概述
我们工作中，并发编程必然离不开jdk提供的j.u.c中的线程池 ，假设让我们自己去设计一个线程池，该从哪几个方面来着手考虑呢？
首先： 既然是线程池 ， 那必然 有个初始化的线程数量 和 最大数量 ----> 两个属性 ： init 和 max
其次：当线程池中的线程达到了 init 数量，但还没到max 数量的时候，将任务放入任务队列中，而不是直接开辟新的线程，是不是一种更好的设计？ 所以 引入 任务队列，存放任务
最后：当线程池中的线程 把 任务队列也塞满了，也达到了Max的值，新的线程过来，我们改如何处理呢？ 是不是指定拒绝策略更合适呢？ 比如 直接拒绝，抛出异常，放入临时队列等等。所以 拒绝策略也是需要的
总结一下
- init （coreSize） / max , 线程池初始化大小和最大线程数
- 任务队列
- 拒绝策略
- 。。。
示例
逐步实现该需求， 先把基本的流程调通，这里我们先实现定义线程池数量的功能 ，代码如下
package com.artisan.test; import java.util.ArrayList; import java.util.LinkedList; import java.util.List; import java.util.stream.IntStream; public class SimpleThreadPool { // 线程数 private int size; // 默认线程数 private static int DEFAULT_SIZE = 10; // 任务队列 private final static LinkedList<Runnable> TASK_QUEUE = new LinkedList(); // 工作线程集合 private final static List<WorkTask> THREAD_QUEUE = new ArrayList<>(); private static volatile int seq = 0; private static final ThreadGroup GROUP = new ThreadGroup("Pool_Group"); private static final String THREAD_PREFIX = "SIMPLE_THREAD_POOL-"; /** * 默认构造函数 * 默认10个线程 */ public SimpleThreadPool() { this(DEFAULT_SIZE); } /** * 线程池中的线程数量 * * @param size */ public SimpleThreadPool(int size) { this.size = size; // 初始化 init(); } /** * 初始化 */ private void init() { for (int i = 0; i < size; i++) { createWorkTask(); } } /** * 创建工作线程 */ private void createWorkTask() { // 创建工作线程 WorkTask workTask = new WorkTask(GROUP, THREAD_PREFIX + (seq++)); // 启动 workTask.start(); // 加入到工作线程集合 THREAD_QUEUE.add(workTask); } /** * 将任务runnable 提交到 TASK_QUEUE * * @param runnable */ public void submit(Runnable runnable) { synchronized (TASK_QUEUE) { // 添加到队列的最后面 TASK_QUEUE.addLast(runnable); // 唤醒所有正在等待的线程 TASK_QUEUE.notifyAll(); } } /** * 线程状态 */ enum TaskState { FREE, RUNNING, BLOCKED, DEAD } /** * private 私有类 * 继承Thread 的 工作线程 */ private static class WorkTask extends Thread { // 默认状态 FREE private volatile TaskState state = TaskState.FREE; public WorkTask(ThreadGroup threadGroup, String name) { super(threadGroup, name); } /** * 获取线程状态 * * @return */ public TaskState getTaskState() { return this.state; } /** * 重写run方法 */ @Override public void run() { // 跳出while的标识 OUTER: // 只要不是DEAD 状态，一直从队列中获取消息 while (state != TaskState.DEAD) { Runnable runnable; // 对队列进行加锁操作 synchronized (TASK_QUEUE) { // 如果队列为空，wait while (TASK_QUEUE.isEmpty()) { try { // 更改状态 state = TaskState.BLOCKED; // wait 放弃该锁 TASK_QUEUE.wait(); } catch (InterruptedException e) { // 如果有个线程在wait,终止该线程会被InterruptedException异常捕获， // break OUTER 跳出while循环，从而达到终止这个线程 break OUTER; } } // TASK_QUEUE不为空 从队列中获取一个Runnable对象 // Removes and returns the first element from this list runnable = TASK_QUEUE.removeFirst(); } if (runnable != null) { // 运行状态 state = TaskState.RUNNING; // 运行 runnable.run(); // 运行结束后 置为FREE state = TaskState.FREE; } } } /** * 关闭线程 */ public void close() { this.state = TaskState.DEAD; } } /** * 测试 * * @param args */ public static void main(String[] args) { SimpleThreadPool simpleThreadPool = new SimpleThreadPool(); // for (int i = 0; i < 50; i++) { // simpleThreadPool.submit(()->{ // try { // System.out.println(Thread.currentThread().getName() + " is running" ); // Thread.sleep(1_000); // System.out.println(Thread.currentThread().getName() + " finished" ); // } catch (InterruptedException e) { // e.printStackTrace(); // } // }); // } IntStream.range(0, 50) .forEach(i -> simpleThreadPool.submit(() -> { try { System.out.println(Thread.currentThread().getName() + "____" + i + " start"); Thread.sleep(1_000); System.out.println(Thread.currentThread().getName() + "____" + i +" finished"); } catch (InterruptedException e) { e.printStackTrace(); } })); } }
