通過使用策略模式模擬實現Java線程池邏輯和研究線程池的實現原理，自己一定要動手寫一下才知道里面的奧妙。

前幾天寫了一個固定大小的連接池，今天通過學習又整理一下線程池的實現邏輯，看完這片代碼，Java線程池的基本思想你就能完全hold住，離着高級程序員又近一步，歡迎大家參考和交流。 

package com.smallfan.connectionpool;

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;

/**
 * @PACKAGE_NAME: com.smallfan.connectionpool
 * @NAME: TestThreadPool
 * @USER: dell
 * @DATE: 2020/5/29
 * @PROJECT_NAME: aboutthread
 */
@Slf4j
public class TestThreadPool {
    public static void main(String[] args) {
        ThreadPool threadPool = new ThreadPool(1, 1000,
                TimeUnit.MILLISECONDS, 1, ((queue, task) -> {
            //死等        
//            queue.takeQueue();
            //設置超時
//            Object o = queue.takeQueueForTime(500, TimeUnit.MILLISECONDS);
            //捨棄
//            log.info("不干預，放棄 {}",task);
            //拋出異常
//            throw new RuntimeException("拋出異常"+task);
            //交給主線程執行
            task.run();
        }));
        for (int i = 0; i < 5; i++) {
            int j = i;
            threadPool.execute(() ->
                    {
                        try {
                            Thread.sleep(1000L);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                        log.info("執行第" + j);
                    }
            );
        }
    }
}

@Slf4j
class ThreadPool {
    //任務對列
    private BlockingQueue<Runnable> taskQueue;
    //線程集合
    private HashSet workers = new HashSet<Worker>();
    //線程數
    private int threadSize;
    //超時時間
    private long timeout;
    //時間單位
    private TimeUnit timeUnit;
    //拒絕策略
    private RejectPolicy<Runnable> policy;

    public ThreadPool(int threadSize, long timeout, TimeUnit timeUnit, int capacity, RejectPolicy<Runnable> policy) {
        this.threadSize = threadSize;
        this.timeout = timeout;
        this.timeUnit = timeUnit;
        taskQueue = new BlockingQueue<>(capacity);
        this.policy = policy;
    }

    public void execute(Runnable task) {
        synchronized (workers) {//公共資源保證線程安全
            //如果任務數小於threadSize時直接執行
            //否則加入到線程對列
            if (workers.size() < threadSize) {
                log.info("新增worker{}", task);
                Worker worker = new Worker(task);
                workers.add(worker);
                worker.start();
            } else {

                //taskQueue.putQueue(task);
                /**
                 * 考慮問題
                 * 1對列滿了死等
                 * 2設置超時時間
                 * 3捨棄
                 * 4主線程執行
                 * 5拋出異常
                 * 使用設計模式的策略模式解決
                 */
                taskQueue.tryPut(policy, task);
            }

        }
    }

    @FunctionalInterface
    interface RejectPolicy<T> {
        void reject(BlockingQueue<T> queue, T task);
    }

    class Worker extends Thread {
        private Runnable runnable;

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

        @Override
        public void run() {
            /**
             * 執行任務
             * 1.當runnable直接執行
             * 2.當對列裏面存在任務時執行
             */
//            while (runnable != null || (runnable = taskQueue.takeQueue()) != null) {
            while (runnable != null || (runnable = taskQueue.takeQueueForTime(timeout, timeUnit)) != null) {
                try {
                    log.info("執行worker{}", runnable);
                    runnable.run();
                } catch (Exception e) {
                    e.printStackTrace();
                } finally {
                    runnable = null;//執行後置空
                }
            }
            synchronized (workers) {
                log.info("移除worker{}", this);
                workers.remove(this);
            }
        }
    }
}

//模擬阻塞隊列
@Slf4j
class BlockingQueue<T> {

    //1.定義隊列大小
    private int capacity;
    //2.定義雙向鏈表，當做容器
    private Deque<T> deque = new ArrayDeque<T>();
    //3.定義鎖
    private ReentrantLock lock = new ReentrantLock();
    //4.定義空條件變量
    private Condition emptyWaitSet = lock.newCondition();
    //5.定義滿條件變量
    private Condition fullWaitSet = lock.newCondition();

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

    //定義獲取方法
    public T takeQueue() {
        lock.lock();
        try {
            while (deque.isEmpty()) {//若還沒有
                try {
                    emptyWaitSet.await();//空等待放入時喚醒
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            //若已經放入
            T t = deque.removeFirst();
            fullWaitSet.signal();//喚醒滿條件
            return t;
        } finally {
            lock.unlock();//釋放鎖，避免死鎖
        }
    }

    //定義超時獲取
    public T takeQueueForTime(long timeout, TimeUnit unit) {
        lock.lock();
        long nanos = unit.toNanos(timeout);//統一時間單位
        try {
            while (deque.isEmpty()) {//若還沒有
                try {
                    if (nanos <= 0) {
                        return null;
                    }
                    nanos = emptyWaitSet.awaitNanos(nanos);//防止虛假喚醒 使用等待時間減去消耗時間
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            //若已經放入
            T t = deque.removeFirst();
            fullWaitSet.signal();//喚醒滿條件
            return t;
        } finally {
            lock.unlock();//釋放鎖，避免死鎖
        }
    }

    //定義放入方法
    public void putQueue(T task) {
        lock.lock();
        try {
            while (deque.size() == capacity) {//已經滿了
                try {
                    fullWaitSet.await();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            deque.addLast(task);
            emptyWaitSet.signal();
            log.info("加入隊列 {}", task);
        } finally {
            lock.unlock();
        }
    }

    /**
     * 任務多時，設置添加任務的超時時間
     *
     * @param task
     * @param timeout
     * @param timeUnit
     * @return
     */
    public boolean putQueueForTimeOut(T task, long timeout, TimeUnit timeUnit) {
        lock.lock();
        long nanos = timeUnit.toNanos(timeout);
        try {
            while (deque.size() == capacity) {//已經滿了
                try {
                    if (nanos <= 0) {//添加失敗
                        return false;
                    }
                    nanos = fullWaitSet.awaitNanos(nanos);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            deque.addLast(task);
            emptyWaitSet.signal();
            return true;//添加成功
        } finally {
            lock.unlock();
        }
    }

    //獲取容量
    public int getCapacity() {
        lock.lock();
        try {
            return deque.size();
        } finally {
            lock.unlock();
        }
    }

    public void tryPut(ThreadPool.RejectPolicy<T> policy, T task) {
        lock.lock();
        try {
            if (deque.size() == capacity) {//對列已滿 調用策略 讓調用者決定
                policy.reject(this, task);
            } else {//空閒
                deque.addLast(task);
                emptyWaitSet.signal();
            }
        } finally {
            lock.unlock();
        }
    }
}