本文參考自：《JAVA設(shè)計(jì)模式》之命令模式(Command)
1. 作用
命令模式屬于對(duì)象的行為模式翩伪。命令模式又稱為行動(dòng)(Action)模式或交易(Transaction)模式碰凶。
命令模式把一個(gè)請(qǐng)求或者操作封裝到一個(gè)對(duì)象中钳榨。命令模式允許系統(tǒng)使用不同的請(qǐng)求把客戶端參數(shù)化，對(duì)請(qǐng)求排隊(duì)或者記錄請(qǐng)求日志，可以提供命令的撤銷和恢復(fù)功能购对。
2. 結(jié)構(gòu)
其中有四個(gè)主要角色
-
命令角色(Command)
所有命令類的抽象接口 -
具體命令角色(ConcreteCommand)
定義一個(gè)接收者和行為之間的弱耦合啡浊；實(shí)現(xiàn)execute()方法戳粒，負(fù)責(zé)調(diào)用接收者的相應(yīng)操作。execute()方法通常叫做執(zhí)行方法虫啥。 -
請(qǐng)求者角色(Invoker)
負(fù)責(zé)調(diào)用命令對(duì)象執(zhí)行請(qǐng)求蔚约，相關(guān)的方法叫做行動(dòng)方法。 -
接收者角色(Receiver)
負(fù)責(zé)具體實(shí)施和執(zhí)行一個(gè)請(qǐng)求涂籽。任何一個(gè)類都可以成為接收者苹祟，實(shí)施和執(zhí)行請(qǐng)求的方法叫做行動(dòng)方法。
3. 具體實(shí)現(xiàn)
Receiver
public class Receiver {
public void action() {
System.out.println("action");
}
}
Command
public interface Command {
void execute();
}
ConcreteCommand
public class ConcreteCommand implements Command{
private Receiver receiver;
public ConcreteCommand(Receiver receiver) {
this.receiver = receiver;
}
@Override
public void execute() {
receiver.action();
}
}
Invoker
public class Invoker {
private Command command;
public Invoker(Command command) {
this.command = command;
}
public void invoke() {
command.execute();
}
}
具體實(shí)現(xiàn)
public class CommandPatternMain {
public static void main(String[] args) {
Receiver receiver = new Receiver();
Command command = new ConcreteCommand(receiver);
Invoker invoker = new Invoker(command);
invoker.invoke();
}
}
4. 命令模式實(shí)例-AudioPlayer系統(tǒng)
小女孩茱麗(Julia)有一個(gè)盒式錄音機(jī)，此錄音機(jī)有播音(Play)树枫、倒帶(Rewind)和停止(Stop)功能直焙，錄音機(jī)的鍵盤便是請(qǐng)求者(Invoker)角色；茱麗(Julia)是客戶端角色砂轻，而錄音機(jī)便是接收者角色奔誓。Command類扮演抽象命令角色，而PlayCommand搔涝、StopCommand和RewindCommand便是具體命令類厨喂。茱麗(Julia)不需要知道播音(play)、倒帶(rewind)和停止(stop)功能是怎么具體執(zhí)行的庄呈，這些命令執(zhí)行的細(xì)節(jié)全都由鍵盤(Keypad)具體實(shí)施蜕煌。茱麗(Julia)只需要在鍵盤上按下相應(yīng)的鍵便可以了。
錄音機(jī)是典型的命令模式诬留。錄音機(jī)按鍵把客戶端與錄音機(jī)的操作細(xì)節(jié)分割開來斜纪。
被調(diào)用者，AudioPlayer
public class AudioPlayer {
public void play() {
System.out.println("play");
}
public void replay() {
System.out.println("replay");
}
public void pause() {
System.out.println("pause");
}
public void stop() {
System.out.println("stop");
}
}
抽象命令
public interface Command {
void execute();
}
具體的命令
public class PlayCommand implements Command{
private AudioPlayer audioPlayer;
public PlayCommand(AudioPlayer audioPlayer) {
this.audioPlayer = audioPlayer;
}
@Override
public void execute() {
audioPlayer.play();
}
}
public class ReplayCommand implements Command{
private AudioPlayer audioPlayer;
public ReplayCommand(AudioPlayer audioPlayer) {
this.audioPlayer = audioPlayer;
}
@Override
public void execute() {
audioPlayer.replay();
}
}
public class PauseCommand implements Command{
private AudioPlayer audioPlayer;
public PauseCommand(AudioPlayer audioPlayer) {
this.audioPlayer = audioPlayer;
}
@Override
public void execute() {
audioPlayer.pause();
}
}
public class StopCommand implements Command{
private AudioPlayer audioPlayer;
public StopCommand(AudioPlayer audioPlayer) {
this.audioPlayer = audioPlayer;
}
@Override
public void execute() {
audioPlayer.stop();
}
}
調(diào)用者 Invoker
public class AudioPlayerInvoker {
private PlayCommand playCommand;
private ReplayCommand replayCommand;
private PauseCommand pauseCommand;
private StopCommand stopCommand;
