一、前言
读完单例模式，接着品命令模式，命令模式在工程实践中有应用，如MVC框架中使用了命令模式，还有日志系统（从异常中恢复）和事务系统都可以使用命令模式，命令模式可以解耦命令请求者和命令执行者，下面开始学习命令模式。
二、命令模式定义
定义：命令模式将请求封装成对象，以便使用不同的请求、队列或者日志来参数化其他对象。命令模式也支持撤销的操作。
从定义可以看出，命令模式核心的概念就是讲请求封装成为对象，这样，对象可以被传递，从而解耦请求者和执行者。
命令模式的类图如下
说明：对于类图说明如下
① Client负责创建一个ConcreteCommand，并设置接受者。
② Invoker持有一个命令对象，并在某个时间点调用命令对象的execute方法，将请求付诸实行。
③ Command为所有命令声明了一个接口，调用命令对象的execute方法，就可以让接受者进行相关的动作，这个接口也具备一个undo方法。
④ ConcreteCommand定义了动作和接受者之间的绑定关系，调用者只要调用execute方法就可以发出请求，然后由ConcreteCommand调用接受者的一个或多个动作。
⑤ Receiver为接受者，实现请求具体的动作，任何类都可以当接受者。
三、示例说明
设想如下场景，家里的电器都通过一个遥控器来控制，按下按钮即可控制电器的开关，并且只要在遥控的相关插口插入对应的电器模块，即可控制相应的电器，即可很方便的进行替换。
3.1 v1.0
Command
package com.hust.grid.leesf.command; interface Command { void execute(); void undo(); }
LightOnCommand
package com.hust.grid.leesf.command; public class LightOnCommand implements Command { private Receiver light; public LightOnCommand(Receiver receiver) { this.light = receiver; } @Override public void execute() { light.on(); } @Override public void undo() { light.off(); } }
LightOffCommand
package com.hust.grid.leesf.command; public class LightOffCommand implements Command { private Receiver light; public LightOffCommand(Receiver receiver) { light = receiver; } @Override public void execute() { light.off(); } @Override public void undo() { light.on(); } }
CeilingFanOnCommand
package com.hust.grid.leesf.command; public class CeilingFanOnCommand implements Command { private Receiver ceilingFan; public CeilingFanOnCommand(Receiver receiver) { ceilingFan = receiver; } @Override public void execute() { ceilingFan.on(); } @Override public void undo() { ceilingFan.off(); } }
CeilingFanOffCommand
package com.hust.grid.leesf.command; public class CeilingFanOffCommand implements Command { private Receiver ceilingFan; public CeilingFanOffCommand(Receiver receiver) { ceilingFan = receiver; } @Override public void execute() { ceilingFan.off(); } @Override public void undo() { ceilingFan.on(); } }
Receiver
package com.hust.grid.leesf.command; public abstract class Receiver { protected String name; public Receiver(String name) { this.name = name; } public void on() { System.out.println(this.name + " is on"); } public void off() { System.out.println(this.name + " is off"); } }
Light
package com.hust.grid.leesf.command; public class Light extends Receiver { public Light(String name) { super(name); } }
CeilingFan
package com.hust.grid.leesf.command; public class CeilingFan extends Receiver { public CeilingFan(String name) { super(name); } }
RemoteController
package com.hust.grid.leesf.command; public class RemoteController { Command[] onCommands; Command[] offCommands; Command undoCommand; public RemoteController(int slotSize) { onCommands = new Command[slotSize]; offCommands = new Command[slotSize]; } public void setCommand(int slot, Command onCommand, Command offCommand) { onCommands[slot] = onCommand; offCommands[slot] = offCommand; } public void onButtonWasPushed(int slot) { undoCommand = onCommands[slot]; onCommands[slot].execute(); } public void offButtonWasPushed(int slot) { undoCommand = offCommands[slot]; offCommands[slot].execute(); } public void undoButtonWasPushed() { undoCommand.undo(); } }
Main（用于测试）
