怎樣解析完整的ThreadPoolExecutor

怎樣解析完整的ThreadPoolExecutor，針對這個(gè)問題，這篇文章詳細(xì)介紹了相對應(yīng)的分析和解答，希望可以幫助更多想解決這個(gè)問題的小伙伴找到更簡單易行的方法。

ThreadPoolExecutor源碼解析

今天為了給一個(gè)朋友做一份文檔，從源碼層級解析一下ThreadPoolExecutor。然后就直接在源碼上寫備注的形式解析。

1、常用變量的解釋

// 1. `ctl`，可以看做一個(gè)int類型的數(shù)字，高3位表示線程池狀態(tài)，低29位表示worker數(shù)量
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
// 2. `COUNT_BITS`，`Integer.SIZE`為32，所以`COUNT_BITS`為29
private static final int COUNT_BITS = Integer.SIZE - 3;
// 3. `CAPACITY`，線程池允許的最大線程數(shù)。1左移29位，然后減1，即為 2^29 - 1
private static final int CAPACITY   = (1 << COUNT_BITS) - 1;

// runState is stored in the high-order bits
// 4. 線程池有5種狀態(tài)，按大小排序如下：RUNNING < SHUTDOWN < STOP < TIDYING < TERMINATED
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
// 5. `runStateOf()`，獲取線程池狀態(tài)，通過按位與操作，低29位將全部變成0
private static int runStateOf(int c)     { return c & ~CAPACITY; }
// 6. `workerCountOf()`，獲取線程池worker數(shù)量，通過按位與操作，高3位將全部變成0
private static int workerCountOf(int c)  { return c & CAPACITY; }
// 7. `ctlOf()`，根據(jù)線程池狀態(tài)和線程池worker數(shù)量，生成ctl值
private static int ctlOf(int rs, int wc) { return rs | wc; }

/*
 * Bit field accessors that don't require unpacking ctl.
 * These depend on the bit layout and on workerCount being never negative.
 */
// 8. `runStateLessThan()`，線程池狀態(tài)小于xx
private static boolean runStateLessThan(int c, int s) {
    return c < s;
}
// 9. `runStateAtLeast()`，線程池狀態(tài)大于等于xx
private static boolean runStateAtLeast(int c, int s) {
    return c >= s;
}

2、構(gòu)造方法

public ThreadPoolExecutor(int corePoolSize,
                          int maximumPoolSize,
                          long keepAliveTime,
                          TimeUnit unit,
                          BlockingQueue<Runnable> workQueue,
                          ThreadFactory threadFactory,
                          RejectedExecutionHandler handler) {
    // 基本類型參數(shù)校驗(yàn)
    if (corePoolSize < 0 ||
        maximumPoolSize <= 0 ||
        maximumPoolSize < corePoolSize ||
        keepAliveTime < 0)
        throw new IllegalArgumentException();
    // 空指針校驗(yàn)
    if (workQueue == null || threadFactory == null || handler == null)
        throw new NullPointerException();
    this.corePoolSize = corePoolSize;
    this.maximumPoolSize = maximumPoolSize;
    this.workQueue = workQueue;
    // 根據(jù)傳入?yún)?shù)`unit`和`keepAliveTime`，將存活時(shí)間轉(zhuǎn)換為納秒存到變量`keepAliveTime `中
    this.keepAliveTime = unit.toNanos(keepAliveTime);
    this.threadFactory = threadFactory;
    this.handler = handler;
}

3、提交執(zhí)行task的過程

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();
    // worker數(shù)量比核心線程數(shù)小，直接創(chuàng)建worker執(zhí)行任務(wù)
    if (workerCountOf(c) < corePoolSize) {
        if (addWorker(command, true))
            return;
        c = ctl.get();
    }
    // worker數(shù)量超過核心線程數(shù)，任務(wù)直接進(jìn)入隊(duì)列
    if (isRunning(c) && workQueue.offer(command)) {
        int recheck = ctl.get();
        // 線程池狀態(tài)不是RUNNING狀態(tài)，說明執(zhí)行過shutdown命令，需要對新加入的任務(wù)執(zhí)行reject()操作。
        // 這兒為什么需要recheck，是因?yàn)槿蝿?wù)入隊(duì)列前后，線程池的狀態(tài)可能會(huì)發(fā)生變化。
        if (! isRunning(recheck) && remove(command))
            reject(command);
        // 這兒為什么需要判斷0值，主要是在線程池構(gòu)造方法中，核心線程數(shù)允許為0
        else if (workerCountOf(recheck) == 0)
            addWorker(null, false);
    }
    // 如果線程池不是運(yùn)行狀態(tài)，或者任務(wù)進(jìn)入隊(duì)列失敗，則嘗試創(chuàng)建worker執(zhí)行任務(wù)。
    // 這兒有3點(diǎn)需要注意：
    // 1. 線程池不是運(yùn)行狀態(tài)時(shí)，addWorker內(nèi)部會(huì)判斷線程池狀態(tài)
    // 2. addWorker第2個(gè)參數(shù)表示是否創(chuàng)建核心線程
    // 3. addWorker返回false，則說明任務(wù)執(zhí)行失敗，需要執(zhí)行reject操作
    else if (!addWorker(command, false))
        reject(command);
}