public void setPlayCommand(PlayCommand playCommand) {
this.playCommand = playCommand;
}
public void setReplayCommand(ReplayCommand replayCommand) {
this.replayCommand = replayCommand;
}
public void setPauseCommand(PauseCommand pauseCommand) {
this.pauseCommand = pauseCommand;
}
public void setStopCommand(StopCommand stopCommand) {
this.stopCommand = stopCommand;
}
public void play() {
playCommand.execute();
}
public void replay() {
replayCommand.execute();
}
public void pause() {
pauseCommand.execute();
}
public void stop() {
stopCommand.execute();
}
}
具體實(shí)現(xiàn)
public class NormalCommandPatternDemoMain {
public static void main(String[] args) {
AudioPlayer audioPlayer = new AudioPlayer();
PlayCommand playCommand = new PlayCommand(audioPlayer);
ReplayCommand replayCommand = new ReplayCommand(audioPlayer);
PauseCommand pauseCommand = new PauseCommand(audioPlayer);
StopCommand stopCommand = new StopCommand(audioPlayer);
AudioPlayerInvoker invoker = new AudioPlayerInvoker();
invoker.setPlayCommand(playCommand);
invoker.setReplayCommand(replayCommand);
invoker.setPauseCommand(pauseCommand);
invoker.setStopCommand(stopCommand);
invoker.play();
invoker.replay();
invoker.pause();
invoker.stop();
}
}
5. 宏命令模式
所謂宏命令簡單點(diǎn)說就是包含多個(gè)命令的命令文兑，是一個(gè)命令的組合盒刚。
設(shè)想茱麗的錄音機(jī)有一個(gè)記錄功能，可以把一個(gè)一個(gè)的命令記錄下來绿贞，再在任何需要的時(shí)候重新把這些記錄下來的命令一次性執(zhí)行伪冰，這就是所謂的宏命令集功能。因此樟蠕，茱麗的錄音機(jī)系統(tǒng)現(xiàn)在有四個(gè)鍵贮聂，分別為播音、倒帶寨辩、停止和宏命令功能吓懈。此時(shí)系統(tǒng)的設(shè)計(jì)與前面的設(shè)計(jì)相比有所增強(qiáng)，主要體現(xiàn)在Julia類現(xiàn)在有了一個(gè)新方法靡狞，用以操作宏命令鍵耻警。
在上面例子的基礎(chǔ)上，增加和修改的部分
MacroCommand 宏命令接口
public interface MacroCommand extends Command{
void add(Command command);
void remove(Command command);
}
ConcreteMacroCommand 宏命令接口的具體實(shí)現(xiàn)
public class MacroAudioCommand implements MacroCommand{
private List<Command> commandList;
public MacroAudioCommand() {
commandList = new ArrayList<>();
}
@Override
public void add(Command command) {
commandList.add(command);
}
@Override
public void remove(Command command) {
commandList.remove(command);
}
@Override
public void execute() {
for(Command command:commandList) {
command.execute();
}
}
}
AudioPlayerInvoker的修改
public class AudioPlayerInvoker {
private MacroAudioCommand macroAudioCommand;
public AudioPlayerInvoker() {
macroAudioCommand = new MacroAudioCommand();
}
public void add(Command command) {
macroAudioCommand.add(command);
}
public void remove(Command command) {
macroAudioCommand.remove(command);
}
public void invoke() {
macroAudioCommand.execute();
}
}
具體實(shí)現(xiàn)部分的修改
public class MacroCommandPatternDemoMain {
public static void main(String[] args) {
AudioPlayer audioPlayer = new AudioPlayer();
PlayCommand playCommand = new PlayCommand(audioPlayer);
ReplayCommand replayCommand = new ReplayCommand(audioPlayer);
PauseCommand pauseCommand = new PauseCommand(audioPlayer);
StopCommand stopCommand = new StopCommand(audioPlayer);
AudioPlayerInvoker invoker = new AudioPlayerInvoker();
invoker.add(playCommand);
invoker.add(replayCommand);
invoker.add(pauseCommand);
invoker.add(stopCommand);
invoker.invoke();
}
}
6. 命令模式的優(yōu)點(diǎn)
（1）命令模式使新的命令很容易地被加入到系統(tǒng)里甸怕。
（2）允許接收請(qǐng)求的一方?jīng)Q定是否要否決請(qǐng)求甘穿。
（3）能較容易地設(shè)計(jì)一個(gè)命令隊(duì)列。
（4）可以容易地實(shí)現(xiàn)對(duì)請(qǐng)求的撤銷和恢復(fù)梢杭。
（5）在需要的情況下温兼，可以較容易地將命令記入日志。
