设计模式—— 十五：命令模式-优快云博客

本文链接：https://blog.youkuaiyun.com/mifffy_java/article/details/148957702

什么是命令模式？

命令模式的定义：

Encapsulate a request as an object,thereby letting you parameterize clients with different requests,queue or log requests,and support undoable operations.（将一个请求封装成一个对象，从而让你使用不同的请求把客户端参数化，对请求排队或者记录请求日志，可以提供命令的撤销和恢复功能。）

命令模式的核心在于引入了命令类，通过命令类来降低发送者和接收者的耦合度，请求发送者只需指定一个命令对象，再通过命令对象来调用请求接收者的处理方法。

命令模式通用类图如下：

图15-1:命令模式模式通用类图

在这里插入图片描述

命令模式包含这么几个角色：

Command（抽象命令）：声明需要执行的命令
ConcreteCommand（具体命令）：实现声明的命令
Receive（接收者）：接收者执行与请求相关的操作，它具体实现对请求的业务处理
Invoker（调用者）：调用者即请求发送者，它通过命令对象来执行请求。

命令模式的本质是对请求进行封装，一个请求对应于一个命令，将发出命令的责任和执行命令的责任分割开。每一个命令都是一个操作：请求的一方发出请求要求执行一个操作；接收的一方收到请求，并执行相应的操作。命令模式允许请求的一方和接收的一方独立开来，使得请求的一方不必知道接收请求的一方的接口，更不必知道请求如何被接收、操作是否被执行、何时被执行，以及是怎么被执行的。

命令模式通用代码如下：

Receiver：Receiver类实现具体的业务需求

复制代码

<span style="color:#141418"><span style="background-color:#ffffff"><code class="language-java"><span style="color:#a0a1a7"><em>/**
 * <span style="color:#a626a4">@author</span> 三分恶
 * <span style="color:#a626a4">@date</span> 2020年6月28日
 * <span style="color:#a626a4">@description</span> 接收者
 */</em></span>
<span style="color:#a626a4">public</span> <span style="color:#a626a4">class</span> <span style="color:#4078f2">Receiver</span> {
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">doSomething</span>() {
		<span style="color:#a0a1a7"><em>//具体的业务逻辑</em></span>
	}
}
</code></span></span>

抽象的Command类：

复制代码

<span style="color:#141418"><span style="background-color:#ffffff"><code class="language-java"><span style="color:#a0a1a7"><em>/**
 * <span style="color:#a626a4">@author</span> 三分恶
 * <span style="color:#a626a4">@date</span> 2020年6月28日
 * <span style="color:#a626a4">@description</span> 抽象的Command类
 */</em></span>
<span style="color:#a626a4">public</span> <span style="color:#a626a4">abstract</span> <span style="color:#a626a4">class</span> <span style="color:#4078f2">Command</span> {
	<span style="color:#a0a1a7"><em>// 每个命令类都必须有一个执行命令的方法</em></span>
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">abstract</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">execute</span>();
}

</code></span></span>

具体的Command类：根据需求，具体的命令类可以有多个：

复制代码

<span style="color:#141418"><span style="background-color:#ffffff"><code class="language-java"><span style="color:#a0a1a7"><em>/**
 * <span style="color:#a626a4">@author</span> 三分恶
 * <span style="color:#a626a4">@date</span> 2020年6月28日
 * <span style="color:#a626a4">@description</span> 具体的命令类
 */</em></span>
<span style="color:#a626a4">public</span> <span style="color:#a626a4">class</span> <span style="color:#4078f2">ConcreteCommand</span> <span style="color:#a626a4">extends</span> <span style="color:#4078f2">Command</span> {
	<span style="color:#a0a1a7"><em>// 维持一个对请求接收者对象的引用</em></span>
	<span style="color:#a626a4">private</span> Receiver receiver; 

	<span style="color:#a0a1a7"><em>//构造函数传递接收者</em></span>
	<span style="color:#a626a4">public</span> <span style="color:#4078f2">ConcreteCommand</span>(Receiver receiver) {
		<span style="color:#c18401">this</span>.receiver = receiver;
	}

	<span style="color:#a626a4">public</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">execute</span>() {
		 <span style="color:#a0a1a7"><em>// 调用请求接收者的业务处理方法doSomething()</em></span>
		receiver.doSomething();
	}
}
</code></span></span>

