四种线程池类型

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

前两个大小有限，第一个线程maximumPoolSize为5，第二个为1，后两个大小无限，newCachedThreadPool为阻塞队列，newScheduledThreadPool为优先队列