code: public static void init(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit timeUnit) {
          ThreadPool.corePoolSize = corePoolSize;
          ThreadPool.maximumPoolSize = maximumPoolSize;
          ThreadPool.keepAliveTime = keepAliveTime;
          ThreadPool.timeUnit = timeUnit;
          init();
      }
docs: 初始化线程池
  
  @param corePoolSize 初始Size
  @param maximumPoolSize 最大Size
  @param keepAliveTime 存活时长
  @param timeUnit 时间单位