调用者Invoker类：

复制代码

<span style="color:#141418"><span style="background-color:#ffffff"><code class="language-java"><span style="color:#a0a1a7"><em>/**
 * <span style="color:#a626a4">@author</span> 三分恶
 * <span style="color:#a626a4">@date</span> 2020年6月28日
 * <span style="color:#a626a4">@description</span> 调用者
 */</em></span>
<span style="color:#a626a4">public</span> <span style="color:#a626a4">class</span> <span style="color:#4078f2">Invoker</span> {
	<span style="color:#a626a4">private</span> Command command;
	
	<span style="color:#a0a1a7"><em>// 设值注入</em></span>
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">setCommand</span>(Command command) {
		<span style="color:#c18401">this</span>.command = command;
	}

	<span style="color:#a0a1a7"><em>// 执行命令</em></span>
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">action</span>() {
		<span style="color:#c18401">this</span>.command.execute();
	}
}
</code></span></span>

场景类：

复制代码

<span style="color:#141418"><span style="background-color:#ffffff"><code class="language-java"><span style="color:#a0a1a7"><em>/**
 * <span style="color:#a626a4">@author</span> 三分恶
 * <span style="color:#a626a4">@date</span> 2020年6月28日 
 * <span style="color:#a626a4">@description</span>  
 */</em></span>
<span style="color:#a626a4">public</span> <span style="color:#a626a4">class</span> <span style="color:#4078f2">Client</span> {

	<span style="color:#a626a4">public</span> <span style="color:#a626a4">static</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">main</span>(String[] args) {
		<span style="color:#a0a1a7"><em>//首先声明调用者Invoker </em></span>
		<span style="color:#986801">Invoker</span> <span style="color:#986801">invoker</span> <span style="color:#ab5656">=</span> <span style="color:#a626a4">new</span> <span style="color:#4078f2">Invoker</span>(); 
		<span style="color:#a0a1a7"><em>//定义接收者 </em></span>
		<span style="color:#986801">Receiver</span> <span style="color:#986801">receiver</span> <span style="color:#ab5656">=</span> <span style="color:#a626a4">new</span> <span style="color:#4078f2">Receiver</span>(); 
		<span style="color:#a0a1a7"><em>//定义一个发送给接收者的命令 </em></span>
		<span style="color:#986801">Command</span> <span style="color:#986801">command</span> <span style="color:#ab5656">=</span> <span style="color:#a626a4">new</span> <span style="color:#4078f2">ConcreteCommand</span>(receiver); 
		<span style="color:#a0a1a7"><em>//把命令交给调用者去执行 </em></span>
		invoker.setCommand(command); 
		invoker.action();
	}

}
</code></span></span>

为什么要用命令模式？

假设有一个这样的业务场景：一个正在开发的软件项目，分为三个组：需求组、美工组、开发组。在开发的过程中，客户需要和厂商沟通，和需求组讨论需求、和美工讨论页面、和代码组讨论实现，告诉他们修改、删除、增加各种内容等。

使用命令模式前

上面的业务场景抽象成类图：

图15-2:甲方要求业务场景初始类图

在这里插入图片描述

根据类图进行具体的编码实现：

Group:抽象类，定义业务：

复制代码

<span style="color:#141418"><span style="background-color:#ffffff"><code class="language-java"><span style="color:#a0a1a7"><em>/**
 * <span style="color:#a626a4">@author</span> 三分恶
 * <span style="color:#a626a4">@date</span> 2020年6月28日
 * <span style="color:#a626a4">@description</span> 抽象Group
 */</em></span>
<span style="color:#a626a4">public</span> <span style="color:#a626a4">abstract</span> <span style="color:#a626a4">class</span> <span style="color:#4078f2">Group</span> {
	<span style="color:#a0a1a7"><em>// 甲乙双方分开办公，如果甲方要和某个组讨论，首先要找到这个组</em></span>
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">abstract</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">find</span>();

	<span style="color:#a0a1a7"><em>// 要求增加功能</em></span>
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">abstract</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">add</span>();

	<span style="color:#a0a1a7"><em>// 要求删除功能</em></span>
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">abstract</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">delete</span>();

	<span style="color:#a0a1a7"><em>// 要求修改功能</em></span>
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">abstract</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">change</span>();

	<span style="color:#a0a1a7"><em>// 要求给出所有的变更计划</em></span>
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">abstract</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">plan</span>();
}

