线程池5th卷:不畏浮云遮望眼

随着业务发展的需要，总公司集团老总(Doug Lea ) 提出了能支持预定功能(scheduled)的经营模式(ScheduledExecutorService)。PS： schedule:安排，计划，预定

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1.战略：ScheduledExecutorService

观摩一下其内部新增的功能

1. schedule(Runnable/Callale, delay, TimeUnit )：在指定延迟之后执行一次(one-shot)任务
2. scheduleAtFixedRate(Runnable , initialDelay, period,TimeUnit )：在initDelay后，以period的固定间隔执行任务
3. scheduleWithFixedDelay(Runnable , initialDelay, delay, TimeUnit )：在initDelay后，以固定的delay延时执行任务

根据上面的描述，我们可以发现ScheduledExecutorService主要推出的功能是：延时，定时，周期性服务、。

2.战术：ScheduledThreadPoolExecutor

正所谓“万变不离其宗”,新模式(**ScheduledExecutorService)**也是基于旧模式(ThreadPoolExecutor)而来的。都说“青出于蓝而胜于蓝”，不过眼见为实…

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

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

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

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

通过上面的构造函数(经营模板)，我们会发现，总公司应该是结合以往的经验，对新模式进行了比较大的优化。

1. 必须指定正式工(corePoolSize)；
2. 临时工的数量无限制(maxiumPoolSize = Integer.MAX_VALUE),结合成本只能keepAliveTime = 0.
3. 排队方式(使用DelayedWorkQueue)进行优化. (PS:初始容量为16，基于数据结构堆(heap-based)而设计，加速离队速度(O(n) -> O(log n)),减少城管的骚扰(减少垃圾回收)
4. 开放拒绝策略(RejectedExecutionHandler)
5. 支持选择厨师培训学校(ThreadFactory)

3. 实施：Executors

如果普通客户来连锁店，一般都会来找Executors…只此一家，别无分店.

  /** Cannot instantiate. 不能实例化
	*/
    private Executors() {}

着重强调一下：Executors一般都只会开放corePoolSize，ThreadFactory两个选项。

为客户着想使我们应该做的,

单厨师套餐

public static ScheduledExecutorService newSingleThreadScheduledExecutor() {
        return new DelegatedScheduledExecutorService
            (new ScheduledThreadPoolExecutor(1));
    }

 public static ScheduledExecutorService newSingleThreadScheduledExecutor(ThreadFactory threadFactory) {
        return new DelegatedScheduledExecutorService
            (new ScheduledThreadPoolExecutor(1, threadFactory));
    }

多厨师套餐

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

 public static ScheduledExecutorService newScheduledThreadPool(
            int corePoolSize, ThreadFactory threadFactory) {
        return new ScheduledThreadPoolExecutor(corePoolSize, threadFactory);
    }

后面我们会专门出一篇来讲总代理(Executors)以及VIP客户定制化，圆你一个开店做老板的梦…