ThreadPoolExecutor 源码分析-优快云博客

本文链接：https://blog.youkuaiyun.com/feivirus/article/details/104231388

一.原理

源码的判断过程参考下面二中的执行任务一节.

二.源码细节

(一).构造方法

ThreadPoolExecutor的构造方法只是给成员变量赋值,没有多余逻辑.主要成员变量是corePoolSize,maximumPoolSize,workQueue,keepAliveTime线程允许的空闲时间,threadFactory,RejectedExecutionHandler(拒绝策略有四种,CallerRunsPolicy,默认AbortPolicy,DiscardPolicy,DiscardOldestPolicy).
下面源码调试以ThreadPoolExecutor(3, 6, 5, TimeUnit.SECONDS, new ArrayBlockingQueue<>(4))为例.

(二).执行任务.

进入ThreadPoolExecutor#execute.代码如下:

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.
     */
    //ctl定义 private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
    int c = ctl.get();
    //如果线程池内的正在执行的线程数量(成员变量ctl的低29位的值,Worker的数量)小于corePoolSize,则创建线程
    if (workerCountOf(c) < corePoolSize) {
        //如果线程池内的正在执行的线程数量小于corePoolSize,创建线程执行
        if (addWorker(command, true))
        	//线程池内的正在执行的线程数量小于corePoolSize时,这里直接返回.
            return;
        c = ctl.get();
    }
    //如果线程池内线程数量大于等于corePoolSize,新来的线程入队列.比如第4,5个线程.
    //进入ArrayBlockingQueue#offer()入队.队列长度为前面构造函数设置的4.
    //队列满了，则不进入这个分支
    if (isRunning(c) && workQueue.offer(command)) {
        int recheck = ctl.get();
        //如果此时线程池不是running状态,则从队列移除.double check
        if (! isRunning(recheck) && remove(command))
            reject(command);
        else if (workerCountOf(recheck) == 0)
        	//如果正在执行的线程数为0时，添加工作线程.
            addWorker(null, false);
    }
    //如果线程池内线程数量小于等于maxPoolSize(corePoolSize小于maxPoolSize),则创建线程执行
    //大于时，执行拒绝策略
    else if (!addWorker(command, false))
    	//线程池的线程数量大于maxPoolSize时,执行拒绝策略
        reject(command);
}

(三).创建线程执行

private boolean addWorker(Runnable firstTask, boolean core) {
    retry:
    //两层死循环
    for (;;) {
        int c = ctl.get();
        //获取当前线程池的状态
        int rs = runStateOf(c);

        // Check if queue empty only if necessary.
        //线程池不是running状态且(线程池关闭或者队列为空),则不创建新线程
        if (rs >= SHUTDOWN &&
            ! (rs == SHUTDOWN &&
               firstTask == null &&
               ! workQueue.isEmpty()))
            return false;

        for (;;) {
            int wc = workerCountOf(c);
            //CAPACITY高3位为0，低29位全是1，即29位的最大整数。如果线程数量大于大于这个数，则退出不创建线程，基本不可能满足.
            //core进来时，如果是corePoolSize这步判断进来是写死的true.如果是maxPoolSize那步,进来是false.取maximumPoolSize
            //如果线程数量大于等于corePoolSize或者maximumPoolSize,退出不创建线程
            if (wc >= CAPACITY ||
                wc >= (core ? corePoolSize : maximumPoolSize))
                return false;
            //走到这一步正常去创建Worker线程
            //对成员变量ctl的值加1，即线程数量+1
            if (compareAndIncrementWorkerCount(c))
                break retry;
            c = ctl.get();  // Re-read ctl
            //如果当前线程池状态和刚开始进来外层for循环时的状态不一样，重新判断是否能创建线程
            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 {
    	//Worker内部封装了线程,创建线程,Worker构造方法中new Thread
        w = new Worker(firstTask);
        final Thread t = w.thread;
        if (t != null) {
        	//获取线程池的主锁,执行这段代码的线程是主线程。多个主线程操作同一个线程池,做并发控制
            final ReentrantLock mainLock = this.mainLock;
            mainLock.lock();
            try {
                // Recheck while holding lock.
                // Back out on ThreadFactory failure or if
                // shut down before lock acquired.
                //获取当前线程池状态，第二次检查前面两层for循环的最后做的第一次检查
                int rs = runStateOf(ctl.get());

