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策略模式定义

比如对象的某个行为，在不同场景有不同实现方式，可以将这些行为的具体实现定义为一组策略，每个实现类实现种策略，在不同场景使用不同的实现，并且可以自由切换策略。

策略模式结构

策略模式需要一个策略接口，不同的策略实现不同的实现类，在具体业务环境中仅持有该策略接口，根据不同的场景使用不同的实现类即可。

面向接口编程，而不是面向实现。

优点

1、干掉繁琐的 if、switch 判断逻辑；

2、代码优雅、可复用、可读性好；

3、符合开闭原则(对修改关闭, 对扩展开放)，扩展性好、便于维护；

缺点

1、策略如果很多的话，会造成策略类膨胀；

2、使用者必须清楚所有的策略类及其用途；

策略模式代码示例

• 基础登录接口

//基础登录接口
public interface BaseLoginService {void login(BaseLoginContext context);
}

• 策略上下文实现

@Data
public class BaseLoginContext {private String userName;private String password;private BaseLoginService baseLoginService;public BaseLoginContext(String userName, String password, BaseLoginService baseLoginService) {this.userName = userName;this.password = password;this.baseLoginService = baseLoginService;}public void login(){baseLoginService.login(this);}
}

• 定义具体的策略实现类

1. 实现类1

public class SimpleLoginServiceImpl implements BaseLoginService {@Overridepublic void login(BaseLoginContext context) {System.out.println("简单登录，当前登录的人：" + context.getUserName() + "，密码：" + context.getPassword());}
}

2. 实现类2

public class HardLoginServiceImpl implements BaseLoginService {@Overridepublic void login(BaseLoginContext context) {System.out.println("复杂登录，当前登录的人：" + context.getUserName() + "，密码：" + context.getPassword());}
}

• 调用

public static void main(String[] args) {BaseLoginService simple = new SimpleLoginServiceImpl();BaseLoginContext simpleContext = new BaseLoginContext("simple","simple",simple);simpleContext.login();BaseLoginService hard = new HardLoginServiceImpl();BaseLoginContext hardContext = new BaseLoginContext("hard","hard",hard);hardContext.login();}

• 结果
  

扩展策略

方式1

在策略的算法实现上添加自己需要的数据的方式

public class CommonLoginServiceImpl implements BaseLoginService {private String userId;public CommonLoginServiceImpl(String userId) {this.userId = userId;}public String getUserId() {return userId;}@Overridepublic void login(BaseLoginContext context) {System.out.println("复杂登录，当前登录的人：" + context.getUserName() + "，账号：" + getUserId() + "，密码：" + context.getPassword());}
}

• 调用

public static void main(String[] args) {BaseLoginService simple = new SimpleLoginServiceImpl();BaseLoginContext simpleContext = new BaseLoginContext("simple","simple",simple);simpleContext.login();BaseLoginService hard = new HardLoginServiceImpl();BaseLoginContext hardContext = new BaseLoginContext("hard","hard",hard);hardContext.login();BaseLoginService common = new CommonLoginServiceImpl("001");BaseLoginContext commonContext = new BaseLoginContext("common","common",common);commonContext.login();}

• 结果
  

方式2

扩展上下文的方式

public class NewLoginContext extends BaseLoginContext{private String userId;public String getUserId() {return userId;}public void setUserId(String userId) {this.userId = userId;}public NewLoginContext(String userName, String password, String userId, BaseLoginService baseLoginService) {super(userName, password, baseLoginService);this.userId = userId;}
}

• 实现类

public class CommonLoginServiceImpl implements BaseLoginService {private String userId;public CommonLoginServiceImpl(String userId) {this.userId = userId;}public String getUserId() {return userId;}@Overridepublic void login(BaseLoginContext context) {NewLoginContext newLoginContext = (NewLoginContext)context;
//        System.out.println("common1登录，当前登录的人：" + context.getUserName() + "，账号：" + getUserId() + "，密码：" + context.getPassword());System.out.println("common2登录，当前登录的人：" + context.getUserName() + "，账号：" + newLoginContext.getUserId() + "，密码：" + context.getPassword());}
}

• 调用

public static void main(String[] args) {BaseLoginService simple = new SimpleLoginServiceImpl();BaseLoginContext simpleContext = new BaseLoginContext("simple","simple",simple);simpleContext.login();BaseLoginService hard = new HardLoginServiceImpl();BaseLoginContext hardContext = new BaseLoginContext("hard","hard",hard);hardContext.login();//        BaseLoginService common1 = new CommonLoginServiceImpl("001");
//        BaseLoginContext commonContext1 = new BaseLoginContext("common","common",common1);
//        commonContext1.login();BaseLoginService common2 = new CommonLoginServiceImpl("001");NewLoginContext commonContext2 = new NewLoginContext("common","common","002",common2);commonContext2.login();}

• 结果