命令模式（请求与具体实现解耦）

目录

前言

UML

plantuml

类图

实战代码

模板

Command

Invoker

Receiver

Client

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前言

命令模式解耦了命令请求者（Invoker）和命令执行者（receiver），使得 Invoker 不再直接引用 receiver，而是依赖于抽象的命令接口。具体的命令类则直接引用 receiver，通过调用 receiver 的方法来执行命令。

解耦之后，具体命令的增删改不再影响 Invoker，同时，抽象出来的命令请求还能队列化，从而实现撤销和重试功能，或做排列组合合成复杂的命令。

在 Invoker 中，也体现了桥接模式的思想，将命令的抽象和具体的实现分离，使 Invoker 中命令的添加和扩展更加简单。不过具体的命令类并不直接实现处理逻辑，而是交给了 receiver 来实现，这也是命令模式和桥接模式最主要的区别。

UML

plantuml

@startuml
'https://plantuml.com/class-diagraminterface Command {+ execute() : void
}class CommandA {- receiver : Receiver+ CommandA(Receiver)+ execute() : void
}class CommandB {- receiver : Receiver+ CommandB(Receiver)+ execute() : void
}class Receiver {+ executeA() : void+ executeB() : void
}class Invoker {- commands : List<Command>+ addCommand(Command) : void+ execute(Command) : void+ executes() : void
}class Client {}Command <|.. CommandA
Command <|.. CommandBCommandA "1" --> "1" Receiver
CommandB "1" --> "1" ReceiverInvoker "1" --> "n" CommandClient ..> Invoker
Client ..> Command@enduml

类图

实战代码

模板

Command

public interface Command {void execute();
}public class ConcreteCommand1 implements Command {private Receiver receiver;public ConcreteCommand1(Receiver receiver) {this.receiver = receiver;}public void execute() {receiver.execute1();}
}public class ConcreteCommand2 implements Command {private Receiver receiver;public ConcreteCommand2(Receiver receiver) {this.receiver = receiver;}public void execute() {receiver.execute2();}
}

Invoker

public class Invoker {private List<Command> commands = new ArrayList<Command>();public void addCommand(Command command){commands.add(command);}public void execute(Command command){command.execute();}public void executes(){for(Command command : commands){command.execute();}commands.clear();}
}

Receiver

public class Receiver {public void execute1() {System.out.println("execute1.");}public void execute2() {System.out.println("execute2.");}
}

Client

public class Test {public static void main(String[] args) {Receiver receiver = new Receiver();Invoker invoker = new Invoker();invoker.execute(new Command1(receiver));invoker.addAction(new Command1(receiver));invoker.addAction(new Command2(receiver));invoker.executes();}
}