Worker的实现 

总体来说我们首先需要实现一个Worker线程类，让他的线程实例拿取任务然后执行任务。

//worker主要是从jobs中拿工作，拿不到等，拿到了执行。class Worker implements Runnable{private volatile boolean running = true;@Overridepublic void run() {while (running){Job job = null;synchronized (jobs){while (jobs.isEmpty()){try{jobs.wait();} catch (Exception e){Thread.currentThread().interrupt();return;}}job = jobs.removeFirst();}if (job != null){job.run();}}}public void shutdown(){running = false;}}

由此我们引出了，需要一个集合来保存worker。

private final List<Worker> workers = Collections.synchronizedList(new ArrayList<Worker>());

同时worker需要拿取任务，任务需要保存在一个队列中，所以引出来了jobs队列。

private final LinkedList<Job> jobs =new LinkedList<>();

线程池的初始化

前提工作准备好了，我们可以开始去实现一个简易的线程池。

初始化就免不了要提起那设定好初始化的数值。

private static final int DEFAULT_WORKER_NUMBERS = 5;private  int workerNum = DEFAULT_WORKER_NUMBERS;private static final int MAX_WORKER_NUMBERS = 10;private static final int MIN_WORKER_NUMBERS = 1;

初始工人为5个，对应的就是线程池中的核心线程数为5.

wokerNum用来即时记录工人数，也就是线程池中存活的线程数。

并且限制了最大工人数为10，最少工人数为1。对应的是最大线程数为10，最少线程数为1。

接下来就可以执行初始化了。

public ThreadExcutor(){InitialWorkers(DEFAULT_WORKER_NUMBERS);}//将worker放入wokers集合中，然后开始让worker线程执行。private void InitialWorkers(int num){for (int i = 0; i < num; i++) {Worker worker = new Worker();workers.add(worker);Thread thread = new Thread(worker,"ThreadPool-Worker-"+ i);thread.start();}}//判断num是否在最大和最少工人范围之间，在的话将工人数量赋值num，不在的话num大于max就复制max，小于max就赋值minpublic ThreadExcutor(int num){this.workerNum = num > MAX_WORKER_NUMBERS ? MAX_WORKER_NUMBERS : Math.max(num, MIN_WORKER_NUMBERS);InitialWorkers(workerNum);}

如果我们调用构造方法时，没有传入参数，他就直接默认生成5个工人，并将它依次加入工人集合中，并开启这个工人线程。

如果我们调用传入参数的方法，就需要判断传入的参数是否在max和min之间，如果在就赋值给workerNum进行记录，然后初始化相应的工人线程。如果不在就考虑大于max就初始化max的数量，如果小于min就初始化min数量。

线程池的五大基础方法实现

以下四种操作都会涉及到

执行方法

言简意赅就是将job放入jobs队列中，然后唤醒jobs队列。

//如果工作不为空，就将工作放入工作队列中，然后唤醒工作队列public void execute(Job job){if (job != null){synchronized (jobs){jobs.addLast(job);jobs.notify();}}}

添加工人（添加线程）

先判断是否超出最大工人数，超出了的话，强制初始化剩下的工人数。最后别忘了workerNum的即时记录。

//这里初始化时对jobs有操作，所以也对jobs进行加锁。添加工人时需要判断加上原来的工人是否超过了最大的工人数，//如果超过了就将num赋值成不超过最大工人的最大数量，然后去用num数初始化，并且将工人数更新成最新。public void addWorks(int num){synchronized (jobs){if (num + workerNum > MAX_WORKER_NUMBERS){num = MAX_WORKER_NUMBERS - workerNum;}InitialWorkers(num);workerNum += num;}}

移除工人（删除工作线程）

这个就是不断地从工人集合中取出工人，然后将他们shutdown掉。别忘了workerNum的即时记录。

//先检查删除数是否在工人数之内，不在就抛出超出范围的异常。然后在工人集合中不断地取工人，然后进行shutdown，最后workerNum更新成剩下的工人数。public void removeWorkers(int num){synchronized (jobs){if(num >= workerNum){throw new IllegalArgumentException("超出范围");}int count = 0;while (count < num){Worker worker = workers.get(count);if (workers.remove(worker)){worker.shutdown();count++;}}workerNum -= count;}}

获取工作数

//获取工作数public int getJobSize(){return jobs.size();}

停止工人线程

对工人集合进行遍历，逐一调用shutdown方法。

 //遍历工人集合，一个一个关闭public void shutdown(){for (Worker worker : workers) {worker.shutdown();}}

总结

这样一个建议的线程池就实现了。

总的来说增加和构造函数都比较需要初始化方法，所以这个记忆要深刻一些。

最后奉上完整代码

public class ThreadExcutor<Job extends Runnable> {private final List<Worker> workers = Collections.synchronizedList(new ArrayList<Worker>());private final LinkedList<Job> jobs =new LinkedList<>();private static final int DEFAULT_WORKER_NUMBERS = 5;private  int workerNum = DEFAULT_WORKER_NUMBERS;private static final int MAX_WORKER_NUMBERS = 10;private static final int MIN_WORKER_NUMBERS = 1;public ThreadExcutor(){InitialWorkers(DEFAULT_WORKER_NUMBERS);}//将worker放入wokers集合中，然后开始让worker线程执行。private void InitialWorkers(int num){for (int i = 0; i < num; i++) {Worker worker = new Worker();workers.add(worker);Thread thread = new Thread(worker,"ThreadPool-Worker-"+ i);thread.start();}}//判断num是否在最大和最少工人范围之间，在的话将工人数量赋值num，不在的话num大于max就复制max，小于max就赋值minpublic ThreadExcutor(int num){this.workerNum = num > MAX_WORKER_NUMBERS ? MAX_WORKER_NUMBERS : Math.max(num, MIN_WORKER_NUMBERS);InitialWorkers(workerNum);}//如果工作不为空，就将工作放入工作队列中，然后唤醒工作队列public void execute(Job job){if (job != null){synchronized (jobs){jobs.addLast(job);jobs.notify();}}}//遍历工人集合，一个一个关闭public void shutdown(){for (Worker worker : workers) {worker.shutdown();}}//这里初始化时对jobs有操作，所以也对jobs进行加锁。添加工人时需要判断加上原来的工人是否超过了最大的工人数，//如果超过了就将num赋值成不超过最大工人的最大数量，然后去用num数初始化，并且将工人数更新成最新。public void addWorks(int num){synchronized (jobs){if (num + workerNum > MAX_WORKER_NUMBERS){num = MAX_WORKER_NUMBERS - workerNum;}InitialWorkers(num);workerNum += num;}}//先检查删除数是否在工人数之内，不在就抛出超出范围的异常。然后在工人集合中不断地取工人，然后进行shutdown，最后workerNum更新成剩下的工人数。public void removeWorkers(int num){synchronized (jobs){if(num >= workerNum){throw new IllegalArgumentException("超出范围");}int count = 0;while (count < num){Worker worker = workers.get(count);if (workers.remove(worker)){worker.shutdown();count++;}}workerNum -= count;}}//获取工作数public int getJobSize(){return jobs.size();}//worker主要是从jobs中拿工作，拿不到等，拿到了执行。class Worker implements Runnable{private volatile boolean running = true;@Overridepublic void run() {while (running){Job job = null;synchronized (jobs){while (jobs.isEmpty()){try{jobs.wait();} catch (Exception e){Thread.currentThread().interrupt();return;}}job = jobs.removeFirst();}if (job != null){job.run();}}}public void shutdown(){running = false;}}
}