线程池之ThreadPoolExecutor使用

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ThreadPoolExecutor提供了四个构造方法:


ThreadPoolExecutor构造方法.png

我们以最后一个构造方法(参数最多的那个),对其参数进行解释:

 public ThreadPoolExecutor(int corePoolSize, // 1
                              int maximumPoolSize,  // 2
                              long keepAliveTime,  // 3
                              TimeUnit unit,  // 4
                              BlockingQueue<Runnable> workQueue, // 5
                              ThreadFactory threadFactory,  // 6
                              RejectedExecutionHandler handler ) { //7
        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;
    }
序号 名称 类型 含义
1 corePoolSize int 核心线程池大小
2 maximumPoolSize int 最大线程池大小
3 keepAliveTime long 线程最大空闲时间
4 unit TimeUnit 时间单位
5 workQueue BlockingQueue<Runnable> 线程等待队列
6 threadFactory ThreadFactory 线程创建工厂
7 handler RejectedExecutionHandler 拒绝策略

如果对这些参数作用有疑惑的请看 ThreadPoolExecutor概述
知道了各个参数的作用后,我们开始构造符合我们期待的线程池。首先看JDK给我们预定义的几种线程池:

一、预定义线程池
  1. FixedThreadPool
    public static ExecutorService newFixedThreadPool(int nThreads) {
        return new ThreadPoolExecutor(nThreads, nThreads,
                                      0L, TimeUnit.MILLISECONDS,
                                      new LinkedBlockingQueue<Runnable>());
    }
  • corePoolSize与maximumPoolSize相等,即其线程全为核心线程,是一个固定大小的线程池,是其优势;
  • keepAliveTime = 0 该参数默认对核心线程无效,而FixedThreadPool全部为核心线程;
  • workQueue 为LinkedBlockingQueue(无界阻塞队列),队列最大值为Integer.MAX_VALUE。如果任务提交速度持续大余任务处理速度,会造成队列大量阻塞。因为队列很大,很有可能在拒绝策略前,内存溢出。是其劣势;
  • FixedThreadPool的任务执行是无序的;

适用场景:可用于Web服务瞬时削峰,但需注意长时间持续高峰情况造成的队列阻塞。

  1. CachedThreadPool
     public static ExecutorService newCachedThreadPool() {
        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                      60L, TimeUnit.SECONDS,
                                      new SynchronousQueue<Runnable>());
    }
  • corePoolSize = 0,maximumPoolSize = Integer.MAX_VALUE,即线程数量几乎无限制;
  • keepAliveTime = 60s,线程空闲60s后自动结束。
  • workQueue 为 SynchronousQueue 同步队列,这个队列类似于一个接力棒,入队出队必须同时传递,因为CachedThreadPool线程创建无限制,不会有队列等待,所以使用SynchronousQueue;

适用场景:快速处理大量耗时较短的任务,如Netty的NIO接受请求时,可使用CachedThreadPool。

  1. SingleThreadExecutor
    public static ExecutorService newSingleThreadExecutor() {
        return new FinalizableDelegatedExecutorService
            (new ThreadPoolExecutor(1, 1,
                                    0L, TimeUnit.MILLISECONDS,
                                    new LinkedBlockingQueue<Runnable>()));
    }

咋一瞅,不就是newFixedThreadPool(1)吗?定眼一看,这里多了一层FinalizableDelegatedExecutorService包装,这一层有什么用呢,写个dome来解释一下:

    public static void main(String[] args) {
        ExecutorService fixedExecutorService = Executors.newFixedThreadPool(1);
        ThreadPoolExecutor threadPoolExecutor = (ThreadPoolExecutor) fixedExecutorService;
        System.out.println(threadPoolExecutor.getMaximumPoolSize());
        threadPoolExecutor.setCorePoolSize(8);
        