package com.hust.grid.leesf.command; public class Main { public static void main(String[] args) { // 遥控器 RemoteController remoteController = new RemoteController(3); Light livingRoomLight = new Light("LivingRoomLight"); Light kitchenLight = new Light("KitchenLight"); CeilingFan ceilingFan = new CeilingFan("CeilingFan"); LightOnCommand livingRoomLightOnCommand = new LightOnCommand(livingRoomLight); LightOffCommand livingRoomLightOffCommand = new LightOffCommand(livingRoomLight); LightOnCommand kitchenLightOnCommand = new LightOnCommand(kitchenLight); LightOnCommand kitchenLightOffCommand = new LightOnCommand(kitchenLight); CeilingFanOnCommand ceilingFanOnCommand = new CeilingFanOnCommand(ceilingFan); CeilingFanOffCommand ceilingFanOffCommand = new CeilingFanOffCommand(ceilingFan); remoteController.setCommand(0, livingRoomLightOnCommand, livingRoomLightOffCommand); remoteController.setCommand(1, kitchenLightOnCommand, kitchenLightOffCommand); remoteController.setCommand(2, ceilingFanOnCommand, ceilingFanOffCommand); // 模拟遥控按下按钮 remoteController.onButtonWasPushed(0); remoteController.onButtonWasPushed(1); remoteController.onButtonWasPushed(2); remoteController.offButtonWasPushed(2); // 撤销操作 remoteController.undoButtonWasPushed(); } }
运行结果
LivingRoomLight is on KitchenLight is on CeilingFan is on CeilingFan is off CeilingFan is on
说明：使用了命令模式，一切都运行良好，并且可进行撤销操作。
若需要添加这样的功能，当按下按钮时，指定的所有电器都开或者关闭，即宏命令。
3.2 v2.0
添加宏命令，即一次性可以执行多个命令
MacroCommand
package com.hust.grid.leesf.command; public class MacroCommand implements Command { private Command[] commands; public MacroCommand(Command[] commands) { this.commands = commands; } @Override public void execute() { for (int i = 0; i < commands.length; i++) { commands[i].execute(); } } @Override public void undo() { for (int i = 0; i < commands.length; i++) { commands[i].undo(); } }
Main（用作测试）
package com.hust.grid.leesf.command; public class Main { public static void main(String[] args) { // 遥控器 RemoteController remoteController = new RemoteController(4); Light livingRoomLight = new Light("LivingRoomLight"); Light kitchenLight = new Light("KitchenLight"); CeilingFan ceilingFan = new CeilingFan("CeilingFan"); LightOnCommand livingRoomLightOnCommand = new LightOnCommand(livingRoomLight); LightOffCommand livingRoomLightOffCommand = new LightOffCommand(livingRoomLight); LightOnCommand kitchenLightOnCommand = new LightOnCommand(kitchenLight); LightOnCommand kitchenLightOffCommand = new LightOnCommand(kitchenLight); CeilingFanOnCommand ceilingFanOnCommand = new CeilingFanOnCommand(ceilingFan); CeilingFanOffCommand ceilingFanOffCommand = new CeilingFanOffCommand(ceilingFan); Command[] partyOn = {livingRoomLightOnCommand, kitchenLightOnCommand, ceilingFanOnCommand}; Command[] partyOff = {livingRoomLightOffCommand, kitchenLightOffCommand, ceilingFanOffCommand}; MacroCommand macroOnCommand = new MacroCommand(partyOn); MacroCommand macroOffCommand = new MacroCommand(partyOff); remoteController.setCommand(0, livingRoomLightOnCommand, livingRoomLightOffCommand); remoteController.setCommand(1, kitchenLightOnCommand, kitchenLightOffCommand); remoteController.setCommand(2, ceilingFanOnCommand, ceilingFanOffCommand); remoteController.setCommand(3, macroOnCommand, macroOffCommand); // 模拟遥控按下按钮 remoteController.onButtonWasPushed(3); // 撤销操作 remoteController.undoButtonWasPushed(); } }
运行结果
LivingRoomLight is on KitchenLight is on CeilingFan is on LivingRoomLight is off KitchenLight is off CeilingFan is off
说明：可以看到，调用了宏命令后，一次性执行了多条命令，并且执行撤消后，会恢复到之前的状态。
命令模式的要点如下：
① 命令模式将发出的请求的对象和执行请求的对象解耦
② 在被解耦的两者之间是通过命令对象进行沟通的。命令对象封装了接受者和一个或一组动作。
③ 调用者通过调用命令对象的execute方法发出请求，这会使得接受者的动作被调用。
④ 调用者可以接受命令当做参数，甚至在运行时动态地进行。
