线程学习--线程池

Posted 2020-09-22

一、Executor框架

为了更好的控制多线程，jdk提供了一套线程框架Executor，帮助开发人员有效地进行线程控制。他们都在java.util.concurrent包中，是jdk并发包的核心。其中有一个比较重要的类：Executors，他扮演者线程工厂的角色，我们通过Executors创建特定功能的线程池。

Executors创建线程池方法：

newFixedThreadPool()方法，该方法返回一个固定数量的线程池，该方法的线程数始终不变，当有一个任务提交时，若线程池中空闲，则立即执行，若没有，则会被暂缓在一个任务队列中等待有 空闲的线程去执行

newSingleThreadExecutor()方法，创建一个线程的线程池，若空闲则执行，若没有空闲线程则暂缓在任务队列中。

newCachedThreadPool()方法，返回一个可根据实际情况调整线程个数的线程池，不限制最大线程数量，若用空闲的线程则执行任务，若无任务则不创建线程。并且每一个空闲线程会在60秒后自动回收。

newScheduledThreadPool方法，该方法返回一个SchededExecutorService对象，但该线程池可以指定线程的数量。以下为此用法

package com.bjsxt.height.concurrent017;

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

class Temp extends Thread {
    public void run() {
        System.out.println("run");
    }
}

public class ScheduledJob {
    
    public static void main(String args[]) throws Exception {
    
        Temp command = new Temp();
        ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);
        
        ScheduledFuture<?> scheduleTask = scheduler.scheduleWithFixedDelay(command, 5, 1, TimeUnit.SECONDS);
    
    }
}

二、自定义线程池

若Executors工厂类无法满足我们的需求，可以自己去创建自定义的线程池，其实Executor工厂类里面的创建线程方法其内部实现均用了ThreadPoolExecutor这个类，这个类可以自定义线程。构造方法如下

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

}

创建有限队列

package threadpoolexecutor;

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class UseThreadPoolExecutor1 {
    public static void main(String[] args) {
        /**
         * 在使用有界队列时，若有新的任务需要执行，如果线程池实际线程数小于corePoolSize，则优先创建线程，
         * 若大于corePoolSize，则会将任务加入队列，
         * 若队列已满，则在总线程数不大于maximumPoolSize的前提下，创建新的线程，
         * 若线程数大于maximumPoolSize，则执行拒绝策略。或其他自定义方式。
         * 
         */    
        ThreadPoolExecutor pool = new ThreadPoolExecutor(
                1,                 //coreSize
                2,                 //MaxSize
                60,             //60
                TimeUnit.SECONDS, 
                new ArrayBlockingQueue<Runnable>(3)            //指定一种队列 （有界队列）
                //new LinkedBlockingQueue<Runnable>()
                , new MyRejected()
                //, new DiscardOldestPolicy()
                );
        
        MyTask mt1 = new MyTask(1, "任务1");
        MyTask mt2 = new MyTask(2, "任务2");
        MyTask mt3 = new MyTask(3, "任务3");
        MyTask mt4 = new MyTask(4, "任务4");
        MyTask mt5 = new MyTask(5, "任务5");
        MyTask mt6 = new MyTask(6, "任务6");
        
        pool.execute(mt1);
        pool.execute(mt2);
        pool.execute(mt3);
        pool.execute(mt4);
        pool.execute(mt5);
        pool.execute(mt6);
        
        pool.shutdown();
        
    }
}

View Code

package threadpoolexecutor;
public class MyTask implements Runnable {
    private int taskId;
    private String taskName;
    
    public MyTask(int taskId, String taskName){
        this.taskId = taskId;
        this.taskName = taskName;
    }
    
    public int getTaskId() {
        return taskId;
    }

    public void setTaskId(int taskId) {
        this.taskId = taskId;
    }

    public String getTaskName() {
        return taskName;
    }

    public void setTaskName(String taskName) {
        this.taskName = taskName;
    }

    @Override
    public void run() {
        try {
            System.out.println("run taskId =" + this.taskId);
            Thread.sleep(5*1000);
            //System.out.println("end taskId =" + this.taskId);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }        
    }
    
    public String toString(){
        return Integer.toString(this.taskId);
    }
}

View Code

package threadpoolexecutor;
import java.net.HttpURLConnection;
import java.util.concurrent.RejectedExecutionHandler;
import java.util.concurrent.ThreadPoolExecutor;

public class MyRejected implements RejectedExecutionHandler{

    
    public MyRejected(){
    }
    
    @Override
    public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
        System.out.println("自定义处理..");
        System.out.println("当前被拒绝任务为：" + r.toString());
        

    }

}

View Code

创建无线队列

package com.bjsxt.height.concurrent018;

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;

public class UseThreadPoolExecutor2 implements Runnable{

    private static AtomicInteger count = new AtomicInteger(0);
    
    @Override
    public void run() {
        try {
            int temp = count.incrementAndGet();
            System.out.println("任务" + temp);
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
    
    public static void main(String[] args) throws Exception{
        //System.out.println(Runtime.getRuntime().availableProcessors());
        BlockingQueue<Runnable> queue = 
                //new LinkedBlockingQueue<Runnable>();
                new ArrayBlockingQueue<Runnable>(10);
        ExecutorService executor  = new ThreadPoolExecutor(
                    5,         //core
                    10,     //max
                    120L,     //2fenzhong
                    TimeUnit.SECONDS,
                    queue);
        
        for(int i = 0 ; i < 20; i++){
            executor.execute(new UseThreadPoolExecutor2());
        }
        Thread.sleep(1000);
        System.out.println("queue size:" + queue.size());        //10
        Thread.sleep(2000);
    }


}

View Code