</code></span></span>

RequirementGroup（需求组）：

复制代码

<span style="color:#141418"><span style="background-color:#ffffff"><code class="language-java"><span style="color:#a626a4">public</span> <span style="color:#a626a4">class</span> <span style="color:#4078f2">RequirementGroup</span> <span style="color:#a626a4">extends</span> <span style="color:#4078f2">Group</span>{

	<span style="color:#4078f2">@Override</span>
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">find</span>() {
		System.out.println(<span style="color:#50a14f">"找到需求组..."</span>);
	}

	<span style="color:#4078f2">@Override</span>
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">add</span>() {
		System.out.println(<span style="color:#50a14f">"客户要求增加一项需求..."</span>);
	}

	<span style="color:#4078f2">@Override</span>
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">delete</span>() {
		System.out.println(<span style="color:#50a14f">"客户要求删除一项需求..."</span>);
	}

	<span style="color:#4078f2">@Override</span>
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">change</span>() {
		System.out.println(<span style="color:#50a14f">"客户要求修改一项需求..."</span>);
	}

	<span style="color:#4078f2">@Override</span>
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">plan</span>() {
		System.out.println(<span style="color:#50a14f">"客户要求需求变更计划..."</span>);
	}

}
</code></span></span>

PageGroup（美工组）：

复制代码

<span style="color:#141418"><span style="background-color:#ffffff"><code class="language-java"><span style="color:#a626a4">public</span> <span style="color:#a626a4">class</span> <span style="color:#4078f2">PageGroup</span> <span style="color:#a626a4">extends</span> <span style="color:#4078f2">Group</span>{

	<span style="color:#4078f2">@Override</span>
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">find</span>() {
		System.out.println(<span style="color:#50a14f">"找到美工组..."</span>);
	}

	<span style="color:#4078f2">@Override</span>
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">add</span>() {
		System.out.println(<span style="color:#50a14f">"客户要求增加一个页面..."</span>);
	}

	<span style="color:#4078f2">@Override</span>
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">delete</span>() {
		System.out.println(<span style="color:#50a14f">"客户要求删除一个页面..."</span>);
	}

	<span style="color:#4078f2">@Override</span>
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">change</span>() {
		System.out.println(<span style="color:#50a14f">"客户要求改变一个页面..."</span>);
	}

	<span style="color:#4078f2">@Override</span>
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">plan</span>() {
		System.out.println(<span style="color:#50a14f">"客户要求页面变更计划..."</span>);
	}

}
</code></span></span>

CodeGroup（开发组）：

复制代码

<span style="color:#141418"><span style="background-color:#ffffff"><code class="language-java"><span style="color:#a626a4">public</span> <span style="color:#a626a4">class</span> <span style="color:#4078f2">CodeGroup</span> <span style="color:#a626a4">extends</span> <span style="color:#4078f2">Group</span>{

	<span style="color:#4078f2">@Override</span>
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">find</span>() {
		System.out.println(<span style="color:#50a14f">"找到代码组..."</span>);
	}

	<span style="color:#4078f2">@Override</span>
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">add</span>() {
		System.out.println(<span style="color:#50a14f">"客户要求增加一项功能..."</span>);
	}

	<span style="color:#4078f2">@Override</span>
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">delete</span>() {
		System.out.println(<span style="color:#50a14f">"客户要求删除一项功能..."</span>);
	}

	<span style="color:#4078f2">@Override</span>
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">change</span>() {
		System.out.println(<span style="color:#50a14f">"客户要求修改一项功能..."</span>);
	}

	<span style="color:#4078f2">@Override</span>
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">plan</span>() {
		System.out.println(<span style="color:#50a14f">"客户要求代码变更计划..."</span>);
	}

}
</code></span></span>

场景类：

复制代码

<span style="color:#141418"><span style="background-color:#ffffff"><code class="language-java"><span style="color:#a626a4">public</span> <span style="color:#a626a4">class</span> <span style="color:#4078f2">Client</span> {

	<span style="color:#a0a1a7"><em>/**
	 * <span style="color:#a626a4">@param</span> args
	 */</em></span>
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">static</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">main</span>(String[] args) {
		<span style="color:#a0a1a7"><em>//首先客户找到需求组说，过来谈需求，并修改</em></span>
		System.out.println(<span style="color:#50a14f">"-----------客户要求增加一项需求---------------"</span>); 
		<span style="color:#986801">Group</span> <span style="color:#986801">rg</span> <span style="color:#ab5656">=</span> <span style="color:#a626a4">new</span> <span style="color:#4078f2">RequirementGroup</span>();
		<span style="color:#a0a1a7"><em>//找到需求组</em></span>
		rg.find(); 
		<span style="color:#a0a1a7"><em>//增加一个需求 </em></span>
		rg.add(); 
		<span style="color:#a0a1a7"><em>//要求变更计划 </em></span>
		rg.plan();
	}

