Executors

提供了工厂方法：

Factory and utility methods for Executor, ExecutorService, ScheduledExecutorService, ThreadFactory, and Callable classes defined in this package. This class supports the following kinds of methods:

• Methods that create and return an ExecutorService set up with commonly useful configuration settings.
• Methods that create and return a ScheduledExecutorService set up with commonly useful configuration settings.
• Methods that create and return a "wrapped" ExecutorService, that disables reconfiguration by making implementation-specific methods inaccessible.
• Methods that create and return a ThreadFactory that sets newly created threads to a known state.
• Methods that create and return a Callable out of other closure-like forms, so they can be used in execution methods requiring Callable。

1、ExecutorService   (为ThreadPoolExecutor设置了一些通用的参数来生成一些线程池，使用的BlockingQueue都是inbound的queue)

    1、ScheduledThreadPool

    2、singledThreadPool（线程的顺序执行，使用该线程池，可以保证线程的顺序性，不会竞争资源）

    3、cachedThreadPool(默认是线程处于idle 6s就自动回收，适合大量而且小的异步任务)

    4、FixedThreadPool