参考：
https://blog.csdn.net/lixiangchibang/article/details/110456200

要点：

Java类继承关系：
public class Executors
public interface ExecutorService extends Executor
public interface ScheduledExecutorService extends ExecutorService

创建单个线程的线程池
ExecutorService executorService = Executors.newSingleThreadExecutor();

创建固定个数线程的线程池，线程个数3。
ExecutorService executorService1 = Executors.newFixedThreadPool(3);

创建一个按照规定计划执行的线程池，线程个数3。
ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(3);

创建一个自动增长的线程池。
ExecutorService executorService2 = Executors.newCachedThreadPool();

创建一个抢占式的线程池。
ExecutorService executorService3 = Executors.newWorkStealingPool();