核心池大小与ThreadPoolExecutor中的最大池大小

当我们谈论时，核心池大小和最大池大小之间到底有什么区别ThreadPoolExecutor？
可以借助示例进行解释吗？

从这篇博客文章：

举这个例子。起始线程池大小为1，核心池大小为5，最大池大小为10，队列为100。

随着请求的到来，将最多创建5个线程，然后将任务添加到队列中，直到达到100个。当队列已满时，将最多创建线程maxPoolSize。一旦所有线程都被使用并且队列已满，任务将被拒绝。随着队列的减少，活动线程的数量也会减少。

如果正在运行的线程> corePoolSize＆<maxPoolSize，则如果Total任务队列已满并且有新线程到达，则创建一个新Thread。

表单文档：（如果正在运行的线程数超过corePoolSize但少于maximumPoolSize，则仅在队列已满时才创建新线程。）

现在，举一个简单的例子，

ThreadPoolExecutor executorPool = new ThreadPoolExecutor(5, 10, 3, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(50));

在这里，5是corePoolSize-意味着Jvm将为前5个任务的新任务创建新线程。其他任务将被添加到队列中，直到队列已满（50个任务）。

10是maxPoolSize -JVM最多可以创建10个线程。意味着如果已经有5个任务/线程正在运行，并且队列中有50个待处理的任务已满，并且如果队列中又有一个新的请求/任务到达，那么JVM将创建最多10个新线程（总线程数=前5个线程+ 5个新线程） ;

new ArrayBlockingQueue（50） =是队列的总大小-它可以在其中排队50个任务。

一旦所有10个线程都运行并且如果有新任务到达，则该新任务将被拒绝。

SUN内部创建线程的规则：

1. 如果线程数小于corePoolSize，则创建一个新线程以运行新任务。

2. 如果线程数等于（或大于）corePoolSize，则将任务放入队列。

3. 如果队列已满，并且线程数小于maxPoolSize，请创建一个新线程来运行任务。

4. 如果队列已满，并且线程数大于或等于maxPoolSize，则拒绝该任务。

希望这是HelpFul ..如果我错了，请纠正我...

从文档：

当在方法execute（java.lang.Runnable）中提交新任务，并且正在运行的线程少于corePoolSize线程时，即使其他工作线程处于空闲状态，也会创建一个新线程来处理请求。如果正在运行的线程大于corePoolSize但小于maximumPoolSize，则仅在队列已满时才创建新线程。

此外：

通过将corePoolSize和maximumPoolSize设置为相同，可以创建固定大小的线程池。通过将maximumPoolSize设置为一个本质上不受限制的值（例如Integer.MAX_VALUE），可以允许池容纳任意数量的并发任务。最典型地，核心和最大池大小仅在构造时设置，但也可以使用setCorePoolSize（int）和setMaximumPoolSize（int）动态更改。

如果决定ThreadPoolExecutor手动创建而不是使用Executors工厂类，则需要使用其构造函数之一来创建和配置一个。此类的最广泛的构造方法是：

public ThreadPoolExecutor(
    int corePoolSize,
    int maxPoolSize,
    long keepAlive,
    TimeUnit unit,
    BlockingQueue<Runnable> workQueue,
    RejectedExecutionHandler handler
);

如您所见，您可以配置：

• 核心池大小（线程池将尝试使用的大小）。
• 最大池大小。
• 保持活动时间，即空闲线程有资格被拆除之后的时间。
• 工作队列中包含等待执行的任务。
• 任务提交被拒绝时要应用的策略。

限制排队的任务数

限制正在执行的并发任务的数量，调整线程池的大小，对于应用程序及其执行环境的可预测性和稳定性而言，是一个巨大的好处：无限制的线程创建最终将耗尽运行时资源，结果您的应用程序可能会遇到严重的性能问题，甚至可能导致应用程序不稳定。

这只是解决问题的一部分：您限制了正在执行的任务数量，但没有限制可以提交和排队供以后执行的作业数量。该应用程序将在以后遇到资源短缺的问题，但是如果提交率始终超过执行率，它将最终遇到这种情况。

