线程池

线程池：
问题：在JDK中什么代表线程池？Executor
线程池其实就是一个生产者消费者模型
生产者：提交任务的线程
消费者：处理任务的线程
产品：任务

问题：大家以前是如何创建线程池的？
Executors里面的静态方法
注意事项：
工作中不要使用Executors里面的静态方法去创建线程里。
原因：这样创建的线程池，里面的阻塞队列都是LinkedBlockingDeque, 可以’无限’缓存任务。
在并发量比较高的场景中，容易导致OOM.
最好使用ThreadPoolExecutor去创建线程池

public class MyThreadPool implements Executor {
    private static final int DEFAULT_CAPACITY = 100;
    private static final int MAX_CAPACITY = Integer.MAX_VALUE - 8;

    private BlockingQueue<Runnable> tasks;
    private int size = 10; // 处理任务的线程数目

    public MyThreadPool() {
        tasks = new ArrayBlockingQueue<>(DEFAULT_CAPACITY);
        init(); // 创建线程，并就绪
    }

    public MyThreadPool(int capacity) {
        if (capacity <= 0 || capacity > MAX_CAPACITY) {
            throw new IllegalArgumentException("capacity=" + capacity);
        }
        tasks = new ArrayBlockingQueue<>(capacity);
        init();
    }

    private void init() {
        for (int i = 0; i < size; i++) {
            new WorkThread().start();
        }
    }


    @Override
    public void execute(Runnable command) {
        try {
            tasks.put(command); // 提交任务
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    private class WorkThread extends Thread{
        @Override
        public void run() {
            while (true) { // 死循环：回收线程
                try {
                    tasks.take().run();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

/* JDK：BlockingQueue<E> |-- ArrayBlockingQueue: 容量大小固定 |-- LinkedBlockingDeque: 容量大小步固定，除非指定大小。 */
public class MyBlockingQueue<E> {
    private static final int DEFAULT_CAPACITY = 10;
    private static final int MAX_CAPACITY = Integer.MAX_VALUE - 8;
    // 属性
    private int front;
    private int rear;
    private int size;
    private E[] elements;

    @SuppressWarnings("unchecked")
    public MyBlockingQueue() {
        elements = (E[]) new Object[DEFAULT_CAPACITY];
    }

    @SuppressWarnings("unchecked")
    public MyBlockingQueue(int initialCapacity) {
        if (initialCapacity <= 0 || initialCapacity > MAX_CAPACITY) {
            throw new IllegalArgumentException("initialCapacity=" + initialCapacity);
        }
        elements = (E[]) new Object[initialCapacity];
    }

    public synchronized void enqueue(E e) {
        // 判断队列是否满
        // if (size == elements.length) {
        while (size == elements.length) { // Caution! 不能够使用if
            try {
                wait();// t1, t2
            } catch (InterruptedException e1) {
                e1.printStackTrace();
            }
        }
        // 添加元素
        elements[rear] = e;
        rear = (rear + 1) % elements.length;
        size++;
        // 队列不空, 需要唤醒其它线程
        notifyAll();
    }

    public synchronized E dequeue() {
        // 判断队列是否为空
        // if (size == 0) {
        while (size == 0) {
            try {
                wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        // 删除元素
        E removeValue = elements[front];
        elements[front] = null;
        front = (front + 1) % elements.length;
        size--;
        // 队列不满，唤醒其它线程
        notifyAll();
        return removeValue;
    }

    public synchronized E peek() {
        while (size == 0) {
            try {
                wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        return elements[front];
    }

    public synchronized boolean isEmpty() {
        return size == 0;
    }

    public synchronized int size() {
        return size;
    }
}