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();
} View Code
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();
}
} 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。
四、总结
对于命令模式而言，其应用也相对比较广泛，在工程应用中有适合应用此种模式的地方，不妨使用，所有源代码已经上传至github，欢迎fork，谢谢各位园友的观看~
PS:如果您觉得阅读本文对您有帮助，请点一下
“推荐”按钮，您的
，将会是我不竭的动力！
作者：
leesf 掌控之中，才会成功；掌控之外，注定失败。出处：http://www.cnblogs.com/leesf456/
本文版权归作者和博客园共有，欢迎转载，但未经作者同意必须保留此段声明，且在文章页面明显位置给出原文连接，否则保留追究法律责任的权利。
如果觉得本文对您有帮助，您可以请我喝杯咖啡!