线程

1. Thread

1. A thread is a thread of execution in a program,The java Virtual Machine allows an application to have multiple threads of execution running concurrenttly
  java虚拟机允许一个应用可以存在多个线程
2. Thread 实现的接口Runnable,属性
    name//线程名称 volatile String
    priority//优先级(1,5,10) 默认是5
    target//执行目标
3. 面试中经常询问的问题sleep(),睡眠，不会释放锁