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

---

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-diagram

interface 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 <|.. CommandB

CommandA "1" --> "1" Receiver
CommandB "1" --> "1" Receiver

Invoker "1" --> "n" Command

Client ..> 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();
    }
}