}
</code></span></span>

运行结果：

在这里插入图片描述

场景类中客户找到了需求组，向需求组提出了增加需求和需求变更的要求。

这样存在什么问题呢？甲方对乙方做一些要求的时候，要找到对应的组，但是不能希望甲方是非常专业的，可能提需求提到了开发组头上，要求页面变更提到了需求组头上。

使用命令模式后

这是为什么项目需要项目经理的原因之一。

甲方不管你什么需求、美工、开发，只管把项目经理叫过去，告诉项目经理他们的要求。项目经理再根据甲方的要求向各个组下达命令。对应的，就可以引入命令模式。

图15-3:甲方要求业务场景引入命令模式的类图

在这里插入图片描述

来看看具体的代码：

抽象命令类：

复制代码

<span style="color:#141418"><span style="background-color:#ffffff"><code class="language-java"><span style="color:#a626a4">public</span> <span style="color:#a626a4">abstract</span>  <span style="color:#a626a4">class</span> <span style="color:#4078f2">Command</span> {
	<span style="color:#a0a1a7"><em>//把三个组都定义好，子类可以直接使用 </em></span>
	<span style="color:#a626a4">protected</span> <span style="color:#986801">RequirementGroup</span> <span style="color:#986801">rg</span> <span style="color:#ab5656">=</span> <span style="color:#a626a4">new</span> <span style="color:#4078f2">RequirementGroup</span>();   <span style="color:#a0a1a7"><em>//需求组</em></span>
	<span style="color:#a626a4">protected</span> <span style="color:#986801">PageGroup</span> <span style="color:#986801">pg</span> <span style="color:#ab5656">=</span> <span style="color:#a626a4">new</span> <span style="color:#4078f2">PageGroup</span>(); <span style="color:#a0a1a7"><em>//美工组 </em></span>
	<span style="color:#a626a4">protected</span> <span style="color:#986801">CodeGroup</span> <span style="color:#986801">cg</span> <span style="color:#ab5656">=</span> <span style="color:#a626a4">new</span> <span style="color:#4078f2">CodeGroup</span>(); <span style="color:#a0a1a7"><em>//代码组 </em></span>
	<span style="color:#a0a1a7"><em>//只有一个方法，你要我做什么事情 </em></span>
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">abstract</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">execute</span>();
}
</code></span></span>

增加需求的命令：

复制代码

<span style="color:#141418"><span style="background-color:#ffffff"><code class="language-java"><span style="color:#a626a4">public</span> <span style="color:#a626a4">class</span> <span style="color:#4078f2">AddRequirementCommand</span> <span style="color:#a626a4">extends</span> <span style="color:#4078f2">Command</span>{

	<span style="color:#4078f2">@Override</span>
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">execute</span>() {
		<span style="color:#a0a1a7"><em>//找到需求组 </em></span>
		<span style="color:#c18401">super</span>.rg.find(); 
		<span style="color:#a0a1a7"><em>//增加一份需求</em></span>
		<span style="color:#c18401">super</span>.rg.add(); 
		<span style="color:#a0a1a7"><em>//给出计划 </em></span>
		<span style="color:#c18401">super</span>.rg.plan();
	}

}
</code></span></span>

删除页面的命令：

复制代码

<span style="color:#141418"><span style="background-color:#ffffff"><code class="language-java"><span style="color:#a626a4">public</span> <span style="color:#a626a4">class</span> <span style="color:#4078f2">DeletePageCommand</span> <span style="color:#a626a4">extends</span> <span style="color:#4078f2">Command</span>{

	<span style="color:#4078f2">@Override</span>
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">execute</span>() {
		<span style="color:#a0a1a7"><em>//找到页面组 </em></span>
		<span style="color:#c18401">super</span>.pg.find(); 
		<span style="color:#a0a1a7"><em>//删除一个页面 </em></span>
		<span style="color:#c18401">super</span>.rg.delete();
		<span style="color:#a0a1a7"><em>//给出计划 </em></span>
		<span style="color:#c18401">super</span>.rg.plan();
	}

