下面就是一个简版的线程池

package cn.liang.mybatis.test.dao;


import lombok.extern.slf4j.Slf4j;

import java.util.ArrayDeque;
import java.util.Deque;
import java.util.HashSet;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;


@Slf4j
public class ThreadPoolDemo {
    public static void main(String[] args) {
        ThreadPool threadPool = new ThreadPool(1, 1000,
                TimeUnit.MILLISECONDS, 2, (queue, task) -> {
            queue.offer(task,3500,TimeUnit.MILLISECONDS);
        });
        for (int i = 0; i < 4; i++) {
            int j = i;
            threadPool.execute(() -> {
                try {
                    Thread.sleep(200);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                log.debug("{}", j);
            });
        }
    }
}


// 自定义线程池
@Slf4j
class ThreadPool{
    // 任务队列
    private BlockingQueue<Runnable> taskQueue;
    // 线程集合
    private HashSet<Worker> workers = new HashSet<Worker>();
    // 核心线程数
    private int coreSize;
    // 获取任务时间
    private long timeout;
    // 时间单位
    private TimeUnit timeUnit;
    // 拒绝策略
    private RejectPolicy<Runnable> rejectPolicy;

    public ThreadPool(int coreSize, long timeout, TimeUnit timeUnit, int queueCapcity,RejectPolicy<Runnable> rejectPolicy) {
        this.coreSize = coreSize;
        this.timeout = timeout;
        this.timeUnit = timeUnit;
        this.taskQueue = new BlockingQueue<>(queueCapcity);
        this.rejectPolicy = rejectPolicy;
    }

    // 执行任务
    public void execute(Runnable task){
        // 当任务数没有超过coreSize 直接交给worker对象执行
        // 如果任务超过coreSize时 加入任务队列暂存
        synchronized (workers){
            if(workers.size() < coreSize){
                Worker worker = new Worker(task);
                log.debug("新增 worker{},{}",worker,task);
                workers.add(worker);
                worker.start();
            }else {
                // 1、死等
                // 2、带超时等待
                // 3、让调用者放弃任务执行
                // 4、让调用者抛出异常
                // 5、让调用者自己执行任务
                taskQueue.tryPut(rejectPolicy,task);
            }
        }
    }

    class Worker extends  Thread{
        private  Runnable task;
        public Worker(Runnable task){
            this.task = task;
        }

        @Override
        public void run() {
            // 执行任务
            // 当task不为空 执行任务
            // 当task 执行完毕 在接着从任务队列中获取任务并执行

            while (task != null || (task = taskQueue.poll(timeout,timeUnit)) != null){
                log.debug("正在执行....{}",task);
                try{
                    task.run();
                }catch (Exception e){
                    e.printStackTrace();
                }finally {
                    task = null;
                }
            }
            synchronized (workers){
                log.debug("worker被移除{}",this);
                workers.remove(this);
            }

        }
    }
}


@FunctionalInterface  // 拒绝策略
interface RejectPolicy<T>{
    void reiect(BlockingQueue<T> queue,T task);
}

@Slf4j
class BlockingQueue<T>{
    // 1、任务队列
    private Deque<T> queue = new ArrayDeque<>();
    // 2、锁
    private ReentrantLock lock = new ReentrantLock();
    // 3、生产者条件变量
    private Condition fullWaitSet = lock.newCondition();
    // 4、消费者条件变量
    private Condition emptyWaitSet = lock.newCondition();
    // 5、容量
    private int capcity;

    public BlockingQueue(int capcity) {
        this.capcity = capcity;
    }

    // 带超时阻塞获取
    public T poll(long timeout, TimeUnit unit){
        lock.lock();
        try{
            // 将timeout统一转换为纳秒
            long nanos = unit.toNanos(timeout);
            while (queue.isEmpty()){
                // 返回值是剩余时间
                try{
                    if(nanos <= 0){
                        return null;
                    }
                    nanos = emptyWaitSet.awaitNanos(nanos);
                }catch (InterruptedException e){
                    e.printStackTrace();
                }
            }
            T t = queue.removeFirst();
            fullWaitSet.signal();
            return t;
        }finally {
            lock.unlock();
        }
    }

    // 带超时时间
    public boolean offer(T task,long timeout,TimeUnit timeUnit){
        lock.lock();
        try{
            long nanos = timeUnit.toNanos(timeout);
            while (queue.size() == capcity){
                try{
                    if (nanos <= 0) {
                        return false;
                    }
                    log.debug("等待任务加入任务队列{}..",task);
                    nanos = fullWaitSet.awaitNanos(nanos);
                }catch (InterruptedException e){
                    e.printStackTrace();
                }
            }
            log.debug("加入任务队列{}",task);
            queue.addLast(task);
            emptyWaitSet.signal();
            return true;
        }finally {
            lock.unlock();
        }
    }

    public int size(){
        lock.lock();
        try{
            return queue.size();
        }finally {
            lock.unlock();
        }
    }

    public void tryPut(RejectPolicy<T> rejectPolicy,T task){
        lock.lock();
        try {
            // 判断队列是否满
            if (queue.size() == capcity) {
                rejectPolicy.reiect(this,task);
            }else { // 有空闲
                log.debug("加入任务队列{}",task);
                queue.addLast(task);
                emptyWaitSet.signal();
            }
        }finally {
            lock.unlock();
        }

    }




}