135

当我们谈论时，核心池大小和最大池大小之间究竟有什么区别ThreadPoolExecutor？
可以借助一个例子来解释吗？

4

11 回答 11

160

从这篇博文：

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。

当请求进来时，最多会创建 5 个线程，然后将任务添加到队列中，直到达到 100 个。当队列满时，将创建最多maxPoolSize. 一旦所有线程都在使用并且队列已满，任务将被拒绝。随着队列的减少，活动线程的数量也会减少。

于 2013-07-16T06:40:12.200 回答
72

IF running threads > corePoolSize & < maxPoolSize，如果 Total task queue 已满且新线程到达，则创建一个新线程。

Form doc：（如果有超过corePoolSize但小于maximumPoolSize的线程在运行，只有队列满时才会创建一个新线程。）

现在，举一个简单的例子，

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

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，直到队列满（50 个任务）。

10 是 maxPoolSize - JVM 最多可以创建 10 个线程。表示如果已经有 5 个任务/线程正在运行并且队列已满，有 50 个待处理任务，并且如果还有一个新的请求/任务到达队列，那么 JVM 将创建最多 10 个新线程（总线程数 = 之前的 5 个 + 新的 5 个） ;

new ArrayBlockingQueue(50) = 是总队列大小 - 它可以在其中排队 50 个任务。

一旦所有 10 个线程都在运行并且如果新任务到达，那么该新任务将被拒绝。

SUN 内部创建线程的规则：

  1. 如果线程数小于 corePoolSize，则创建一个新的 Thread 来运行一个新的任务。

  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，则创建一个新线程来运行任务。

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

希望，这是有帮助的......如果我错了，请纠正我......

于 2018-01-27T06:12:38.173 回答
21

从文档：

当在方法 execute(java.lang.Runnable) 中提交了一个新任务，并且运行的线程少于 corePoolSize 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该请求。如果运行的线程数多于 corePoolSize 但少于 maximumPoolSize，则仅当队列已满时才会创建新线程。

此外：

通过将 corePoolSize 和 maximumPoolSize 设置为相同，您可以创建一个固定大小的线程池。通过将 maximumPoolSize 设置为基本上无界的值，例如 Integer.MAX_VALUE，您允许池容纳任意数量的并发任务。最典型的是，核心和最大池大小仅在构造时设置，但它们也可以使用 setCorePoolSize(int) 和 setMaximumPoolSize(int) 动态更改。

于 2013-07-15T16:53:15.453 回答
21

资源

ThreadPoolExecutor 池大小的规则

池大小的规则ThreadPoolExecutor's通常被误解，因为它没有按照您认为应该的方式或您希望的方式工作。

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：当请求进入时，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。当队列满时，将创建最多 5 个新线程maxPoolSize。一旦所有线程都在使用并且队列已满，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：当请求进入线程时，将创建最多 10 个线程，然后将任务添加到队列中，直到达到 100 个时它们被拒绝。线程数将重命名为最大值，直到队列为空。当队列为空时，线程将消失，直到corePoolSize剩下。

不同之处在于用户希望更早开始增加池大小并希望队列更小，而 Sun 方法希望保持池大小较小，并且仅在负载变得很大时才增加它。

以下是 Sun 的线程创建规则，简单来说：

  1. 如果线程数小于corePoolSize，则创建一个新线程来运行一个新任务。
  2. 如果线程数等于（或大于）corePoolSize，则将任务放入队列中。
  3. 如果队列已满，并且线程数少于maxPoolSize，则创建一个新线程来运行任务。
  4. 如果队列已满，且线程数大于等于maxPoolSize，则拒绝该任务。总而言之，新线程仅在队列填满时创建，因此如果您使用无界队列，则线程数不会超过corePoolSize.

要获得更全面的解释，请从马口中获取：ThreadPoolExecutorAPI 文档。

有一个非常好的论坛帖子，它通过ThreadPoolExecutor代码示例的工作方式与您交谈： http://forums.sun.com/thread.jspa?threadID=5401400& tstart=0

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

于 2020-01-03T14:06:21.097 回答
10

如果您决定ThreadPoolExecutor手动创建一个而不是使用Executors工厂类，则需要使用其构造函数之一来创建和配置一个。这个类最广泛的构造函数是：

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（线程池将尝试坚持的大小）。
  • 最大池大小。
  • 保持活动时间，这是一个空闲线程有资格被拆除的时间。
  • 保存等待执行的任务的工作队列。
  • 任务提交被拒绝时应用的策略。

限制排队任务的数量

限制正在执行的并发任务的数量，调整线程池的大小，在可预测性和稳定性方面对您的应用程序及其执行环境带来巨大的好处：无限的线程创建最终会耗尽运行时资源，您的应用程序可能会因此而遇到问题, 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以供以后执行的作业数量。应用程序稍后会遇到资源短缺，但如果提交率持续超过执行率，它最终会遇到这种情况。

解决这个问题的方法是：给执行者提供一个阻塞队列来保存等待中的任务。如果队列已满，提交的任务将被“拒绝”。当RejectedExecutionHandler任务提交被拒绝时调用 ，这就是为什么在前一项中引用了动词被拒绝的原因。您可以实施自己的拒绝策略或使用框架提供的内置策略之一。

默认拒绝策略让执行者抛出一个RejectedExecutionException. 但是，其他内置策略允许您：

  • 默默地放弃工作。
  • 放弃最旧的作业并尝试重新提交最后一个作业。
  • 在调用者的线程上执行被拒绝的任务。
于 2015-08-25T11:52:22.173 回答
5

在图片中，只考虑添加任务正在发生

在此处输入图像描述

于 2021-06-11T18:47:24.350 回答
3

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

上面的链接有你问题的答案。但是，只是为了说清楚。应用程序将继续创建线程，直到达到 corePoolSize。我认为这里的想法是这些线程应该足以处理任务的流入。如果在创建 corePoolSize 线程之后有新任务出现，则任务将排队。一旦队列已满，执行程序将开始创建新线程。这是一种平衡。它本质上的意思是，任务的流入量大于处理能力。因此，Executor 将再次开始创建新线程，直到达到最大线程数。同样，当且仅当队列已满时，才会创建一个新线程。

于 2013-09-02T10:40:44.057 回答
3

这个博客中的很好的解释：

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
于 2018-05-27T21:55:12.663 回答
1

来自Java concurency Essentials一书：

CorePoolSize：ThreadPoolExecutor 有一个属性 corePoolSize，它决定了它会启动多少个线程，直到队列满时才启动新线程

MaximumPoolSize：这个属性决定了最多启动多少个线程。您可以将其设置为整数。MAX_VALUE 为了没有上限

于 2019-08-26T13:08:16.930 回答
0

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

让：

  • N是池中的线程数，getPoolSize(). 活动线程+空闲线程。
  • T是提交给执行者/池的任务数量。
  • C是核心池大小，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，空闲线程不会被分配新的传入任务，而是创建一个新线程。
  • N > C如果有空闲线程，则在那里分配新任务。
  • 如果没有空闲线程，N > C则将新任务放入队列中。此处未创建新线程。
  • 当队列已满时，我们创建新的线程直到M. 如果M达到，我们拒绝任务。重要的是不要在这里创建新线程，直到队列已满！

资料来源：

例子

队列容量为corePoolSize = 0和的示例。maximumPoolSize = 1050

这将导致池中只有一个活动线程，直到队列中有 50 个项目。

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.

队列容量为corePoolSize = 10和的示例。maximumPoolSize = 1050

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`.
于 2019-12-05T19:12:50.500 回答
0

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);
    }
于 2018-10-15T11:40:48.073 回答