运行：
SIMPLE_THREAD_POOL-9____0 start SIMPLE_THREAD_POOL-4____5 start SIMPLE_THREAD_POOL-0____9 start SIMPLE_THREAD_POOL-5____4 start SIMPLE_THREAD_POOL-6____3 start SIMPLE_THREAD_POOL-8____1 start SIMPLE_THREAD_POOL-7____2 start SIMPLE_THREAD_POOL-1____8 start SIMPLE_THREAD_POOL-3____6 start SIMPLE_THREAD_POOL-2____7 start SIMPLE_THREAD_POOL-4____5 finished SIMPLE_THREAD_POOL-6____3 finished SIMPLE_THREAD_POOL-5____4 finished SIMPLE_THREAD_POOL-3____6 finished SIMPLE_THREAD_POOL-0____9 finished SIMPLE_THREAD_POOL-9____0 finished SIMPLE_THREAD_POOL-0____13 start SIMPLE_THREAD_POOL-2____7 finished SIMPLE_THREAD_POOL-5____12 start SIMPLE_THREAD_POOL-1____8 finished SIMPLE_THREAD_POOL-6____11 start SIMPLE_THREAD_POOL-7____2 finished SIMPLE_THREAD_POOL-4____10 start SIMPLE_THREAD_POOL-8____1 finished SIMPLE_THREAD_POOL-7____17 start SIMPLE_THREAD_POOL-1____16 start SIMPLE_THREAD_POOL-2____15 start SIMPLE_THREAD_POOL-9____14 start SIMPLE_THREAD_POOL-8____18 start SIMPLE_THREAD_POOL-3____19 start SIMPLE_THREAD_POOL-0____13 finished SIMPLE_THREAD_POOL-0____20 start SIMPLE_THREAD_POOL-5____12 finished SIMPLE_THREAD_POOL-5____21 start SIMPLE_THREAD_POOL-6____11 finished SIMPLE_THREAD_POOL-6____22 start SIMPLE_THREAD_POOL-4____10 finished SIMPLE_THREAD_POOL-4____23 start SIMPLE_THREAD_POOL-7____17 finished SIMPLE_THREAD_POOL-7____24 start SIMPLE_THREAD_POOL-1____16 finished SIMPLE_THREAD_POOL-1____25 start SIMPLE_THREAD_POOL-2____15 finished SIMPLE_THREAD_POOL-2____26 start SIMPLE_THREAD_POOL-9____14 finished SIMPLE_THREAD_POOL-9____27 start SIMPLE_THREAD_POOL-8____18 finished SIMPLE_THREAD_POOL-8____28 start SIMPLE_THREAD_POOL-3____19 finished SIMPLE_THREAD_POOL-3____29 start SIMPLE_THREAD_POOL-5____21 finished SIMPLE_THREAD_POOL-2____26 finished SIMPLE_THREAD_POOL-1____25 finished SIMPLE_THREAD_POOL-7____24 finished SIMPLE_THREAD_POOL-4____23 finished SIMPLE_THREAD_POOL-0____20 finished SIMPLE_THREAD_POOL-6____22 finished SIMPLE_THREAD_POOL-0____35 start SIMPLE_THREAD_POOL-4____34 start SIMPLE_THREAD_POOL-7____33 start SIMPLE_THREAD_POOL-1____32 start SIMPLE_THREAD_POOL-8____28 finished SIMPLE_THREAD_POOL-3____29 finished SIMPLE_THREAD_POOL-2____31 start SIMPLE_THREAD_POOL-9____27 finished SIMPLE_THREAD_POOL-5____30 start SIMPLE_THREAD_POOL-9____39 start SIMPLE_THREAD_POOL-3____38 start SIMPLE_THREAD_POOL-8____37 start SIMPLE_THREAD_POOL-6____36 start SIMPLE_THREAD_POOL-8____37 finished SIMPLE_THREAD_POOL-2____31 finished SIMPLE_THREAD_POOL-5____30 finished SIMPLE_THREAD_POOL-3____38 finished SIMPLE_THREAD_POOL-9____39 finished SIMPLE_THREAD_POOL-6____36 finished SIMPLE_THREAD_POOL-9____44 start SIMPLE_THREAD_POOL-0____35 finished SIMPLE_THREAD_POOL-4____34 finished SIMPLE_THREAD_POOL-3____43 start SIMPLE_THREAD_POOL-4____47 start SIMPLE_THREAD_POOL-7____33 finished SIMPLE_THREAD_POOL-7____48 start SIMPLE_THREAD_POOL-5____42 start SIMPLE_THREAD_POOL-1____32 finished SIMPLE_THREAD_POOL-2____41 start SIMPLE_THREAD_POOL-8____40 start SIMPLE_THREAD_POOL-1____49 start SIMPLE_THREAD_POOL-0____46 start SIMPLE_THREAD_POOL-6____45 start SIMPLE_THREAD_POOL-3____43 finished SIMPLE_THREAD_POOL-5____42 finished SIMPLE_THREAD_POOL-7____48 finished SIMPLE_THREAD_POOL-4____47 finished SIMPLE_THREAD_POOL-9____44 finished SIMPLE_THREAD_POOL-6____45 finished SIMPLE_THREAD_POOL-0____46 finished SIMPLE_THREAD_POOL-1____49 finished SIMPLE_THREAD_POOL-8____40 finished SIMPLE_THREAD_POOL-2____41 finished
可知： 虽然定义了50个任务，但是根据日志来看，是由10个线程来完成的，符合预期。