解决此问题的方法是：向执行者提供一个阻塞队列，以保留等待的任务。如果队列已满，提交的任务将被“拒绝”。在RejectedExecutionHandler当任务提交被拒绝被调用，这就是为什么拒绝动词前一个项目中被引述。您可以实施自己的拒绝策略，也可以使用框架提供的内置策略之一。

默认拒绝策略使执行程序抛出RejectedExecutionException。但是，其他内置策略可让您：

• 静默丢弃作业。
• 丢弃最旧的作业，然后尝试重新提交最后一份。
• 在调用者的线程上执行被拒绝的任务。

资源

ThreadPoolExecutor池大小的规则

ThreadPoolExecutor's池大小的规则通常会被误解，因为它无法按照您认为的方式或所需的方式运行。

举这个例子。起始线程池大小为1，核心池大小为5，最大池大小为10，队列为100。

Sun的方式：随着请求的到来，最多将创建5个线程，然后将任务添加到队列中，直到达到100个。当队列已满时，将最多创建一个新线程maxPoolSize。一旦所有线程都被使用并且队列已满，任务将被拒绝。随着队列的减少，活动线程的数量也会减少。

用户预期的方式：随着请求的进入，最多将创建10个线程，然后将任务添加到队列中，直到达到100个为止，此时它们被拒绝。线程数将最大重命名，直到队列为空。当队列为空时，线程将死掉直到corePoolSize剩下。

不同之处在于，用户希望更早地开始增加池的大小，并希望队列变小，而Sun方法则希望保持池的大小小，并且只有在负载变得很大时才增加池的大小。

简单来说，这是Sun的线程创建规则：

1. 如果线程数小于corePoolSize，则创建一个新线程以运行新任务。
2. 如果线程数等于（或大于）corePoolSize，则将任务放入队列。
3. 如果队列已满，并且线程数少于maxPoolSize，请创建一个新线程来运行任务。
4. 如果队列已满，并且线程数大于或等于maxPoolSize，则拒绝任务。总而言之，就是只有在队列填满时才创建新线程，因此，如果您使用的是无界队列，则线程数不会超过corePoolSize。

要获得更全面的解释，请直接从以下渠道获取：ThreadPoolExecutorAPI文档。

有一篇非常不错的论坛帖子，向您介绍了如何ThreadPoolExecutor与代码示例配合使用：http : //forums.sun.com/thread.jspa?threadID=5401400&tstart=0

更多信息：http : //forums.sun.com/thread.jspa?threadID=5224557&tstart=450

您可以在javadoc中找到术语corepoolsize和maxpoolsize的定义。http://docs.oracle.com/javase/6/docs/api/java/util/concurrent/ThreadPoolExecutor.html

上面的链接提供了您问题的答案。但是，只是为了清楚起见。应用程序将继续创建线程，直到达到corePoolSize为止。我认为这里的想法是，许多线程应该足以处理任务的流入。如果在创建corePoolSize线程之后出现新任务，则该任务将排队。一旦队列已满，执行器将开始创建新线程。这是一种平衡。从本质上讲，这意味着任务的流入大于处理能力。因此，Executor将再次开始创建新线程，直到达到最大线程数。同样，仅当队列已满时，才会创建新线程。

这个博客很好的解释：

插图

public class ThreadPoolExecutorExample {

    public static void main (String[] args) {
        createAndRunPoolForQueue(new ArrayBlockingQueue<Runnable>(3), "Bounded");
        createAndRunPoolForQueue(new LinkedBlockingDeque<>(), "Unbounded");
        createAndRunPoolForQueue(new SynchronousQueue<Runnable>(), "Direct hand-off");
    }

    private static void createAndRunPoolForQueue (BlockingQueue<Runnable> queue,
                                                                      String msg) {
        System.out.println("---- " + msg + " queue instance = " +
                                                  queue.getClass()+ " -------------");

