線程池：
- 避免系統頻繁的創建和銷燬線程；
- 提高創建線程進行復用；
ThreadPoolExecutor
| 方法 | 返回值 | 解釋 |
| newFixedThreadPool(int nThreads) | ExecutorService | |
| newSingleThreadExecutor() | ExecutorService | |
| newCachedThreadPool() | ExecutorService | |
| newSingleThreadScheduledExecutor() | ScheduleExecutorService | |
| newScheduledThreadPool(int corePoolSize) | ScheduledExecutorService |