Head First 设计模式 Design Pattern 5-6
Section 5 单件模式 Singleton
创建独一无二的, 只能有一个实例的对象. 延迟实例化 Lazy Instantiaze
单件模式 确保一个类只有一个实例, 并提供一个全局访问点
处理多线程
>Java 利用同步, 缺点是降低性能;
>Java 利用急切 Eagerly方式创建单件, JVM保证在任何线程访问instance之前先创建实例
1
public static synchronized Singleton getInstance()1234567public class Singleton{ private static Singleton instance = new Singleton(); private Singleton(); public static Singleton getInstance(){ return instance; }}>双重检查加锁 double checked locking, Java - volatile, synchronized
>Java 单件模式要避免多个类加载器
12345678910111213class Singleton{public: static Singleton* instance(); void Print();private: Singleton() {}; void lockForSync() {}; void unLockForSync() {}; private: static Singleton* mpInstance;};123456789101112131415161718Singleton* Singleton::mpInstance = NULL; //static member must be initialized Singleton* Singleton::instance(){ if ( !mpInstance ) { lockForSync(); if ( !mpInstance ) mpInstance = new Singleton; unLockForSync(); } return mpInstance;} void Singleton::Print(){ cout << "Print the instance" << endl;}1Singleton::instance()->Print();Summary
>单件模式 确保一个类只有一个实例, 并提供全局访问点
>要点 确保程序中一个类只有一个实例; 提供访问这个实例的全局点; 私有构造器, 静态方法, 静态变量; 多线程问题; 双重检查加锁;
Java避免多个JVM加载器产生多个实例; Java单件注册表;
GlobalAccessPoint, DoubleChecked, Lazy, Private, ClassLoaders, Statically, GarbageCollector, Constructor, MultiThreading, Instance
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Section 6 命令模式 Command
封装方法调用 Method Invocation
命令模式 将请求封装成对象, 以便使用不同的请求, 队列或者日志来参数化其他对象. 支持可撤销的操作.
>Client创建一个ConcereteCommand, 设置Receiver; Invoke持有一个Command对象, 可调用execute方法执行请求; Command为所有命令声明了接口, 调用
命令对象的execute()方法, 让接收者进行相应的操作, 接口也具备undo()方法; 任何类可以作为接收者; ConcreteCommand定义了动作和接收者之间的绑定关系,
调用者通过execute()发出请求, 由ConcreteCommand调用接收者的一个或多个动作.
Party模式 MacrcoCommand, MetaCommandPattern
>队列请求 将运算打包, 在不同的线程中调用, example 线程池, 日程安排, 工作队列
>日志请求 store() load(), JAVA 对象序列化 Serialization; 在执行命令时将历史记录储存在磁盘上, 在重新启动时将命令对象重新加载, 依次调用 (持久化 Persistence).
从检查点 checkpoint 开始应用这些操作. 事务处理 Transaction, 一整批操作需必须全部完成, 否则退回原点, 没有做任何操作.