        ThreadPoolExecutor e = new ThreadPoolExecutor(2, 5, Long.MAX_VALUE,
                                 TimeUnit.NANOSECONDS, queue);

        for (int i = 0; i < 10; i++) {
            try {
                e.execute(new Task());
            } catch (RejectedExecutionException ex) {
                System.out.println("Task rejected = " + (i + 1));
            }
            printStatus(i + 1, e);
        }

        e.shutdownNow();

        System.out.println("--------------------\n");
    }

    private static void printStatus (int taskSubmitted, ThreadPoolExecutor e) {
        StringBuilder s = new StringBuilder();
        s.append("poolSize = ")
         .append(e.getPoolSize())
         .append(", corePoolSize = ")
         .append(e.getCorePoolSize())
         .append(", queueSize = ")
         .append(e.getQueue()
                  .size())
         .append(", queueRemainingCapacity = ")
         .append(e.getQueue()
                  .remainingCapacity())
         .append(", maximumPoolSize = ")
         .append(e.getMaximumPoolSize())
         .append(", totalTasksSubmitted = ")
         .append(taskSubmitted);

        System.out.println(s.toString());
    }

    private static class Task implements Runnable {

        @Override
        public void run () {
            while (true) {
                try {
                    Thread.sleep(1000000);
                } catch (InterruptedException e) {
                    break;
                }
            }
        }
    }
}

输出：

---- Bounded queue instance = class java.util.concurrent.ArrayBlockingQueue -------------
poolSize = 1, corePoolSize = 2, queueSize = 0, queueRemainingCapacity = 3, maximumPoolSize = 5, totalTasksSubmitted = 1
poolSize = 2, corePoolSize = 2, queueSize = 0, queueRemainingCapacity = 3, maximumPoolSize = 5, totalTasksSubmitted = 2
poolSize = 2, corePoolSize = 2, queueSize = 1, queueRemainingCapacity = 2, maximumPoolSize = 5, totalTasksSubmitted = 3
poolSize = 2, corePoolSize = 2, queueSize = 2, queueCapacity = 1, maximumPoolSize = 5, totalTasksSubmitted = 4
poolSize = 2, corePoolSize = 2, queueSize = 3, queueRemainingCapacity = 0, maximumPoolSize = 5, totalTasksSubmitted = 5
poolSize = 3, corePoolSize = 2, queueSize = 3, queueRemainingCapacity = 0, maximumPoolSize = 5, totalTasksSubmitted = 6
poolSize = 4, corePoolSize = 2, queueSize = 3, queueRemainingCapacity = 0, maximumPoolSize = 5, totalTasksSubmitted = 7
poolSize = 5, corePoolSize = 2, queueSize = 3, queueRemainingCapacity = 0, maximumPoolSize = 5, totalTasksSubmitted = 8
Task rejected = 9
poolSize = 5, corePoolSize = 2, queueSize = 3, queueRemainingCapacity = 0, maximumPoolSize = 5, totalTasksSubmitted = 9
Task rejected = 10
poolSize = 5, corePoolSize = 2, queueSize = 3, queueRemainingCapacity = 0, maximumPoolSize = 5, totalTasksSubmitted = 10
--------------------

---- Unbounded queue instance = class java.util.concurrent.LinkedBlockingDeque -------------
poolSize = 1, corePoolSize = 2, queueSize = 0, queueRemainingCapacity = 2147483647, maximumPoolSize = 5, totalTasksSubmitted = 1
poolSize = 2, corePoolSize = 2, queueSize = 0, queueRemainingCapacity = 2147483647, maximumPoolSize = 5, totalTasksSubmitted = 2
poolSize = 2, corePoolSize = 2, queueSize = 1, queueRemainingCapacity = 2147483646, maximumPoolSize = 5, totalTasksSubmitted = 3
poolSize = 2, corePoolSize = 2, queueSize = 2, queueRemainingCapacity = 2147483645, maximumPoolSize = 5, totalTasksSubmitted = 4
poolSize = 2, corePoolSize = 2, queueSize = 3, queueRemainingCapacity = 2147483644, maximumPoolSize = 5, totalTasksSubmitted = 5
poolSize = 2, corePoolSize = 2, queueSize = 4, queueRemainingCapacity = 2147483643, maximumPoolSize = 5, totalTasksSubmitted = 6
poolSize = 2, corePoolSize = 2, queueSize = 5, queueRemainingCapacity = 2147483642, maximumPoolSize = 5, totalTasksSubmitted = 7
poolSize = 2, corePoolSize = 2, queueSize = 6, queueRemainingCapacity = 2147483641, maximumPoolSize = 5, totalTasksSubmitted = 8
poolSize = 2, corePoolSize = 2, queueSize = 7, queueRemainingCapacity = 2147483640, maximumPoolSize = 5, totalTasksSubmitted = 9
poolSize = 2, corePoolSize = 2, queueSize = 8, queueRemainingCapacity = 2147483639, maximumPoolSize = 5, totalTasksSubmitted = 10
--------------------

