廖雪峰Java11多线程编程-3高级concurrent包-6ExecutorService

Java语言内置多线程支持：

• 创建线程需要操作系统资源（线程资源，栈空间）
• 频繁创建和销毁线程需要消耗大量时间

如果可以复用一个线程

线程池：

• 线程池维护若干个线程，处于等待状态
• 如果有新任务，就分配一个空闲线程执行
• 如果所有线程都处于忙碌状态，新任务放入队列等待

ExecutorService

JDK提供了ExecutorService接口表示线程池：

    ExecutorService executor = Executors.newFixedThreadPool(4); //固定大小的线程池
    executor.submit(task1); //提交任务到线程池
    executor.submit(task2);
    executor.submit(task3)

常用的ExecutorService：

• FixedThreadPool：线程数固定
• CachedThreadPool：线程数根据任务动态调整
• SingleThreadExecutor：仅单线程执行

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

class PrintTask implements Runnable{
    String name;
    public PrintTask(String name){
        this.name = name;
    }
    public void run(){
        for(int i=0;i<3;i++){
            System.out.println(i+" Hello,"+name+"!");
            try{
                Thread.sleep(1000);
            }catch (InterruptedException e){}
        }
    }
}
public class ThreadPool {
    public static void main(String[] args) throws InterruptedException{
        ExecutorService executor = Executors.newFixedThreadPool(3); //指定线程数为3
        executor.submit(new PrintTask("Bob"));
        executor.submit(new PrintTask("Alice"));
        executor.submit(new PrintTask("Tim"));
        executor.submit(new PrintTask("Robot"));
        Thread.sleep(10000);
        executor.shutdown(); //结束线程池
    }
}

        //单个线程
        ExecutorService executor = Executors.newSingleThreadExecutor();

        //动态调整的线程池
        ExecutorService executor = Executors.newCachedThreadPool();

查看newCachedThreadPool源码，发现其实现的是ThreadPoolExecutor的构造方法，

    public static ExecutorService newCachedThreadPool() {
        return new ThreadPoolExecutor(0, //初始化线程池的大小
                                      Integer.MAX_VALUE,//线程池的最大值
                                      60L, TimeUnit.SECONDS,
                                      new SynchronousQueue<Runnable>());
    }

        //设置最大数量为10的动态线程池
        ExecutorService executor = new ThreadPoolExecutor(0, 10,
                60L, TimeUnit.SECONDS,
                new SynchronousQueue<Runnable>());

ScheduledThreadPool：一个任务可以定期反复执行

执行模式：

• Fixed Rate：固定的间隔，任务就会执行。例如每隔3秒任务就会启动，而不管这个任务执行多长、是否结束
• Fixed Delay：当任务执行完毕以后，等待1秒钟再继续执行。无论任务执行多久，只有在任务结束以后，等待1秒钟才会开始执行下一次的任务。
  注意：ScheduledExecutorService不会自动停止

import java.time.LocalTime;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

class HelloTask implements Runnable{
    String name;
    public HelloTask(String name){
        this.name = name;
    }
    public void run(){
        System.out.println("Hello,"+name+" ! It is "+LocalTime.now());
        try{
            Thread.sleep(1000);
        }catch (InterruptedException e){}
        System.out.println("Goodbye, "+name+"! It is "+LocalTime.now());
    }

}
public class SchedulePool {
    public static void main(String[] args) throws Exception{
        ScheduledExecutorService executor = Executors.newScheduledThreadPool(3);
        executor.scheduleAtFixedRate(new HelloTask("Bob"),2,5,TimeUnit.SECONDS);
        executor.scheduleWithFixedDelay(new HelloTask("Alice"),2,5,TimeUnit.SECONDS);
    }
}

Bob的执行频率比Alice高的多，任务开始的时间差也越来越大
问题：
1.FixedRate模式下，如果任务执行时间过长，后续任务会不会并发执行？

不会

import java.time.LocalTime;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

class HelloTask implements Runnable{
    String name;
    public HelloTask(String name){
        this.name = name;
    }
    public void run(){
        System.out.println("Hello,"+name+" ! It is "+LocalTime.now());
        try{
            Thread.sleep(10000);
        }catch (InterruptedException e){}
        System.out.println("Goodbye, "+name+"! It is "+LocalTime.now());
    }

}
public class SchedulePool {
    public static void main(String[] args) throws Exception{
        ScheduledExecutorService executor = Executors.newScheduledThreadPool(3);
        executor.scheduleAtFixedRate(new HelloTask("Bob"),2,1,TimeUnit.SECONDS);

    }
}

2.如果任务抛出了异常，后续任务是否继续执行?
不会

import java.time.LocalTime;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

class HelloTask implements Runnable{
    String name;
    int count;
    public HelloTask(String name,int count){
        this.name = name;
        this.count = count;
    }
    public void run(){
        System.out.println("Hello,"+name+" ! It is "+LocalTime.now()+" "+count);
        try{
            if(count == 3){
                throw new RuntimeException("我是故意的");
            }
            Thread.sleep(1000);
        }catch (InterruptedException e){}
        System.out.println("Goodbye, "+name+"! It is "+LocalTime.now());
        count++;
    }

}
public class SchedulePool {
    public static void main(String[] args) throws Exception{
        ScheduledExecutorService executor = Executors.newScheduledThreadPool(3);
        executor.scheduleAtFixedRate(new HelloTask("Bob",0),2,5,TimeUnit.SECONDS);
    }
}

java.util.Timer 也可以定期执行任务

• 一个Timer对应一个Thread，只能定期执行一个任务。如果要执行多个定时任务，就必须要启动多个Timer。
• 必须在主线程结束时跳用Timer.cancel()
  而一个ScheduledPool就可以调度多个任务，所以完全可以用新的Scheduled取代Timer类。

总结：

• JDK提供了ExecutorService实现了线程池功能
• 线程池内部维护一组线程，可以搞笑执行大量小任务
• Executors提供了静态方法创建不同类型的ExecutorService
• 必须调用shutdown()关闭ExecutorService
• ScheduledThreadPool可以定期调度多个任务