一、Executors提供四种线程池配置方案

1、构造一个固定线程数目的线程池，核心线程数与最大线程数相同，同时使用了一个无界LinkedBlockingQueue存放阻塞任务，因此多余的任务将存在阻塞队列，不会由RejectedExecutionHandler处理

//固定线程数目的线程池源码
public static ExecutorService newFixedThreadPool(int nThreads) {  
        return new ThreadPoolExecutor(nThreads, nThreads,  
                                      0L, TimeUnit.MILLISECONDS,  
                                      new LinkedBlockingQueue());  
 } 
 //无界LinkedBlockingQueue源码 默认是Integer.MAX_VALUE大小，也可以指定大小
 public LinkedBlockingQueue() {
        this(Integer.MAX_VALUE);
 } 
 public LinkedBlockingQueue(int capacity) {
        if (capacity <= 0) throw new IllegalArgumentException();
        this.capacity = capacity;
        last = head = new Node(null);
 }

2、构造一个缓冲功能的线程池，配置核心线程数corePoolSize=0，最大线程数maximumPoolSize=Integer.MAX_VALUE，keepAliveTime=60s,以及一个无容量的阻塞队列 SynchronousQueue，因此任务提交之后，将会创建新的线程执行；线程空闲超过60s将会销毁

//缓冲功能的线程池源码
public static ExecutorService newCachedThreadPool() {  
        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,  
                                      60L, TimeUnit.SECONDS,  
                                      new SynchronousQueue());  
}  

3、构造一个只支持一个线程的线程池，配置corePoolSize=maximumPoolSize=1，无界阻塞队列LinkedBlockingQueue；保证任务由一个线程串行执行

public static ExecutorService newSingleThreadExecutor() {  
        return new FinalizableDelegatedExecutorService  
            (new ThreadPoolExecutor(1, 1,  
                                    0L, TimeUnit.MILLISECONDS,  
                                    new LinkedBlockingQueue()));  
    }  

4、构造有定时功能的线程池，配置corePoolSize，无界延迟阻塞队列DelayedWorkQueue；有意思的是：maximumPoolSize=Integer.MAX_VALUE，由于DelayedWorkQueue是无界队列，所以这个值是没有意义的

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

注意：阿里巴巴开发手册强调在日常编写代码时不要使用Executors显示的去创建线程池。

 二 线程池执行流程

 ThreadPoolExecutor最核心的构造方法源码

public ThreadPoolExecutor(int corePoolSize, //核心线程数
                          int maximumPoolSize, //最大线程数
                          long keepAliveTime,  //线程池中超过核心线程数目的空闲线程最大存活时间
                          TimeUnit unit,        //时间单位
                          BlockingQueue workQueue,//阻塞任务队列
                          ThreadFactory threadFactory, //新建线程工厂
                          RejectedExecutionHandler handler //拒绝策略
                          //当提交任务数超过最大线程数+队列(maxmumPoolSize+workQueue)之和时，
                          //任务会交给RejectedExecutionHandler来处理
                          ) {
        if (corePoolSize < 0 ||
            maximumPoolSize <= 0 ||
            maximumPoolSize < corePoolSize ||
            keepAliveTime < 0)
            throw new IllegalArgumentException();
        if (workQueue == null || threadFactory == null || handler == null)
            throw new NullPointerException();
        this.acc = System.getSecurityManager() == null ?
                null :
                AccessController.getContext();
        this.corePoolSize = corePoolSize;
        this.maximumPoolSize = maximumPoolSize;
        this.workQueue = workQueue;
        this.keepAliveTime = unit.toNanos(keepAliveTime);
        this.threadFactory = threadFactory;
        this.handler = handler;
    }

//四种拒绝策略
RejectedExecutionHandler rejected = null;
rejected = new ThreadPoolExecutor.AbortPolicy();//默认，队列满了丢任务抛出异常
rejected = new ThreadPoolExecutor.DiscardPolicy();//队列满了丢任务不异常
rejected = new ThreadPoolExecutor.DiscardOldestPolicy();//将最早进入队列的任务删，之后再尝试加入队列
rejected = new ThreadPoolExecutor.CallerRunsPolicy();//如果添加到线程池失败，那么主线程会自己去执行该任务

