线程池的原理分析

1.为什么要创建线程池？

　　为了避免频繁创建线程对系统的资源开销

　　通过控制线程池的线程大小来从而达到限流的效果

2.快速构建线程池

ExecutorService executorService1=Executors.newSingleThreadExecutor();
ExecutorService executorService2=Executors.newFixedThreadPool(2);
ExecutorService executorService3=Executors.newCachedThreadPool();
ExecutorService executorService4=Executors.newScheduledThreadPool();