如果系统要运行多个线程,大量反复的启动创建和回收线程会非常占用系统资源,导致性能下降.

提交一个任务到线程池中,线程池的处理流程如下:
1.判断线程池里的核心线程是否都在执行任务,如果不是(核心线程空闲或者核心线程没有被创建)则创建一个新的工作线程来执行任务.如果核心线程都在执行任务,则进入下个流程.
2.线程池判断工作队列是否已满,如果工作路径没有满,则新提交的任务储存在这个工作队列里.如果工作队列满了,则进入下个流程.
3.判断线程池里的线程是否都处于工作状态,如果没有,则创建一个新的工作线程来执行任务.如果已经满了,则交给饱和策略来处理这个任务.

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

newCachedThreadPool
创建一个可缓存线程池，如果线程池长度超过处理需要，可灵活回收空闲线程，
若无可回收，则新建线程。

package cn.qbz.thread;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class Test111907 {
    public static void main(String[] args) {
        ExecutorService executorService = Executors.newCachedThreadPool();
        for (int i = 0; i < 10; i++) {
            final int temp = i;
            executorService.execute(new Runnable() {
                @Override
                public void run() {
                    try {
                        Thread.sleep(1000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println(Thread.currentThread().getName() + "   i=" + temp);
                }
            });
        }
    }
}

    public static ExecutorService newCachedThreadPool() {
        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                      60L, TimeUnit.SECONDS,
                                      new SynchronousQueue<Runnable>());
    }

newFixedThreadPool
创建一个定长线程池，可控制线程最大并发数，超出的线程会在队列中等待。

package cn.qbz.thread;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class Test111907 {
    public static void main(String[] args) {
        ExecutorService executorService = Executors.newFixedThreadPool(3);
        for (int i = 0; i < 10; i++) {
            final int temp = i;
            executorService.execute(new Runnable() {
                @Override
                public void run() {
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println(Thread.currentThread().getName() + "   i=" + temp);
                }
            });
        }
    }
}


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

newScheduledThreadPool
创建一个定长线程池，支持定时及周期性任务执行。

package cn.qbz.thread;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

public class Test111907 {
    public static void main(String[] args) {
        final long begin = System.currentTimeMillis();
        ExecutorService executorService = Executors.newScheduledThreadPool(3);
        for (int i = 0; i < 10; i++) {
            final int temp = i;
            final long time = begin;
            executorService.schedule(new Runnable() {
                @Override
                public void run() {
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println(Thread.currentThread().getName() + "   i=" + temp + "   time=" + (System.currentTimeMillis() - time));
                }
            }, 5, TimeUnit.SECONDS);
        }
    }
}
    public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
        return new ScheduledThreadPoolExecutor(corePoolSize);
    }
    public ScheduledThreadPoolExecutor(int corePoolSize) {
        super(corePoolSize, Integer.MAX_VALUE, 0, TimeUnit.NANOSECONDS,
              new DelayedWorkQueue());
    }

newSingleThreadExecutor
创建一个单线程化的线程池，它只会用唯一的工作线程来执行任务，
保证所有任务按照指定顺序(FIFO, LIFO, 优先级)执行。
public class Test111907 {
    public static void main(String[] args) {
        ExecutorService executorService = Executors.newSingleThreadExecutor();
        for (int i = 0; i < 10; i++) {
            final int temp = i;
            executorService.execute(new Runnable() {
                @Override
                public void run() {
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println(Thread.currentThread().getName() + "   i=" + temp);
                }
            });
        }
    }
}