自定义线程池和自定义拒绝策略

import java.util.concurrent.*;
import java.util.concurrent.atomic.AtomicInteger;

public class MyThreadPoolExecutor {

    private ThreadPoolExecutor pool = null;

    public void init() {
        pool = new ThreadPoolExecutor(
                1,
                3,
                5,
                TimeUnit.MINUTES,
                new ArrayBlockingQueue(5),
                new CustomThreadFactory(),
                new CustomRejectedExecutionHandler());
    }


    public void destory() {
        if(pool != null) {
            pool.shutdownNow();
        }
    }

    public ExecutorService getMyThreadPoolExecutor() {
        return this.pool;
    }

    /**
     * 自定义创建线程工厂
     */
    private class CustomThreadFactory implements ThreadFactory {

        private AtomicInteger count = new AtomicInteger(0);

        @Override
        public Thread newThread(Runnable r) {
            Thread t = new Thread(r);
            String threadName = MyThreadPoolExecutor.class.getSimpleName() + count.addAndGet(1);
            //System.out.println(threadName);
            t.setName(threadName);
            return t;
        }
    }
    /**
     * 自定义拒绝策略
     */
    private class CustomRejectedExecutionHandler implements RejectedExecutionHandler {

        @Override
        public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
            try {
                // 核心改造点，由blockingqueue的offer改成put阻塞方法  
                executor.getQueue().put(r);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
    

    // 测试构造的线程池    
    public static void main(String[] args) {

        MyThreadPoolExecutor exec = new MyThreadPoolExecutor();
        // 1.初始化    
        exec.init();
        ExecutorService pool = exec.getMyThreadPoolExecutor();
        MyTask task = new MyTask();
        for(int i=0; i<1000; i++) {
           pool.execute(task);
        }
        try {
            Thread.sleep(5000);
            // 2.销毁----留点时间给线程池执行,因为任务没有提交执行完,如果销毁线程池,任务也就无法执行了
            exec.destory();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

}
class MyTask implements Runnable{
    public AtomicInteger i = new AtomicInteger(0);

    @Override
    public void run() {
        System.out.println("第"+i.getAndIncrement()+"次执行任务");
    }
}

执行结果：

//excute执行源码
public void execute(Runnable command) {  
        if (command == null)  
            throw new NullPointerException();  
  
        int c = ctl.get();  
        if (workerCountOf(c) < corePoolSize) {  
            if (addWorker(command, true))  
                return;  
            c = ctl.get();  
        }  
        if (isRunning(c) && workQueue.offer(command)) {  
            int recheck = ctl.get();  
            if (! isRunning(recheck) && remove(command))  
                reject(command);  
            else if (workerCountOf(recheck) == 0)  
                addWorker(null, false);  
        }  
        else if (!addWorker(command, false))  
            // 进入拒绝机制， 我们把runnable任务拿出来，重新用阻塞操作put，来实现提交阻塞功能  
            reject(command);  
    } 

阻塞队列常见操作方法：

总结：
1、用ThreadPoolExecutor自定义线程池，看线程是的用途，如果任务量不大，可以用无界队列，如果任务量非常大，要用有界队列，防止OOM
2、如果任务量很大，还要求每个任务都处理成功，要对提交的任务进行阻塞提交，重写拒绝机制，改为阻塞提交。保证不抛弃一个任务
3、最大线程数一般设为2N+1最好，N是CPU核数，可以通过Runtime.getRuntime().availableProcessors();来获取
4、核心线程数，看应用，如果是任务，一天跑一次，设置为0，合适，因为跑完就停掉了，如果是常用线程池，看任务量，是保留一个核心还是几个核心线程数

转载：https://blog.csdn.net/weixin_44777693/article/details/104569711

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