                //线程池是running状态,才添加
                //线程池是shutdown状态，但是队列中有未完成的任务，也可以添加.
                if (rs < SHUTDOWN ||
                    (rs == SHUTDOWN && firstTask == null)) {
                	//新创建的线程已经被启动.则抛异常.这里谁会启动它?
                    if (t.isAlive()) // precheck that t is startable
                        throw new IllegalThreadStateException();
                    //添加worker到成员变量HashSet中
                    workers.add(w);
                    int s = workers.size();
                    //修改最大线程池大小
                    if (s > largestPoolSize)
                        largestPoolSize = s;
                    workerAdded = true;
                }
            } finally {
                mainLock.unlock();
            }
            if (workerAdded) {
            	//运行新加的线程代码.进入ThreadPoolExecutor#runWorker
                t.start();
                workerStarted = true;
            }
        }
    } finally {
    	//线程启动失败了.从队列中移除这个Worker，减少线程计数,结束该失败线程
        if (! workerStarted)
            addWorkerFailed(w);
    }
    return workerStarted;
}

(四).Worker工作线程类

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. */
    //具体工作线程
    final Thread thread;
    /** Initial task to run.  Possibly null. */
    //执行的任务代码
    Runnable firstTask;
    /** Per-thread task counter */
    volatile long completedTasks;
    ...
 }

构造方法中调用getThreadFactory().newThread(Runnable)创建线程.

Worker(Runnable firstTask) {
    setState(-1); // inhibit interrupts until runWorker
    this.firstTask = firstTask;
    this.thread = getThreadFactory().newThread(this);
}

(五).线程代码执行.

进入ThreadPoolExecutor#runWorker()方法.

final void runWorker(Worker w) {
 	//执行任务的子线程
    Thread wt = Thread.currentThread();
    Runnable task = w.firstTask;
    w.firstTask = null;
    w.unlock(); // allow interrupts
    boolean completedAbruptly = true;
    try {
    	//获取要执行的任务
        while (task != null || (task = getTask()) != null) {
        	//获取任务的锁
            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 {
            	//在线程开始执行前，可以做一些拦截操作,类似aop,继承ThreadPoolExecutor,实现这个方法
                beforeExecute(wt, task);
                Throwable thrown = null;
                try {
                	//执行Runnable任务的run方法代码
                    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;
                //当前Worker执行任务数+1
                w.completedTasks++;
                w.unlock();
            }
        }
        completedAbruptly = false;
    } finally {
        processWorkerExit(w, completedAbruptly);
    }
}

(六).线程池状态和内部的线程数量

CAPACITY的值为0b00011111_11111111_11111111_11111111.参数c就是传进来的成员变量AtomicInteger ctl的值.它的高3位存线程池状态，低29位存线程池内的线程数量。

private static int runStateOf(int c)     { 
 	//取高3位的值
 	return c & ~CAPACITY; 
 }
 private static int workerCountOf(int c)  { 
 	//取低29位的值
 	return c & CAPACITY; 
 }

线程池的几种状态,整数值越来越大:
RUNNNING:0b11100000_00000000_00000000_00000000.十进制表示为-536870912.
SHUTDOWN:0
STOP:0b00100000_00000000_00000000_00000000.十进制为536870912.
TIDYING:0b01000000_00000000_00000000_00000000.
TERMINATED:0b01100000_00000000_00000000_00000000.

(七).从队列获取任务getTask()

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.
        //线程池关闭或者(线程池停止，且没有未完成的队列任务)，则返回没有任务
        if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
            decrementWorkerCount();
            return null;
        }

        int wc = workerCountOf(c);

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

        //线程数量超过最大数量限制，则不再取任务
        if ((wc > maximumPoolSize || (timed && timedOut))
            && (wc > 1 || workQueue.isEmpty())) {
            if (compareAndDecrementWorkerCount(c))
                return null;
            continue;
        }

        try {
            //从任务队列中阻塞式的取出一个任务去执行.keepAliveTime时间内没有取到任务，返回for循环继续执行
            Runnable r = timed ?
                workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                workQueue.take();
            if (r != null)
                return r;
            timedOut = true;
        } catch (InterruptedException retry) {
            timedOut = false;
        }
    }
}

(八).Worker线程任务退出时清理

private void processWorkerExit(Worker w, boolean completedAbruptly) {
    if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
        decrementWorkerCount();

    final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
        //完成任务数加一
        completedTaskCount += w.completedTasks;
        //从HashSet中移除这个Worker
        workers.remove(w);
    } finally {
        mainLock.unlock();
    }

    //判断是否需要停止线程池
    tryTerminate();

    int c = ctl.get();
    if (runStateLessThan(c, STOP)) {
        if (!completedAbruptly) {
            int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
            if (min == 0 && ! workQueue.isEmpty())
                min = 1;
            if (workerCountOf(c) >= min)
                return; // replacement not needed
        }
        addWorker(null, false);
    }
}