基本概念
Command Pattern: Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queues, or log requests, and supports undoable operations.
封装的不是一个实例，而是一个动作（类似 Controller、Mapper 层？）
实现代码
命令
接收者
Receiver
触发者
Invoker
测试类
Q & A
What are the key advantages associated with command patterns?
- Requests for creation and the ultimate execution are decoupled. Clients may not know how an invoker is performing the operations.
- You can create macros (sequence of commands).
- New commands can be added without affecting the existing system.
- Most importantly, you can support the undo/redo operations.
具体应用
java.lang.Runnable
该接口的所有实现（作为接受者）都是命令者模式的例子
Runnable 接口通常用于定义线程执行的任务。当创建一个 Runnable 实现并将其提交给一个线程，实际上是在定义一个要执行的命令，将其交给调用者（Thread）执行。