一,命令模式的实现:
命令模式里边一般都有以下几个角色:客户端,请求者,命令接口,命令实现,接受者,
下边是简单命令模式的实现代码实现:
1![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedBlockStart.gif)
public class Client
{
2![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public static void main(String[] args)
{
3
Receiver receiver = new Receiver();
4
Command commandOne = new ConcreteCommandOne(receiver);
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Command commandTwo = new ConcreteCommandTwo(receiver);
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Invoker invoker = new Invoker(commandOne,commandTwo);
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invoker.actionOne();
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invoker.actionTwo();
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}
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}
11![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedBlockStart.gif)
public class Invoker
{
12
private Command commandOne;
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private Command commandTwo;
14![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public Invoker(Command commandOne,Command commandTwo)
{
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this.commandOne = commandOne;
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this.commandTwo = commandTwo;
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}
18![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public void actionOne()
{
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commandOne.execute();
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}
21![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public void actionTwo()
{
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commandTwo.execute();
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}
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}
25![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedBlockStart.gif)
public interface Command
{
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void execute();
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}
28![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedBlockStart.gif)
public class ConcreteCommandOne implements Command
{
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private Receiver receiver
30![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public ConcreteCommandOne(Receiver receiver)
{
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this.receiver = receiver;
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}
33![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public void execute()
{
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receiver.actionOne();
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}
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}
37![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedBlockStart.gif)
public class ConcreteCommandTwo implements Command
{
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private Receiver receiver
39![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public ConcreteCommandTwo(Receiver receiver)
{
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this.receiver = receiver;
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}
42![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public void execute()
{
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receiver.actionTwo();
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}
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}
46![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedBlockStart.gif)
public class Receiver
{
47![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public Receiver()
{
48
//![](http://www.blogjava.net/Images/dot.gif)
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}
50![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public void actionOne()
{
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System.out.println("ActionOne has been taken.");
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}
53![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public void actionTwo()
{
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System.out.println("ActionTwo has been taken.");
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}
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}
二,命令模式的功能,好处,或者说为什么使用命令模式?
上边的代码是否看起来很傻呢,本来可以这样简单实现的:
1![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedBlockStart.gif)
public class Client
{
2![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public static void main(String[] args)
{
3
Receiver receiver = new Receiver();
4
receiver.actionOne();
5
receiver.actionTwo();
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}
7
}
8![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedBlockStart.gif)
public class Receiver
{
9![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public Receiver()
{
10
//![](http://www.blogjava.net/Images/dot.gif)
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}
12![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public void actionOne()
{
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System.out.println("ActionOne has been taken.");
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}
15![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public void actionTwo()
{
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System.out.println("ActionTwo has been taken.");
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}
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}
看多简洁,如果是像上边如此简单的需求,这个才应该是我们的选择,但是有些情况下这样的写法不能解决的,
或者说解决起来不好,所以引入命令模式.
(1)我们须要Client和Receiver同时开发,而且在开发过程中分别须要不停重购,改名
(2)如果我们要求Redo ,Undo等功能
(3)我们须要命令不按照调用执行,而是按照执行时的情况排序,执行
(4)开发后期,我们发现必须要log哪些方法执行了,如何在尽量少更改代码的情况下实现.并且渐少重复代码
(5)在上边的情况下,我们的接受者有很多,不止一个
解决办法:
情况一,我们可以定义一个接口,让Receiver实现这个接口,Client按照接口调用。
情况二,我们可以让Receiver记住一些状态,例如执行前的自己的状态,用来undo,但自己记录自己的状态
实现起来比较混乱,一般都是一个累记录另一个类的状态.
情况三,很难实现
情况四,,我们须要在每个Action,前后加上log
情况五,相对好实现,但是再加上这个,是否感觉最终的实现很混乱呢
好,我们再来看看命令模式,在命令模式中,我们增加一些过渡的类,这些类就是上边的命名接口和命令实现,
这样就很好的解决了情况一,情况二。我们再加入一个Invoker,这样情况三和情况四就比较好解决了。
如下加入Log和排序后的Invoker
1![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedBlockStart.gif)
public class Invoker
{
2
private List cmdList = new ArrayList();
3![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public Invoker()
{
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}
5![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public add(Command command)
{
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cmdList.add(command);
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}
8![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public remove(Command command)
{
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cmdList.remove(command);
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}
11![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public void action()
{
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Command cmd;
13![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
while((cmd =getCmd()) != null)
{
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log("begin"+cmd.getName());
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cmd.execute();
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log("end"+cmd.getName());
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}
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}
19![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public Command getCmd()
{
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//按照自定义优先级,排序取出cmd
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}
22
}
23![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedBlockStart.gif)
public class Client
{
24![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public static void main(String[] args)
{
25
Receiver receiver = new Receiver();
26
Command commandOne = new ConcreteCommandOne(receiver);
27
Command commandTwo = new ConcreteCommandTwo(receiver);
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Invoker invoker = new Invoker();
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invoker.add(commandOne);
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invoker.add(commandTwo);
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iinvoker.action();
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}
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}
三,命令模式与其它模式的配合使用:
1,看上边的Invoker的实现是否很像代理模式呢,Invoker的这种实现其实就是一种代理模式。
2,需求:有个固定命令组合会多次被执行
解决:加入合成模式,实现方法如下,定义一个宏命令类:
1![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedBlockStart.gif)
public class MacroCommand implements Command
{
2
private List cmdList = new ArrayList();
3![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public add(Command command)
{
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cmdList.add(command);
5
}
6![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public remove(Command command)
{
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cmdList.remove(command);
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}
9![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public void execute()
{
10
Command cmd;
11![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
for(int i=0;i<cmdList.size();i++)
{
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cmd = (Command)cmdList.get(i);
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cmd.execute();
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}
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}
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}
3,需求:须要redo undo
解决:加入备忘录模式,一个简单的实现如下
1![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedBlockStart.gif)
public class ConcreteCommandOne implements Command
{
2
private Receiver receiver
3
private Receiver lastReceiver;
4![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public ConcreteCommandOne(Receiver receiver)
{
5
this.receiver = receiver;
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}
7![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public void execute()
{
8
record();
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receiver.actionOne();
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}
11![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public void undo()
{
12
//恢复状态![](http://www.blogjava.net/Images/dot.gif)
13
}
14![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public void redo()
{
15
lastReceiver.actionOne();
16
//![](http://www.blogjava.net/Images/dot.gif)
17
}
18![](http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public record()
{
19
//记录状态
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}
21
}
4,需求:命令很多类似的地方
解决:使用原型模式,利用clone
这个就不写例子了。
四,命令模式的使用场合
1,须要callback的时候,例如java awt/swing/swt中的Listening的消息方式
2,须要对请求排队执行,命令的发送者和接受者有不同对的生命周期,就是命令执行的时候,可能发出命令的
Client已经不存在了
3,须要Redo Undo等函数
4,须要log每条命令
5,须要支持transaction,封装一组数据命令的时候.
五,最后再次总结一下命令模式的优点和缺点:
优点:
降低Client和命令接受者的耦合,是命令请求和命令执行的对象分割
便于修改和扩张
便于聚合多个命令
缺点:
造成出现过多的具体命令类,太多文件。
五,一个比较有意思的例子,来说明命令模式
Client :看电视的人
Invoker :遥控器
Command :电信号
具体命令 :遥控器上的按键对应的不同的电信号
Receiver :电视机
最后说一句,并不是全部按照模式写一定就好,应该根据你的需求来应用,或者全部应用,或者部分应用,或者根本不用。