小米面试：什么是线程池？工作原理是什么？线程池可以动态修改吗？

public ThreadPoolExecutor threadPoolExecutor() {
        ThreadPoolExecutor executor = new ThreadPoolExecutor(
                // 核心线程池大小，表示线程池常驻线程数量
                30,
                // 最大线程数，表示线程池最多创建的线程数量
                100,
                // 保活时间，表示一个非核心线程多久没有使用，会被回收
                10,
                TimeUnit.MINUTES,
                // 阻塞队列，表示队列最多缓存多少任务，如果队列满了，将触发 RejectedExecutionHandler
                new ArrayBlockingQueue<>(1000),
                // 线程工厂，创建线程时候用的，可以给线程命名等
                new NamedThreadFactory("cust-task")
        );
        // 拒绝策略，当阻塞队列满了之后，会触发这里的handler
        // 默认是丢弃新任务
        executor.setRejectedExecutionHandler((r, executor1) -> {
            log.warn("thread pool is full");
        });
    }

@Slf4j
public class ThreadPoolMonitor {

    private final ThreadPoolExecutor customThreadPool;
    private final String poolName;
    private final ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);

    public ThreadPoolMonitor(ThreadPoolExecutor customThreadPool, String poolName) {
        this.customThreadPool = customThreadPool;
        this.poolName = poolName;
    }

    public void startMonitoring(long period, TimeUnit unit) {
        scheduler.scheduleAtFixedRate(this::monitor, 0, period, unit);
    }

    private void monitor() {
        //核心线程数
        int corePoolSize = customThreadPool.getCorePoolSize();
        //最大线程数
        int maximumPoolSize = customThreadPool.getMaximumPoolSize();
        //活跃线程数
        int activeCount = customThreadPool.getActiveCount();
        //队列任务数
        int queueSize = customThreadPool.getQueue().size();
        //已执行完成任务数
        long completedTaskCount = customThreadPool.getCompletedTaskCount();
        //队列任务数峰值
        int largestPoolSize = customThreadPool.getLargestPoolSize();

        //上报监控数据
        sendToKafka(corePoolSize,maximumPoolSize, activeCount, queueSize, completedTaskCount, largestPoolSize);
    }

    private void sendToKafka(int corePoolSize,int maximumPoolSize, int activeCount, int queueSize, long completedTaskCount, int largestPoolSize) {
        // 自定义实现发送kafka逻辑或上报到prometheus逻辑
    }
}