4、addworker源碼解析

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

        // 這個(gè)條件寫得比較難懂，我對其進(jìn)行了調(diào)整，和下面的條件等價(jià)
        // (rs > SHUTDOWN) ||
        // (rs == SHUTDOWN && firstTask != null) ||
        // (rs == SHUTDOWN && workQueue.isEmpty())
        // 1. 線程池狀態(tài)大于SHUTDOWN時(shí)，直接返回false
        // 2. 線程池狀態(tài)等于SHUTDOWN，且firstTask不為null，直接返回false
        // 3. 線程池狀態(tài)等于SHUTDOWN，且隊(duì)列為空，直接返回false
        // Check if queue empty only if necessary.
        if (rs >= SHUTDOWN &&
            ! (rs == SHUTDOWN &&
               firstTask == null &&
               ! workQueue.isEmpty()))
            return false;

        // 內(nèi)層自旋
        for (;;) {
            int wc = workerCountOf(c);
            // worker數(shù)量超過容量，直接返回false
            if (wc >= CAPACITY ||
                wc >= (core ? corePoolSize : maximumPoolSize))
                return false;
            // 使用CAS的方式增加worker數(shù)量。
            // 若增加成功，則直接跳出外層循環(huán)進(jìn)入到第二部分
            if (compareAndIncrementWorkerCount(c))
                break retry;
            c = ctl.get();  // Re-read ctl
            // 線程池狀態(tài)發(fā)生變化，對外層循環(huán)進(jìn)行自旋
            if (runStateOf(c) != rs)
                continue retry;
            // 其他情況，直接內(nèi)層循環(huán)進(jìn)行自旋即可
            // 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;
            // worker的添加必須是串行的，因此需要加鎖
            mainLock.lock();
            try {
                // Recheck while holding lock.
                // Back out on ThreadFactory failure or if
                // shut down before lock acquired.
                // 這兒需要重新檢查線程池狀態(tài)
                int rs = runStateOf(ctl.get());

                if (rs < SHUTDOWN ||
                    (rs == SHUTDOWN && firstTask == null)) {
                    // worker已經(jīng)調(diào)用過了start()方法，則不再創(chuàng)建worker
                    if (t.isAlive()) // precheck that t is startable
                        throw new IllegalThreadStateException();
                    // worker創(chuàng)建并添加到workers成功
                    workers.add(w);
                    // 更新`largestPoolSize`變量
                    int s = workers.size();
                    if (s > largestPoolSize)
                        largestPoolSize = s;
                    workerAdded = true;
                }
            } finally {
                mainLock.unlock();
            }
            // 啟動(dòng)worker線程
            if (workerAdded) {
                t.start();
                workerStarted = true;
            }
        }
    } finally {
        // worker線程啟動(dòng)失敗，說明線程池狀態(tài)發(fā)生了變化（關(guān)閉操作被執(zhí)行），需要進(jìn)行shutdown相關(guān)操作
        if (! workerStarted)
            addWorkerFailed(w);
    }
    return workerStarted;
}

5、線程池worker任務(wù)單元

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;
        // 這兒是Worker的關(guān)鍵所在，使用了線程工廠創(chuàng)建了一個(gè)線程。傳入的參數(shù)為當(dāng)前worker
        this.thread = getThreadFactory().newThread(this);
    }

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

    // 省略代碼...
}

6、核心線程執(zhí)行邏輯-runworker

final void runWorker(Worker w) {
    Thread wt = Thread.currentThread();
    Runnable task = w.firstTask;
    w.firstTask = null;
    // 調(diào)用unlock()是為了讓外部可以中斷
    w.unlock(); // allow interrupts
    // 這個(gè)變量用于判斷是否進(jìn)入過自旋（while循環(huán)）
    boolean completedAbruptly = true;
    try {
        // 這兒是自旋
        // 1. 如果firstTask不為null，則執(zhí)行firstTask；
        // 2. 如果firstTask為null，則調(diào)用getTask()從隊(duì)列獲取任務(wù)。
        // 3. 阻塞隊(duì)列的特性就是：當(dāng)隊(duì)列為空時(shí)，當(dāng)前線程會(huì)被阻塞等待
        while (task != null || (task = getTask()) != null) {
            // 這兒對worker進(jìn)行加鎖，是為了達(dá)到下面的目的
            // 1. 降低鎖范圍，提升性能
            // 2. 保證每個(gè)worker執(zhí)行的任務(wù)是串行的
            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
            // 如果線程池正在停止，則對當(dāng)前線程進(jìn)行中斷操作
            if ((runStateAtLeast(ctl.get(), STOP) ||
                 (Thread.interrupted() &&
                  runStateAtLeast(ctl.get(), STOP))) &&
                !wt.isInterrupted())
                wt.interrupt();
            // 執(zhí)行任務(wù)，且在執(zhí)行前后通過`beforeExecute()`和`afterExecute()`來擴(kuò)展其功能。
            // 這兩個(gè)方法在當(dāng)前類里面為空實(shí)現(xiàn)。
            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 {
                // 幫助gc
                task = null;
                // 已完成任務(wù)數(shù)加一
                w.completedTasks++;
                w.unlock();
            }
        }
        completedAbruptly = false;
    } finally {
        // 自旋操作被退出，說明線程池正在結(jié)束
        processWorkerExit(w, completedAbruptly);
    }
}

關(guān)于怎樣解析完整的ThreadPoolExecutor問題的解答就分享到這里了，希望以上內(nèi)容可以對大家有一定的幫助，如果你還有很多疑惑沒有解開，可以關(guān)注億速云行業(yè)資訊頻道了解更多相關(guān)知識。