线程池ThreadPool

Posted 2021-03-05 hpdblogs

线程池要在执行execute时才会正式创建线程

public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue,
                              ThreadFactory threadFactory,
                              RejectedExecutionHandler handler)

线程池7大参数

corePoolSize：线程池中心的常驻核心线程数

maximumPoolSize：线程池中能够容纳同时执行的最大线程数，此值必须要大于等于！

keepAliveTime：多余的空间线程的存活时间，当前池中线程数量超过corePoolSize时，当空闲时间达到keepAliveTime，多余线程会被销毁直到只剩下corePoolSize个线程为止

TimeUnit：KeepAliveTime的单位 

workQueue：任务队列，被提交但尚未被执行的任务

threadFactory：表示生成线程池中工作线程的线程工厂，用于创建线程，一般默认即可

handler：拒绝策略，表示当前队列满了，并且工作线程大于等于线程池的最大线程数时如何拒绝请求执行的runnable的策略

public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory) {
    return new ThreadPoolExecutor(nThreads, nThreads,
                                  0L, TimeUnit.MILLISECONDS,
                                  new LinkedBlockingQueue<Runnable>(),
                                  threadFactory);
}

public ThreadPoolExecutor(int corePoolSize,
                          int maximumPoolSize,
                          long keepAliveTime,
                          TimeUnit unit,
                          BlockingQueue<Runnable> workQueue,
                          ThreadFactory threadFactory) {
    this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
         threadFactory, defaultHandler);

 
在实际工作中

newFixedThreadPool（固定）

newSingleThreadExecutor（单一）

newCachedThreadPool（可变）

的这三个接口都不用，原因在于
（1）FixedThreadPool 和 SingleThreadPool 允许的请求队列长度为Integer.MAX_VALUE，可能会堆积大量的请求，从而导致OOM

（2）CachedThreadPool 和 ScheduledThreadPool 允许的请求队列长度为Integer.MAX_VALUE，可能会堆积大量的线程，从而导致OOM

例子
public class MyThreadPoolDemo {
    public static void main(String[] args) {
        ExecutorService threadPool = new ThreadPoolExecutor(
                2,
                5,
                3L,
                TimeUnit.SECONDS,
                new ArrayBlockingQueue<Runnable>(3),
                Executors.defaultThreadFactory(),
                new ThreadPoolExecutor.DiscardPolicy()
                //new ThreadPoolExecutor.DiscardOldestPolicy()
                //new ThreadPoolExecutor.CallerRunsPolicy()
                //new ThreadPoolExecutor.AbortPolicy()
                );
        try {
            for (int i = 1; i <=10 ; i++) {
                threadPool.execute(()->{
                    System.out.println(Thread.currentThread().getName()
                            +"	 号业务员办理业务");
                });
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            threadPool.shutdown();
        }

    }

    private static void threadPool() {
        ExecutorService threadPool = Executors.newFixedThreadPool(3);//银行网点3个窗口
        ExecutorService threadPool2 = Executors.newSingleThreadExecutor();//银行网点1个窗口
        ExecutorService threadPool3 = Executors.newCachedThreadPool();//银行网点可扩展窗口

        try {
            for (int i = 1; i <=30 ; i++) {
                threadPool3.execute(()->{
                    System.out.println(Thread.currentThread().getName()
                            +"	 号业务员办理业务");
                });
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            threadPool3.shutdown();
        }
    }


}