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 {
    @Override
    public 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 {
    @Override
    public 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;
    }

    @Override
    public 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;
    }

    @Override
    public 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