}
</code></span></span>

负责人Invoker：负责人只要接到客户的命令，就立刻执行。

复制代码

<span style="color:#141418"><span style="background-color:#ffffff"><code class="language-java"><span style="color:#a626a4">public</span> <span style="color:#a626a4">class</span> <span style="color:#4078f2">Invoker</span> {
	<span style="color:#a0a1a7"><em>// 什么命令</em></span>
	<span style="color:#a626a4">private</span> Command command;

	<span style="color:#a0a1a7"><em>// 客户发出命令</em></span>
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">setCommand</span>(Command command) {
		<span style="color:#c18401">this</span>.command = command;
	}

	<span style="color:#a0a1a7"><em>// 执行客户的命令</em></span>
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">action</span>() {
		<span style="color:#c18401">this</span>.command.execute();
	}
}
</code></span></span>

场景类：模拟客户增加需求的命令。

复制代码

<span style="color:#141418"><span style="background-color:#ffffff"><code class="language-java"><span style="color:#a626a4">public</span> <span style="color:#a626a4">class</span> <span style="color:#4078f2">Client</span> {
	<span style="color:#a626a4">public</span> <span style="color:#a626a4">static</span> <span style="color:#a626a4">void</span> <span style="color:#4078f2">main</span>(String[] args) {
		<span style="color:#a0a1a7"><em>// 定义调用者</em></span>
		<span style="color:#986801">Invoker</span> <span style="color:#986801">jingli</span> <span style="color:#ab5656">=</span> <span style="color:#a626a4">new</span> <span style="color:#4078f2">Invoker</span>();
		<span style="color:#a0a1a7"><em>// 客户要求增加一项需求</em></span>
		System.out.println(<span style="color:#50a14f">"------------客户要求增加一项需求---------------"</span>);
		<span style="color:#a0a1a7"><em>// 客户下命令</em></span>
		<span style="color:#986801">Command</span> <span style="color:#986801">command</span> <span style="color:#ab5656">=</span> <span style="color:#a626a4">new</span> <span style="color:#4078f2">AddRequirementCommand</span>();
		<span style="color:#a0a1a7"><em>// 调用者接收到命令</em></span>
		jingli.setCommand(command);
		<span style="color:#a0a1a7"><em>// 调用者执行命令</em></span>
		jingli.action();
	}
}
</code></span></span>

场景类就简单了很多，如果客户提出其它要求，如删除页面，只需要：

复制代码

<span style="color:#141418"><span style="background-color:#ffffff"><code class="language-java"><span style="color:#a0a1a7"><em>// 客户下命令</em></span>
<span style="color:#a0a1a7"><em>//Command command = new AddRequirementCommand();</em></span>
<span style="color:#986801">Command</span> <span style="color:#986801">command</span> <span style="color:#ab5656">=</span> <span style="color:#a626a4">new</span> <span style="color:#4078f2">DeletePageCommand</span>();
</code></span></span>

同样比较简单。

这样一来，客户提要求的时候，不需要知道具体谁去完成这个要求，只需求把这个要求提给项目经理即可。

高内聚的要求就被满足了。

命令模式优缺点

优点

● 类间解耦
调用者角色与接收者角色之间没有任何依赖关系，调用者实现功能时只需调用Command 抽象类的execute方法就可以，不需要了解到底是哪个接收者执行。

● 可扩展性 Command的子类可以非常容易地扩展，而调用者Invoker和高层次的模块Client不产生严重的代码耦合。

缺点

使用命令模式可能会导致某些系统有过多的具体命令类。因为针对每一个对请求接收者的调用操作都需要设计一个具体命令类，因此在某些系统中可能需要提供大量的具体命令类，这将影响命令模式的使用。

命令模式使用场景

在以下情况下可以考虑使用命令模式：

系统需要将请求调用者和请求接收者解耦，使得调用者和接收者不直接交互。请求调用者无须知道接收者的存在，也无须知道接收者是谁，接收者也无须关心何时被调用。
系统需要在不同的时间指定请求、将请求排队和执行请求。一个命令对象和请求的初始调用者可以有不同的生命期，换言之，最初的请求发出者可能已经不在了，而命令对象本身仍然是活动的，可以通过该命令对象去调用请求接收者，而无须关心请求调用者的存在性，可以通过请求日志文件等机制来具体实现。
系统需要支持命令的撤销(Undo)操作和恢复(Redo)操作。