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990//Commandclass ICommand{public: enum commandType { TLight = 1, TDoor = 2, } cmdType; virtual void execute() = 0; virtual void undo() = 0;};typedef map <int, ICommand*> CmdMap;typedef vector <ICommand*> CmdVec; class NoCommand : public ICommand{public: NoCommand() {}; virtual void execute() { cout << "NoCommand" << endl ;}; virtual void undo() {};}; class Light{public: void on() { cout << "Light On" << endl; } void off() { cout << "Light Off" << endl; }}; class LightOnCommand : public ICommand{public: LightOnCommand(Light* light); virtual void execute(); virtual void undo();private: Light* mpLight;}; class Door{public: void open() { cout << "Door Open" << endl; } void close() { cout << "Door Close" << endl; }}; class DoorOpenCommand : public ICommand{public: DoorOpenCommand(Door* door); virtual void execute(); virtual void undo();private: Door* mpDoor;}; class SimpleRemoteControl{public: SimpleRemoteControl() {}; void setCommand(ICommand* cmd); void buttonPressed();private: ICommand* mpCmdSlot;}; class RemoteControl{public: RemoteControl(); void setOnCommand(int id, ICommand* onCmd); void ButtonOnPressed(int id); void ButtonUndoPressed();private: CmdMap mOnCommands; ICommand* undoCommand;}; class MacroCommand : public ICommand{public: MacroCommand() {}; void addCommand(ICommand* cmd); virtual void execute(); virtual void undo();private: CmdVec mCommands; CmdVec mUndoCmds;};123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293//CommandLightOnCommand::LightOnCommand(Light* light){ mpLight = light;} void LightOnCommand::execute(){ mpLight->on();} void LightOnCommand::undo(){ mpLight->off();} void SimpleRemoteControl::setCommand(ICommand* cmd){ mpCmdSlot = cmd;} void SimpleRemoteControl::buttonPressed(){ mpCmdSlot->execute();} DoorOpenCommand::DoorOpenCommand(Door* door){ mpDoor = door;} void DoorOpenCommand::execute(){ mpDoor->open();} void DoorOpenCommand::undo(){ mpDoor->close();} RemoteControl::RemoteControl(){ undoCommand = new NoCommand();} void RemoteControl::setOnCommand(int id, ICommand* onCmd){ //mOnCommands.insert(pair<int, ICommand*>(id, onCmd)); mOnCommands[id] = onCmd;} void RemoteControl::ButtonOnPressed(int id){ CmdMap::iterator iter = mOnCommands.find(id); if (iter != mOnCommands.end()) { ICommand* pCmd = iter->second; pCmd->execute(); undoCommand = pCmd; } else cout << "Command NOT Found" << endl;} void RemoteControl::ButtonUndoPressed(){ undoCommand->undo();} void MacroCommand::addCommand(ICommand* cmd){ mCommands.push_back(cmd);} void MacroCommand::execute(){ CmdVec::iterator iter = mCommands.begin(); for (; iter != mCommands.end(); iter++) { (*iter)->execute(); mUndoCmds.push_back(*iter); }} void MacroCommand::undo(){ while (!mUndoCmds.empty()) { mUndoCmds.back()->undo(); mUndoCmds.pop_back(); }}123456789101112131415161718192021//Command SimpleRemoteControl* sRemoteControl = new SimpleRemoteControl(); Light* light = new Light(); LightOnCommand* lightOnCmd = new LightOnCommand(light); sRemoteControl->setCommand(lightOnCmd); sRemoteControl->buttonPressed(); Door* door = new Door(); DoorOpenCommand* doorOpenCmd = new DoorOpenCommand(door); sRemoteControl->setCommand(doorOpenCmd); sRemoteControl->buttonPressed(); RemoteControl* remoteControl = new RemoteControl(); remoteControl->setOnCommand(ICommand::TLight, lightOnCmd); remoteControl->ButtonOnPressed(ICommand::TLight); remoteControl->ButtonOnPressed(ICommand::TDoor); remoteControl->ButtonUndoPressed(); MacroCommand* partyoCmd = new MacroCommand(); partyoCmd->addCommand(doorOpenCmd); partyoCmd->addCommand(lightOnCmd); partyoCmd->execute(); partyoCmd->undo();Summary
>命令模式 将请求封装成对象, 可以使用不同的请求, 队列, 或者日志请求来参数化其他对象, 支持撤消操作.
>要点 将发出请求的对象和执行请求的对象解耦; 被解耦的两者之间通过命令对象进行沟通, 命令对象封装了接收者的一个或一组动作; 调用者通过调用命令对象的execute()发出请求,
调用接收者的动作; 调用者接受命令当作参数, 可以运行时动态进行; 命令可撤消, 实现undo()来回到执行前状态; 宏命令是命令的简单延伸, 允许调用多个命令, 支持撤消; 聪明命令对象,
直接实现请求, 代替接收者; 日志和事务系统;
Client, Invoker, Binds, Decoupled, VendorClasses, Execute, Receiver, Command, Undo, Request
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