Java线程池（ExecutorService）使用

一、前提

/**

 * 线程运行demo，运行时打出线程id以及传入线程中参数

 */

public class ThreadRunner implements Runnable {

    private final SimpleDateFormat format = new SimpleDateFormat("HH:mm:ss.SSS");

    /**

     * 线程私有属性，创建线程时创建

     */

    private Integer num;

    public ThreadRunner(Integer num) {

        this.num = num;

    }

    @Override

    public void run() {

        System.out.println("thread:" + Thread.currentThread().getName() + ",time:" + format.format(new Date()) + ",num:" + num);

        try {//使线程睡眠，模拟线程阻塞情况

            TimeUnit.SECONDS.sleep(1);

        } catch (InterruptedException e) {

            e.printStackTrace();

        }

    }

}

二、分类

1、FixedThreadPool-有一个固定大小的线程池

public class FixedThreadPoolDemo {

    public static void main(String[] args) {

        ExecutorService pool = Executors.newFixedThreadPool(4);

        for(int i = 0 ; i < 50 ; i++){

            pool.submit(new ThreadRunner((i + 1)));

        }

        pool.shutdown();

    }

}

thread:pool-1-thread-2,time:16:14:45.677,num:2

thread:pool-1-thread-4,time:16:14:45.678,num:4

thread:pool-1-thread-3,time:16:14:45.680,num:3

thread:pool-1-thread-1,time:16:14:45.684,num:1

thread:pool-1-thread-4,time:16:14:46.680,num:5

thread:pool-1-thread-2,time:16:14:46.680,num:6

thread:pool-1-thread-3,time:16:14:46.680,num:7

thread:pool-1-thread-1,time:16:14:46.684,num:8

thread:pool-1-thread-4,time:16:14:47.680,num:9

thread:pool-1-thread-2,time:16:14:47.680,num:10

thread:pool-1-thread-3,time:16:14:47.681,num:11

thread:pool-1-thread-1,time:16:14:47.684,num:12

thread:pool-1-thread-4,time:16:14:48.681,num:13

thread:pool-1-thread-2,time:16:14:48.681,num:14

thread:pool-1-thread-3,time:16:14:48.681,num:15

thread:pool-1-thread-1,time:16:14:48.684,num:16

thread:pool-1-thread-4,time:16:14:49.681,num:17

thread:pool-1-thread-2,time:16:14:49.682,num:18

thread:pool-1-thread-3,time:16:14:49.682,num:19

thread:pool-1-thread-1,time:16:14:49.684,num:20

thread:pool-1-thread-4,time:16:14:50.681,num:21

thread:pool-1-thread-2,time:16:14:50.682,num:22

thread:pool-1-thread-3,time:16:14:50.682,num:23

thread:pool-1-thread-1,time:16:14:50.684,num:24

thread:pool-1-thread-4,time:16:14:51.681,num:25

thread:pool-1-thread-2,time:16:14:51.682,num:26

thread:pool-1-thread-3,time:16:14:51.682,num:27

thread:pool-1-thread-1,time:16:14:51.684,num:28

thread:pool-1-thread-4,time:16:14:52.681,num:29

thread:pool-1-thread-2,time:16:14:52.682,num:30

thread:pool-1-thread-3,time:16:14:52.682,num:31

thread:pool-1-thread-1,time:16:14:52.684,num:32

thread:pool-1-thread-4,time:16:14:53.681,num:33

thread:pool-1-thread-2,time:16:14:53.682,num:34

thread:pool-1-thread-3,time:16:14:53.683,num:35

thread:pool-1-thread-1,time:16:14:53.685,num:36

thread:pool-1-thread-2,time:16:14:54.682,num:38

thread:pool-1-thread-4,time:16:14:54.682,num:37

thread:pool-1-thread-3,time:16:14:54.683,num:39

thread:pool-1-thread-1,time:16:14:54.686,num:40

thread:pool-1-thread-2,time:16:14:55.682,num:41

thread:pool-1-thread-4,time:16:14:55.682,num:42

thread:pool-1-thread-3,time:16:14:55.683,num:43

thread:pool-1-thread-1,time:16:14:55.686,num:44

thread:pool-1-thread-2,time:16:14:56.682,num:45

thread:pool-1-thread-4,time:16:14:56.683,num:46

thread:pool-1-thread-3,time:16:14:56.684,num:47

thread:pool-1-thread-1,time:16:14:56.686,num:48

thread:pool-1-thread-2,time:16:14:57.683,num:49

thread:pool-1-thread-4,time:16:14:57.683,num:50

总结：
- 池中线程数量固定，不会发生变化
- 使用无界的LinkedBlockingQueue，要综合考虑生成与消费能力，生成过剩，可能导致堆内存溢出。
- 适用一些很稳定很固定的正规并发线程，多用于服务器

2、CachedThreadPool

public class CachedThreadPoolDemo {

    public static void main(String[] args) {

        ExecutorService pool = Executors.newCachedThreadPool();

        for(int i = 0 ; i < 50 ; i++){

            pool.submit(new ThreadRunner((i + 1)));

        }

        pool.shutdown();

