JDK1.8 ThreadPoolExecutor （待完结）

按照new对象然后执行方法的顺序，先构造函数，execute执行方法，工作者worker，肃清队列purge，拒绝Handler等等

构造函数 ThreadPoolExecutor

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;
    }

corePoolSize

• the number of threads to keep in the pool, even if they are idle, unless {@code allowCoreThreadTimeOut} is set
• 池中要保留的线程数，即使它们处于空闲状态，除非设置了{@code allowCoreThreadTimeOut}

maximumPoolSize

• the maximum number of threads to allow in the pool
• 池中允许的最大线程数

keepAliveTime

• when the number of threads is greater than the core
• this is the maximum time that excess idle threads will wait for new tasks before terminating
• 当线程数大于内核数时，多余空闲线程在终止前等待新任务的最长时间

unit

• the time unit for the {@code keepAliveTime} argument
• {keepAliveTime}参数的时间单位

workQueue

• the queue to use for holding tasks before they are executed
• This queue will hold only the {@code Runnable} tasks submitted by the {@code execute} method
• 用于在任务完成之前保留任务的队列执行，此队列将仅包含{Runnable}，由{execute}方法提交的任务

threadFactory

• the factory to use when the executor creates a new thread
• 执行器创建新线程时要使用的工厂

handler

• the handler to use when execution is blocked because the thread bounds and queue capacities are reached
• 由于达到线程边界和队列容量而阻止执行时要使用的处理程序

Execute

public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        int c = ctl.get();
        if (workerCountOf(c) < corePoolSize) {
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        if (isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();
            if (! isRunning(recheck) && remove(command))
                reject(command);
            else if (workerCountOf(recheck) == 0)
                addWorker(null, false);
        }
        else if (!addWorker(command, false))
            reject(command);
    }
}

该方法分三步进行：

第一步

如果正在运行的线程少于corePoolSize，请尝试以给定命令作为第一个线程启动新线程任务

对addWorker的调用以原子方式检查运行状态和worker的数量，在添加了不应该添加的线程下通过返回false防止错误警报

第二步

如果任务可以成功排队，进入此方法后，我们仍然需要再次检查是否应该添加线程

因为自上次检查以来，已存在的某些线程已死亡，或者池子已经关闭不再需要检查

所以我们重新检查状态，如果已停止，在有必要的情况下回滚入队或者如果任务执行结束，则启动新线程

第三步

如果我们无法将任务排队，那么我们将尝试添加一个新线程。如果失败了，我们知道我们已经被关闭或饱和，所以拒绝这个任务

Purge 肃清

public void purge() {
    final BlockingQueue<Runnable> q = workQueue;
    try {
        Iterator<Runnable> it = q.iterator();
        while (it.hasNext()) {
            Runnable r = it.next();
            if (r instanceof Future<?> && ((Future<?>)r).isCancelled())
                it.remove();
        }
    } catch (ConcurrentModificationException fallThrough) {
        // Take slow path if we encounter interference during traversal.
// Make copy for traversal and call remove for cancelled entries.
        // The slow path is more likely to be O(N*N).
        for (Object r : q.toArray())
            if (r instanceof Future<?> && ((Future<?>)r).isCancelled())
                q.remove(r);
    }

    tryTerminate(); // In case SHUTDOWN and now empty
}

尝试从工作队列中删除所有{@link Future}已取消的任务，这种方法可以作为一种有用的方法存储回收操作，这对功能性没有其他影响

取消的任务永远不会执行，但可能会在工作队列中累积，直到工作线程可以活动移除它们。相反，调用此方法会尝试删除它们

但是，如果存在其他线程的干扰，此方法可能无法删除任务

拒绝调用操作者

CallerRunsPolicy

public static class CallerRunsPolicy implements RejectedExecutionHandler {
    /**
     * Creates a {@code CallerRunsPolicy}.
     */
    public CallerRunsPolicy() { }

    /**
      * Executes task r in the caller's thread, unless the executor
      * has been shut down, in which case the task is discarded.
      *
      * @param r the runnable task requested to be executed
      * @param e the executor attempting to execute this task
      */
     public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
         if (!e.isShutdown()) {
             r.run();
         }
     }
}

直接在{@code execute}方法的调用线程中执行被拒绝任务，除非执行器已关闭，在这种情况下，任务被丢弃

AbortPolicy

public static class AbortPolicy implements RejectedExecutionHandler {
    /**
      * Creates an {@code AbortPolicy}.
      */
       public AbortPolicy() { }

        /**
         * Always throws RejectedExecutionException.
         *
         * @param r the runnable task requested to be executed
         * @param e the executor attempting to execute this task
         * @throws RejectedExecutionException always
         */
        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
            throw new RejectedExecutionException("Task " + r.toString() +
                                                 " rejected from " +
                                                 e.toString());
        }
    }

用于抛出{@code RejectedExecutionException}

DiscardPolicy

public static class DiscardPolicy implements RejectedExecutionHandler {
   /**
      * Creates a {@code DiscardPolicy}.
      */
    public DiscardPolicy() { }

    /**
       * Does nothing, which has the effect of discarding task r.
       *
       * @param r the runnable task requested to be executed
       * @param e the executor attempting to execute this task
       */
    public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
    }
}

被拒绝任务的处理程序，以静默方式丢弃被拒绝的任务

DiscardOldestPolicy

public static class DiscardOldestPolicy implements RejectedExecutionHandler {
  /**
     * Creates a {@code DiscardOldestPolicy} for the given executor.
     */
    public DiscardOldestPolicy() { }

  /**
     * Obtains and ignores the next task that the executor
     * would otherwise execute, if one is immediately available,
     * and then retries execution of task r, unless the executor
     * is shut down, in which case task r is instead discarded.
     *
     * @param r the runnable task requested to be executed
     * @param e the executor attempting to execute this task
     */
    public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
        if (!e.isShutdown()) {
            e.getQueue().poll();
            e.execute(r);
        }
    }
}

用于丢弃最旧的未处理任务请求，然后重试{@code execute}，除非执行器被关闭，在这种情况下，任务被丢弃

Worker

    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;
            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.
        // The value 1 represents the locked state.

        protected boolean isHeldExclusively() {
            return getState() != 0;
        }

        protected boolean tryAcquire(int unused) {
            if (compareAndSetState(0, 1)) {
                setExclusiveOwnerThread(Thread.currentThread());
                return true;
            }
            return false;
        }

        protected boolean tryRelease(int unused) {
            setExclusiveOwnerThread(null);
            setState(0);
            return true;
        }

        public void lock()        { acquire(1); }
        public boolean tryLock()  { return tryAcquire(1); }
        public void unlock()      { release(1); }
        public boolean isLocked() { return isHeldExclusively(); }

        void interruptIfStarted() {
            Thread t;
            if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
                try {
                    t.interrupt();
                } catch (SecurityException ignore) {
                }
            }
        }
    }

Worker类继承了AQS，实现了Runnable接口，用来创建新线程来执行任务

runWorker

    final void runWorker(Worker w) {
        Thread wt = Thread.currentThread();
        Runnable task = w.firstTask;
        w.firstTask = null;
        w.unlock(); // allow interrupts
        boolean completedAbruptly = true;
        try {
            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 {
                    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();
                }
            }
            completedAbruptly = false;
        } finally {
            processWorkerExit(w, completedAbruptly);
        }
    }

 

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