---- Direct hand-off queue instance = class java.util.concurrent.SynchronousQueue -------------
poolSize = 1, corePoolSize = 2, queueSize = 0, queueRemainingCapacity = 0, maximumPoolSize = 5, totalTasksSubmitted = 1
poolSize = 2, corePoolSize = 2, queueSize = 0, queueRemainingCapacity = 0, maximumPoolSize = 5, totalTasksSubmitted = 2
poolSize = 3, corePoolSize = 2, queueSize = 0, queueRemainingCapacity = 0, maximumPoolSize = 5, totalTasksSubmitted = 3
poolSize = 4, corePoolSize = 2, queueSize = 0, queueRemainingCapacity = 0, maximumPoolSize = 5, totalTasksSubmitted = 4
poolSize = 5, corePoolSize = 2, queueSize = 0, queueRemainingCapacity = 0, maximumPoolSize = 5, totalTasksSubmitted = 5
Task rejected = 6
poolSize = 5, corePoolSize = 2, queueSize = 0, queueRemainingCapacity = 0, maximumPoolSize = 5, totalTasksSubmitted = 6
Task rejected = 7
poolSize = 5, corePoolSize = 2, queueSize = 0, queueRemainingCapacity = 0, maximumPoolSize = 5, totalTasksSubmitted = 7
Task rejected = 8
poolSize = 5, corePoolSize = 2, queueSize = 0, queueRemainingCapacity = 0, maximumPoolSize = 5, totalTasksSubmitted = 8
Task rejected = 9
poolSize = 5, corePoolSize = 2, queueSize = 0, queueRemainingCapacity = 0, maximumPoolSize = 5, totalTasksSubmitted = 9
Task rejected = 10
poolSize = 5, corePoolSize = 2, queueSize = 0, queueRemainingCapacity = 0, maximumPoolSize = 5, totalTasksSubmitted = 10
--------------------


Process finished with exit code 0

从Java并发性基础知识这本书开始：

CorePoolSize：ThreadPoolExecutor具有一个corePoolSize属性，该属性确定要在只有队列已满时才启动新线程之前要启动的线程数。

MaximumPoolSize：此属性确定最大启动多少线程。您可以将其设置为Integer。没有上限的MAX_VALUE

java.util.concurrent.ThreadPoolExecutor

  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();
        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);
    }

了解ThreadPoolExecutor提交新任务时的内部行为有助于我了解如何corePoolSize以及maximumPoolSize不同之处。

让：

• N是池中的线程数，getPoolSize()。活动线程+空闲线程。
• T 提交给执行者/池的任务数量。
• C是核心池的大小getCorePoolSize()。在新任务进入队列之前，每个池最多可以为传入任务创建多少个线程。
• M是最大池大小，getMaximumPoolSize()。池可以分配的最大线程数。

ThreadPoolExecutor提交新任务时Java中的行为：

• 对于 N <= C，没有为空闲线程分配新的传入任务，而是创建了一个新线程。
• 对于N > C并且，如果有空闲线程，则在那里分配新任务。
• 对于N > C如果没有空闲的线程，新任务放入队列。此处未创建新的线程。
• 当队列已满时，我们最多创建新线程M。如果M达成，我们将拒绝任务。重要的是不要在队列已满之前不要创建新线程！