        ExecutorService singleExecutorService = Executors.newSingleThreadExecutor();
//      运行时异常 java.lang.ClassCastException
//      ThreadPoolExecutor threadPoolExecutor2 = (ThreadPoolExecutor) singleExecutorService;
    }

对比可以看出,FixedThreadPool可以向下转型为ThreadPoolExecutor,并对其线程池进行配置,而SingleThreadExecutor被包装后,无法成功向下转型。因此,SingleThreadExecutor被定以后,无法修改,做到了真正的Single。

  1. ScheduledThreadPool
    public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
        return new ScheduledThreadPoolExecutor(corePoolSize);
    }

newScheduledThreadPool调用的是ScheduledThreadPoolExecutor的构造方法,而ScheduledThreadPoolExecutor继承了ThreadPoolExecutor,构造是还是调用了其父类的构造方法。

    public ScheduledThreadPoolExecutor(int corePoolSize) {
        super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
              new DelayedWorkQueue());
    }

对于ScheduledThreadPool本文不做描述,其特性请关注后续篇章。

二、自定义线程池

以下是自定义线程池,使用了有界队列,自定义ThreadFactory和拒绝策略的demo:

public class ThreadTest {

    public static void main(String[] args) throws InterruptedException, IOException {
        int corePoolSize = 2;
        int maximumPoolSize = 4;
        long keepAliveTime = 10;
        TimeUnit unit = TimeUnit.SECONDS;
        BlockingQueue<Runnable> workQueue = new ArrayBlockingQueue<>(2);
        ThreadFactory threadFactory = new NameTreadFactory();
        RejectedExecutionHandler handler = new MyIgnorePolicy();
        ThreadPoolExecutor executor = new ThreadPoolExecutor(corePoolSize, maximumPoolSize, keepAliveTime, unit,
                workQueue, threadFactory, handler);
        executor.prestartAllCoreThreads(); // 预启动所有核心线程
        
        for (int i = 1; i <= 10; i++) {
            MyTask task = new MyTask(String.valueOf(i));
            executor.execute(task);
        }

        System.in.read(); //阻塞主线程
    }

    static class NameTreadFactory implements ThreadFactory {

        private final AtomicInteger mThreadNum = new AtomicInteger(1);

        @Override
        public Thread newThread(Runnable r) {
            Thread t = new Thread(r, "my-thread-" + mThreadNum.getAndIncrement());
            System.out.println(t.getName() + " has been created");
            return t;
        }
    }

    public static class MyIgnorePolicy implements RejectedExecutionHandler {

        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
            doLog(r, e);
        }

        private void doLog(Runnable r, ThreadPoolExecutor e) {
            // 可做日志记录等
            System.err.println( r.toString() + " rejected");
//          System.out.println("completedTaskCount: " + e.getCompletedTaskCount());
        }
    }

    static class MyTask implements Runnable {
        private String name;

        public MyTask(String name) {
            this.name = name;
        }

        @Override
        public void run() {
            try {
                System.out.println(this.toString() + " is running!");
                Thread.sleep(3000); //让任务执行慢点
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

        public String getName() {
            return name;
        }

        @Override
        public String toString() {
            return "MyTask [name=" + name + "]";
        }
    }
}

输出结果如下:


image.png

其中线程线程1-4先占满了核心线程和最大线程数量,然后4、5线程进入等待队列,7-10线程被直接忽略拒绝执行,等1-4线程中有线程执行完后通知4、5线程继续执行。

总结,通过自定义线程池,我们可以更好的让线程池为我们所用,更加适应我的实际场景。

多线程系列目录(不断更新中):
线程启动原理
线程中断机制
多线程实现方式
FutureTask实现原理
线程池之ThreadPoolExecutor概述
线程池之ThreadPoolExecutor使用
线程池之ThreadPoolExecutor状态控制
线程池之ThreadPoolExecutor执行原理
线程池之ScheduledThreadPoolExecutor概述
线程池的优雅关闭实践

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