1.手撕线程池原理图

2.代码实现

// 手撕线程池
public class Main {
    public static void main(String[] args) {
        ThreadPool threadPool = new ThreadPool(1,1000,TimeUnit.MILLISECONDS,1,(queue, task) -> {
            queue.putByTime(task,1500,TimeUnit.MILLISECONDS);
        });
        for (int i = 0; i < 4; i++) {
            int j=i;
            threadPool.execute(()->{
                try{
                    Thread.sleep(3000L);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(j);
            });
        }
    }
}

// 自定义任务队列
class BlockQueue<T>{
    // 任务队列
    private Deque<T> queue=new ArrayDeque<>();

    // 锁
    private ReentrantLock lock=new ReentrantLock();

    // 生产者条件变量
    private Condition fullWaitSet=lock.newCondition();

    // 消费者条件变量
    private Condition emptyWaitSet=lock.newCondition();

    // 容量大小
    private int capacity;

    public BlockQueue(int capacity) {
        this.capacity = capacity;
    }

    // 阻塞获取
    public T take(){
        lock.lock();
        try{
            while(queue.isEmpty()){
                try{
                    emptyWaitSet.await();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            T t=queue.removeFirst();
            fullWaitSet.signal();
            return t;
        }finally {
            lock.unlock();
        }
    }

    // 阻塞添加
    public void put(T task){
        lock.lock();
        try {
            while(queue.size()>=capacity){
                try{
                    System.out.println("等待加入任务队列："+task);
                    fullWaitSet.await();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            System.out.println("加入任务队列："+task);
            queue.addLast(task);
            emptyWaitSet.signal();
        }finally {
            lock.unlock();
        }
    }

    // 带超时时间的阻塞获取
    public T takeByTime(long timeout, TimeUnit unit){
        lock.lock();
        try{
            // 将 timeout 统一转换为 纳秒
            long nanos=unit.toNanos(timeout);
            while(queue.isEmpty()){
                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();
        }
    }

    // 带超时时间阻塞添加
    public boolean putByTime(T task,long timeout, TimeUnit unit){
        lock.lock();
        try {
            long nanos=unit.toNanos(timeout);
            while(queue.size()>=capacity){
                try{
                    if(nanos<=0){
                        return false;
                    }
                    System.out.println("等待加入任务队列："+task);
                    nanos=fullWaitSet.awaitNanos(nanos);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            System.out.println("加入任务队列："+task);
            queue.addLast(task);
            emptyWaitSet.signal();
            return true;
        }finally {
            lock.unlock();
        }
    }

    // 返回队列大小
    public int size(){
        lock.lock();
        try{
            return queue.size();
        }finally {
            lock.unlock();
        }
    }

    // 根据策略添加
    public void tryPut(RejectPolicy<T> rejectPolicy,T task){
        lock.lock();
        try{
            // 判断队列是否满
            if(queue.size()>=capacity){
                rejectPolicy.reject(this,task);
            }else{
                System.out.println("加入任务队列："+task);
                queue.addLast(task);
                emptyWaitSet.signal();
            }
        }finally {
            lock.unlock();
        }
    }
}

// 拒绝策略
// 采用策略模式
@FunctionalInterface
interface RejectPolicy<T>{
    void reject(BlockQueue<T> queue,T task);
}

// 自定义线程池
class ThreadPool{
    // 任务队列
    private BlockQueue<Runnable> taskQueue;

    // 线程集合
    // Worker为内部类
    private HashSet<Worker> workers=new HashSet<Worker>();

    // 核心线程数
    private int coreSize;

    // 获取任务时的超时时间
    private long timeout;
    private TimeUnit timeUnit;

    // 拒绝策略
    private RejectPolicy<Runnable> policy;

    public ThreadPool(int coreSize, long timeout, TimeUnit timeUnit,int queueCapacity, RejectPolicy<Runnable> policy) {
        this.coreSize = coreSize;
        this.timeout = timeout;
        this.timeUnit = timeUnit;
        this.policy = policy;
        this.taskQueue=new BlockQueue<>(queueCapacity);
    }

    // 执行任务
    public void execute(Runnable task){
        // 当任务数没有超过 coreSize 时，直接交给 worker 对象执行
        // 如果任务数超过 coreSize 时，加入任务队列暂存
        synchronized (workers){
            if(workers.size()<coreSize){
                Worker worker = new Worker(task);
                System.out.println("新增worker："+worker+" ,task："+task);
                workers.add(worker);
                worker.start();
            }else{
                taskQueue.tryPut(policy,task);
            }
        }
    }

    class Worker extends Thread{
        private Runnable task;

        public Worker(Runnable task) {
            this.task = task;
        }

        @Override
        public void run() {
            // 执行任务
            // (1) 当 task 不为空，执行任务
            // (2) 当 task 执行完毕，再接着从任务队列获取任务并执行
            while(task!=null||(task= taskQueue.takeByTime(timeout,timeUnit))!=null){
                try{
                    System.out.println("正在执行："+task);
                    task.run();
                } catch (Exception e) {
                    e.printStackTrace();
                }finally {
                    task=null;
                }
            }
            synchronized (workers){
                System.out.println("worker 被移除："+this);
                workers.remove(this);
            }
        }
    }
}

{
task=null;
}
}
synchronized (workers){
System.out.println(“worker 被移除：”+this);
workers.remove(this);
}
}
}
}