExit(w, completedAbruptly);
    }
}

/**
 * Performs cleanup and bookkeeping for a dying worker. Called
 * only from worker threads. Unless completedAbruptly is set,
 * assumes that workerCount has already been adjusted to account
 * for exit.  This method removes thread from worker set, and
 * possibly terminates the pool or replaces the worker if either
 * it exited due to user task exception or if fewer than
 * corePoolSize workers are running or queue is non-empty but
 * there are no workers.
 *
 * @param w the worker
 * @param completedAbruptly if the worker died due to user exception
 */
private void processWorkerExit(Worker w, boolean completedAbruptly) {
    // 如果 completedAbruptly 值为 true，则说明线程执行时出现了异常，需要将 workerCount 减 1；// 如果线程执行时没有出现异常，说明在 getTask()方法中已经已经对 workerCount 进行了减 1 操作，这里就不必再减了。if (completedAbruptly) 
        decrementWorkerCount();

    final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
        completedTaskCount += w.completedTasks; // 统计线程池完成任务数量
        workers.remove(w);                      // 从线程池中移除线程 Worker 对象引用
    } finally {mainLock.unlock();
    }

    tryTerminate(); // 根据线程池状态判断是否结束线程池

    int c = ctl.get();
    // 当线程池状态为 RUNNING 或 SHUTDOWN 时
    // 如果任务为异常结束 completedAbruptly=true, 直接 addWorker 新建线程;
    // 如果 allowCoreThreadTimeOut=true，并且等待队列有任务，至少保留一个 worker；// 如果 allowCoreThreadTimeOut=false，workerCount 不少于 corePoolSize。if (runStateLessThan(c, STOP)) {if (!completedAbruptly) {
            int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
            if (min == 0 && ! workQueue.isEmpty())
                min = 1;
            if (workerCountOf(c) >= min) // 判断当前有效线程是否大于 1，大于的话直接 return，否则会执行 addWorker 函数新建一个线程。return; // replacement not needed
        }
        addWorker(null, false);
    }
}

1. 如果允许核心线程超时，并且阻塞队列中有任务，至少保留一个线程
2. 如果不允许核心线程超时，且 workerCount 不少于 corePoolSize，直接返回。否则新建线程

package ThreadPool;

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * @author Lin YuHang
 * @date 2022/12/1 16:08
*/
public class ThreadPoolDemo {public static void main(String[] args) {
        final int taskCount = 50;
        AtomicInteger integer = new AtomicInteger(0);
        // 初始化线程池
        ThreadPoolExecutor executor = new ThreadPoolExecutor(10,
                30,
                5,
                TimeUnit.SECONDS,
                new ArrayBlockingQueue<Runnable>(30));
        System.out.println(" 总任务数：" + taskCount);
        long start = System.currentTimeMillis();
        // 任务提交
        for (int i = 0; i < taskCount; i++) {Thread thread = new Thread(() -> {
                try {Thread.sleep(500);// 模拟执行耗时
                    System.out.println(" 已执行 " + integer.addAndGet(1) + " 个任务 ");
                } catch (InterruptedException e) {e.printStackTrace();
                }
            });
            try {
                // 注意这里我 try 起来了，默认拒绝策略会报错
                executor.execute(thread);
            } catch (Exception e) {System.out.println(e.getMessage());
            }
        }
        long end = 0;
        while (executor.getCompletedTaskCount() < 50) {end = System.currentTimeMillis();
        }
        System.out.println(" 任务总耗时：" + (end - start));
    }
}