资料来源：

• docs.oracle.com
• 关于该主题的优秀文章。

例子

例如具有corePoolSize = 0和maximumPoolSize = 10与队列容量50。

这将导致池中只有一个活动线程，直到队列中有50个项目为止。

executor.execute(task #1):

before task #1 submitted to executor: java.util.concurrent.ThreadPoolExecutor@c52dafe[Running, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 0]

after task #1 submitted to executor: java.util.concurrent.ThreadPoolExecutor@c52dafe[Running, pool size = 1, active threads = 1, queued tasks = 1, completed tasks = 0]

[task #1 immediately queued and kicked in b/c the very first thread is created when `workerCountOf(recheck) == 0`]

execute(task #2):

before task #2 submitted to executor: java.util.concurrent.ThreadPoolExecutor@c52dafe[Running, pool size = 1, active threads = 1, queued tasks = 0, completed tasks = 0]

after task #2 submitted to executor: java.util.concurrent.ThreadPoolExecutor@c52dafe[Running, pool size = 1, active threads = 1, queued tasks = 1, completed tasks = 0]

[task #2 not starting before #1 is done]

... executed a few tasks...

execute(task #19)

before task #19 submitted to executor: java.util.concurrent.ThreadPoolExecutor@735afe38[Running, pool size = 1, active threads = 1, queued tasks = 17, completed tasks = 0]

after task #19 submitted to executor: java.util.concurrent.ThreadPoolExecutor@735afe38[Running, pool size = 1, active threads = 1, queued tasks = 18, completed tasks = 0]

...

execute(task #51)

before task submitted to executor: java.util.concurrent.ThreadPoolExecutor@735afe38[Running, pool size = 1, active threads = 1, queued tasks = 50, completed tasks = 0]

after task submitted to executor: java.util.concurrent.ThreadPoolExecutor@735afe38[Running, pool size = 2, active threads = 2, queued tasks = 50, completed tasks = 0]

Queue is full.
A new thread was created as the queue was full.

例如具有corePoolSize = 10和maximumPoolSize = 10与队列容量50。

这将导致池中有10个活动线程。当队列中有50个项目时，任务将被拒绝。

execute(task #1)

before task #1 submitted to executor: java.util.concurrent.ThreadPoolExecutor@32d9e072[Running, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 0]

after task #1 submitted to executor: java.util.concurrent.ThreadPoolExecutor@32d9e072[Running, pool size = 1, active threads = 1, queued tasks = 0, completed tasks = 0]

execute(task #2)

before task #2 submitted to executor: java.util.concurrent.ThreadPoolExecutor@32d9e072[Running, pool size = 1, active threads = 1, queued tasks = 0, completed tasks = 0]

after task #2 submitted to executor: java.util.concurrent.ThreadPoolExecutor@32d9e072[Running, pool size = 2, active threads = 2, queued tasks = 0, completed tasks = 0]

execute(task #3)

before task #3 submitted to executor: java.util.concurrent.ThreadPoolExecutor@32d9e072[Running, pool size = 2, active threads = 2, queued tasks = 0, completed tasks = 0]

after task #3 submitted to executor: java.util.concurrent.ThreadPoolExecutor@32d9e072[Running, pool size = 3, active threads = 3, queued tasks = 0, completed tasks = 0]

... executed a few tasks...

execute(task #11)

before task #11 submitted to executor: java.util.concurrent.ThreadPoolExecutor@32d9e072[Running, pool size = 10, active threads = 10, queued tasks = 0, completed tasks = 0]

after task #11 submitted to executor: java.util.concurrent.ThreadPoolExecutor@32d9e072[Running, pool size = 10, active threads = 10, queued tasks = 1, completed tasks = 0]

... executed a few tasks...

execute(task #51)
before task #51 submitted to executor: java.util.concurrent.ThreadPoolExecutor@32d9e072[Running, pool size = 10, active threads = 10, queued tasks = 50, completed tasks = 0]

Task was rejected as we have reached `maximumPoolSize`.