Java并发：Executor与连接池

 概述

首先来说一说java连接池中常用到的几个类：Executor，ExecutorService，ScheduledExecutorService

• Executor

执行已经提交的任务对象。此接口提供了将任务提交和任务执行分离的机制。

• ExecutorService

它是Executor的子接口，可以终止提交新的线程任务，可以中式线程池里现有的所有线程，还可以批量提交线程任务等。它的方法有很多，可以详细阅读相关的api。

• ScheduledExecutorService

可延时执行线程任务

本文中案例中的线程实现如下：

public class ThreadDemo implements Runnable{
    private String threadName = null;
    private boolean flag = true;
    private int count;
    private int counter;
    private long suspend;
    
    /**
     * This is the constructor
     * @param threadName
     * @param count 循环次数
     * @param suspend 线程终端时间，单位毫秒
     */
    public ThreadDemo(String threadName, int count, long suspend) {
        super();
        this.threadName = threadName;
        this.count = count;
        this.suspend = suspend;
    }

    /**
     * run
     */
    @Override
    public void run() {
        while (flag) {
            try {
                Thread.sleep(suspend);
                System.out.println(threadName+"--------------"+counter);
                counter++;
                if(counter>count){
                    flag = false;
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

创建一个无限大的线程池

ExecutorService executorService1 = Executors.newCachedThreadPool();
executorService1.execute(new ThreadDemo("线程B-1", 5, 200));
executorService1.execute(new ThreadDemo("线程B-2", 5, 200));
executorService1.shutdown();
executorService1.shutdown()作用是拒绝接收其它的线程，在线程池里的线程都执行完后，关闭线程池。

创建一个特定大小的线程池

ExecutorService executorService1 = Executors.newFixedThreadPool(2);
executorService1.submit(new ThreadDemo("线程B-1", 5, 200));
executorService1.submit(new ThreadDemo("线程B-2", 5, 200));
executorService1.submit(new ThreadDemo("线程B-3", 5, 200));
executorService1.shutdown();

这里创建的线程池大小为2，若是提交的线程个数大于2，多余的则会在队列里等候

创建一个单线程执行的线程池

ExecutorService executorService1 = Executors.newSingleThreadExecutor();
executorService1.execute(new ThreadDemo("线程A-1", 5, 200));
executorService1.execute(new ThreadDemo("线程A-2", 5, 200));
executorService1.shutdown();

可提交多个线程，但一次只运行一个线程，其它的在队列里等待

创建延迟执行线程的的线程池

ScheduledExecutorService executorService1 = Executors.newScheduledThreadPool(2);
executorService1.schedule(new ThreadDemo("线程B-1", 5, 500),1,TimeUnit.SECONDS);
executorService1.schedule(new ThreadDemo("线程B-2", 5, 500),2,TimeUnit.SECONDS);
executorService1.schedule(new ThreadDemo("线程B-3", 5, 500),3,TimeUnit.SECONDS);
executorService1.shutdown();

关于Executors.newScheduledThreadPool(int corePoolSize)中的corePoolSize：当提交一个任务到线程池时，线程池会创建一个线程来执行任务，即使其他空闲的基本线程能够执行新任务也会创建线程，等到需要执行的任务数大于线程池基本大小时就不再创建。如果调用了线程池的prestartAllCoreThreads方法，线程池会提前创建并启动所有基本线程。

创建定时单线程池

ScheduledExecutorService executorService1 = Executors.newSingleThreadScheduledExecutor();
executorService1.schedule(new ThreadDemo("线程A-1", 5, 500),1,TimeUnit.SECONDS);
executorService1.schedule(new ThreadDemo("线程A-2", 5, 500),2,TimeUnit.SECONDS);
executorService1.shutdown();

每个类有什么方法，看具体的api，以上只是语法范例。