设计模式之命令模式

命令模式：将一个请求封装成一个对象，从而可以使用不同的请求对客户端进行参数化，还可以记录请求日志和撤销请求。

当我们发出一个或者多个请求的时候，如果请求的接收者和执行者重合的话，会导致职责过重，难以对请求进行管理。如果将请求接收者和请求执行者分开，就可以比较好地对请求进行管理和完成。一个典型的例子就是餐厅点餐，服务员负责接受客人点餐，厨师负责完成炒菜。

基本实现

执行者：

public class Executor { public void execute() { System.out.println("Executor executes"); } }

命令类及其具体实现，与执行者相关联，命令的执行就是调用执行者去执行：

public abstract class Command { protected Executor executor; public Command(Executor executor) { this.executor = executor; } public abstract void execute(); } class ConcreteCommand extends Command { public ConcreteCommand(Executor executor) { super(executor); } @Override public void execute() { executor.execute(); } }

命令接收者，依赖于命令，接收者的执行方法就是执行命令：

public class Receiver { private Command command; public void setCommand(Command command) { this.command = command; } public void executeCommand() { command.execute(); } }

测试类：

public class Main { public static void main(String[] args) { final Executor executor = new Executor(); final Command command = new ConcreteCommand(executor); final Receiver receiver = new Receiver(); receiver.setCommand(command); receiver.executeCommand(); } }

输出：

Executor executes

示例实现：

厨师类：

public class Chef { public void kungPowChicken() { System.out.println("老师傅炒出宫保鸡丁这道菜"); } public void seaweedSoup() { System.out.println("老师傅熬制上等紫菜汤"); } public void rice() { System.out.println("老师傅准备特级香米饭"); } }

点菜命令类及其子类：

public abstract class Order { protected Chef chef; public Order(Chef chef) { this.chef = chef; } public abstract void execute(); } class KungPowChickenOrder extends Order { public KungPowChickenOrder(Chef chef) { super(chef); } @Override public void execute() { chef.kungPowChicken(); } @Override public String toString() { return "宫保鸡丁"; } } class SeaweedSoupOrder extends Order { public SeaweedSoupOrder(Chef chef) { super(chef); } @Override public void execute() { chef.seaweedSoup(); } @Override public String toString() { return "海带汤"; } } class RiceOrder extends Order { public RiceOrder(Chef chef) { super(chef); } @Override public void execute() { chef.rice(); } @Override public String toString() { return "香米饭"; } }

服务员类,，可以对接收的命令进行记录，还可以进行判断命令是否有效，以及统一执行命令：

public class Waiter { private List<Order> orders = new ArrayList<>(); public void addOrder(Order order) { orders.add(order); System.out.println("客人点了" + order); } public void deleteOrder(Order order) { orders.remove(order); System.out.println("客人不点" + order + "了"); } public void finishOrder() { for (Order order : orders) { order.execute(); } } }

测试类：

public class Main { public static void main(String[] args) { final Chef chef = new Chef(); final KungPowChickenOrder kungPowChickenOrder = new KungPowChickenOrder(chef); final SeaweedSoupOrder seaweedOrder = new SeaweedSoupOrder(chef); final RiceOrder riceOrder = new RiceOrder(chef); final Waiter waiter = new Waiter(); waiter.addOrder(kungPowChickenOrder); waiter.addOrder(seaweedOrder); waiter.addOrder(riceOrder); System.out.println(); waiter.deleteOrder(riceOrder); System.out.println(); waiter.finishOrder(); } }

输出：

客人点了宫保鸡丁 客人点了海带汤 客人点了香米饭 客人不点香米饭了 老师傅炒出宫保鸡丁这道菜 老师傅熬制上等紫菜汤