    }

}

thread:pool-1-thread-2,time:16:17:21.289,num:2

thread:pool-1-thread-3,time:16:17:21.290,num:3

thread:pool-1-thread-4,time:16:17:21.290,num:4

thread:pool-1-thread-6,time:16:17:21.291,num:6

thread:pool-1-thread-7,time:16:17:21.291,num:7

thread:pool-1-thread-8,time:16:17:21.291,num:8

thread:pool-1-thread-1,time:16:17:21.292,num:1

thread:pool-1-thread-10,time:16:17:21.293,num:10

thread:pool-1-thread-5,time:16:17:21.294,num:5

thread:pool-1-thread-11,time:16:17:21.294,num:11

thread:pool-1-thread-15,time:16:17:21.294,num:15

thread:pool-1-thread-9,time:16:17:21.294,num:9

thread:pool-1-thread-16,time:16:17:21.295,num:16

thread:pool-1-thread-20,time:16:17:21.295,num:20

thread:pool-1-thread-14,time:16:17:21.296,num:14

thread:pool-1-thread-12,time:16:17:21.296,num:12

thread:pool-1-thread-19,time:16:17:21.297,num:19

thread:pool-1-thread-13,time:16:17:21.299,num:13

thread:pool-1-thread-17,time:16:17:21.300,num:17

thread:pool-1-thread-18,time:16:17:21.302,num:18

thread:pool-1-thread-22,time:16:17:21.304,num:22

thread:pool-1-thread-23,time:16:17:21.304,num:23

thread:pool-1-thread-24,time:16:17:21.305,num:24

thread:pool-1-thread-21,time:16:17:21.305,num:21

thread:pool-1-thread-26,time:16:17:21.305,num:26

thread:pool-1-thread-25,time:16:17:21.306,num:25

thread:pool-1-thread-29,time:16:17:21.307,num:29

thread:pool-1-thread-28,time:16:17:21.308,num:28

thread:pool-1-thread-30,time:16:17:21.308,num:30

thread:pool-1-thread-34,time:16:17:21.308,num:34

thread:pool-1-thread-35,time:16:17:21.308,num:35

thread:pool-1-thread-33,time:16:17:21.308,num:33

thread:pool-1-thread-27,time:16:17:21.309,num:27

thread:pool-1-thread-32,time:16:17:21.308,num:32

thread:pool-1-thread-31,time:16:17:21.309,num:31

thread:pool-1-thread-36,time:16:17:21.310,num:36

thread:pool-1-thread-37,time:16:17:21.310,num:37

thread:pool-1-thread-38,time:16:17:21.310,num:38

thread:pool-1-thread-42,time:16:17:21.310,num:42

thread:pool-1-thread-40,time:16:17:21.310,num:40

thread:pool-1-thread-41,time:16:17:21.311,num:41

thread:pool-1-thread-47,time:16:17:21.762,num:47

thread:pool-1-thread-43,time:16:17:21.762,num:43

thread:pool-1-thread-39,time:16:17:21.762,num:39

thread:pool-1-thread-45,time:16:17:21.762,num:45

thread:pool-1-thread-44,time:16:17:21.763,num:44

thread:pool-1-thread-46,time:16:17:21.761,num:46

thread:pool-1-thread-48,time:16:17:21.761,num:48

thread:pool-1-thread-49,time:16:17:21.765,num:49

thread:pool-1-thread-50,time:16:17:21.765,num:50

总结
- 池中线程时随着处理数据增加而增加
- 线程数并不是一直增加，如果有新任务需要执行时，首先查询池中是否有空闲线程并且还为到空闲截止时间，如果有，则使用空闲线程，如果没有，则创建新线程并放入池中。
- 用于执行一些生存期很短的异步型任务。不适用于IO等长延时操作，因为这可能会创建大量线程，导致系统崩溃。
- 使用SynchronousQueue作为阻塞队列，如果有新任务进入队列，必须队列中数据被其他线程处理，否则会等待。

3、SingleThreadExecutor

public class SingleThreadPoolDemo {

    public static void main(String[] args) {

        ExecutorService pool = Executors.newSingleThreadExecutor();

        for(int i = 0 ; i < 50 ; i++){

            pool.submit(new ThreadRunner((i + 1)));

        }

        pool.shutdown();

