//创建单一线程
ExecutorService executorService= Executors.newSingleThreadExecutor();
//创建可伸缩线程
ExecutorService executorService1=Executors.newCachedThreadPool();
//指定创建3个线程
ExecutorService executorService2=Executors.newFixedThreadPool(3);
一般不会用newCachedThreadPool太耗费内存，而newSingleThreadExecutor和newFixedThreadPool
容易积累大量的oom异常
