四種線程池類型

ExecutorService threadPool = Executors.newFixedThreadPool(5);
ExecutorService threadPool4 = Executors.newSingleThreadExecutor();
ExecutorService threadPool2 = Executors.newCachedThreadPool();
ExecutorService threadPool3 = Executors.newScheduledThreadPool(5);

前兩個大小有限，第一個線程maximumPoolSize爲5，第二個爲1，後兩個大小無限，newCachedThreadPool爲阻塞隊列，newScheduledThreadPool爲優先隊列