    }

}

thread:pool-1-thread-1,time:16:20:10.194,num:1

thread:pool-1-thread-1,time:16:20:11.197,num:2

thread:pool-1-thread-1,time:16:20:12.197,num:3

thread:pool-1-thread-1,time:16:20:13.197,num:4

thread:pool-1-thread-1,time:16:20:14.197,num:5

thread:pool-1-thread-1,time:16:20:15.198,num:6

thread:pool-1-thread-1,time:16:20:16.198,num:7

thread:pool-1-thread-1,time:16:20:17.198,num:8

thread:pool-1-thread-1,time:16:20:18.198,num:9

thread:pool-1-thread-1,time:16:20:19.198,num:10

thread:pool-1-thread-1,time:16:20:20.198,num:11

thread:pool-1-thread-1,time:16:20:21.199,num:12

thread:pool-1-thread-1,time:16:20:22.200,num:13

thread:pool-1-thread-1,time:16:20:23.200,num:14

thread:pool-1-thread-1,time:16:20:24.200,num:15

thread:pool-1-thread-1,time:16:20:25.200,num:16

thread:pool-1-thread-1,time:16:20:26.201,num:17

thread:pool-1-thread-1,time:16:20:27.201,num:18

thread:pool-1-thread-1,time:16:20:28.201,num:19

thread:pool-1-thread-1,time:16:20:29.201,num:20

thread:pool-1-thread-1,time:16:20:30.202,num:21

thread:pool-1-thread-1,time:16:20:31.202,num:22

thread:pool-1-thread-1,time:16:20:32.203,num:23

thread:pool-1-thread-1,time:16:20:33.203,num:24

thread:pool-1-thread-1,time:16:20:34.203,num:25

thread:pool-1-thread-1,time:16:20:35.203,num:26

thread:pool-1-thread-1,time:16:20:36.203,num:27

thread:pool-1-thread-1,time:16:20:37.203,num:28

thread:pool-1-thread-1,time:16:20:38.203,num:29

thread:pool-1-thread-1,time:16:20:39.203,num:30

thread:pool-1-thread-1,time:16:20:40.203,num:31

thread:pool-1-thread-1,time:16:20:41.203,num:32

thread:pool-1-thread-1,time:16:20:42.203,num:33

thread:pool-1-thread-1,time:16:20:43.204,num:34

thread:pool-1-thread-1,time:16:20:44.204,num:35

thread:pool-1-thread-1,time:16:20:45.204,num:36

thread:pool-1-thread-1,time:16:20:46.204,num:37

thread:pool-1-thread-1,time:16:20:47.205,num:38

thread:pool-1-thread-1,time:16:20:48.205,num:39

thread:pool-1-thread-1,time:16:20:49.205,num:40

thread:pool-1-thread-1,time:16:20:50.206,num:41

thread:pool-1-thread-1,time:16:20:51.206,num:42

thread:pool-1-thread-1,time:16:20:52.207,num:43

thread:pool-1-thread-1,time:16:20:53.207,num:44

thread:pool-1-thread-1,time:16:20:54.207,num:45

thread:pool-1-thread-1,time:16:20:55.207,num:46

thread:pool-1-thread-1,time:16:20:56.207,num:47

thread:pool-1-thread-1,time:16:20:57.208,num:48

thread:pool-1-thread-1,time:16:20:58.208,num:49

thread:pool-1-thread-1,time:16:20:59.209,num:50

总结：
- 线程中只有一个线程在执行
- 适用于有明确执行顺序但是不影响主线程的任务，压入池中的任务会按照队列顺序执行。
- 使用无界的LinkedBlockingQueue，要综合考虑生成与消费能力，生成过剩，可能导致堆内存溢出。

三、源码

public static ExecutorService newFixedThreadPool(int nThreads) {

        return new ThreadPoolExecutor(nThreads, nThreads,

                                      0L, TimeUnit.MILLISECONDS,

                                      new LinkedBlockingQueue<Runnable>());

public static ExecutorService newCachedThreadPool() {

        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,

                                      60L, TimeUnit.SECONDS,

                                      new SynchronousQueue<Runnable>());

 public static ExecutorService newSingleThreadExecutor() {

        return new FinalizableDelegatedExecutorService

            (new ThreadPoolExecutor(1, 1,

                                    0L, TimeUnit.MILLISECONDS,

                                    new LinkedBlockingQueue<Runnable>()));

    }

ThreadPoolExecutor 构造方法*

public ThreadPoolExecutor(int corePoolSize,

                          int maximumPoolSize,

                          long keepAliveTime,

                          TimeUnit unit,

                          BlockingQueue<Runnable> workQueue) {

    this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,

         Executors.defaultThreadFactory(), defaultHandler);

}

 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.corePoolSize = corePoolSize;

    this.maximumPoolSize = maximumPoolSize;

    this.workQueue = workQueue;

    this.keepAliveTime = unit.toNanos(keepAliveTime);

    this.threadFactory = threadFactory;

    this.handler = handler;

}

corePoolSize:线程池核心线程数量
1. 如果池中线程数量少于核心线程池数量，则直接新建线程处理当前任务。
2. 核心线程池空闲不会被回收。
3. 当池中无空闲线程时，新任务将被添加到阻塞队列
maximumPoolSize：线程池最大线程数量
1. 当阻塞队列已满，并且有新任务还在入队时，创建新的线程处理，直到线程数大于maximumPoolSize。
2. 超出corePoolSize部分的线程超过空闲时间后会被回收
3. 当线程已经超出corePoolSize，并且队列容量已满，则拒绝入队。
keepAliveTime unit：线程存活时间
1. 当线程超出corePoolSize时生效
2. 线程空余keepAliveTime后，将被回收
workQueue：线程使用阻塞队列
threadFactory：创建线程池工厂
1. 用于控制创建线程或者销毁线程时加入其它逻辑
handler：线程池拒绝策略
1. 直接丢弃（DiscardPolicy）
2. 丢弃队列中最老的任务(DiscardOldestPolicy)。
3. 抛异常(AbortPolicy)
4. 将任务分给调用线程来执行(CallerRunsPolicy)

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