【设计模式-行为型】状态模式

一、什么是状态模式

什么是状态模式呢，这里我举一个例子来说明，在自动挡汽车中，挡位的切换是根据驾驶条件（如车速、油门踏板位置、刹车状态等）自动完成的。这种自动切换挡位的过程可以很好地用状态模式来描述。状态模式（State Pattern） 是一种行为型设计模式，它允许一个对象在其内部状态发生变化时（加速或者减速）改变其行为（换挡）。状态模式的核心思想是将对象的行为封装在不同的状态类中，使得对象的行为随着状态的改变而改变。这种模式特别适用于对象的行为依赖于其内部状态的场景。

二、为什么使用状态模式

1. 清晰的职责划分：代码结构清晰，每个状态类的职责单一，易于理解和维护。例如，在自动挡汽车中，每个挡位的行为都封装在独立的状态类中，职责划分明确。

2. 动态改变行为 ：减少了复杂的条件分支（如 if-else 或 switch-case），使代码更加简洁和可读。例如，在自动挡汽车中，换挡逻辑由状态机自动处理，无需在主逻辑中写复杂的条件判断。

3. 扩展性：符合开闭原则（对扩展开放，对修改封闭）。例如，在自动挡汽车中，新增一个挡位（如6挡）时，只需添加一个新的状态类，无需修改现有的换挡逻辑。

4. 低耦合 ：状态类和上下文类之间的依赖关系减少，提高了代码的灵活性和可维护性。例如，在自动挡汽车中，Car 类与各个挡位状态类通过 GearState 接口交互，降低了耦合度。

5. 适应复杂状态逻辑：能够清晰地表达状态转换逻辑，避免代码混乱。例如，在自动挡汽车中，换挡逻辑可能涉及多种条件（如车速、油门位置等），状态机模式能够清晰地表达这些逻辑。

三、状态模式的示例

3.1 状态模式示例及角色

1. **State：**定义状态接口（行为）

定义了所有挡位状态的通用接口，包含所有挡位共有的方法，例如加速、减速等

public interface GearState {
    void accelerate(Car car); // 加速时的行为
    void decelerate(Car car); // 减速时的行为
}

2. ConcreteState：实现具体状态类(档位)

每个挡位（如1挡、2挡、3挡、4挡、5挡等）都实现状态接口，并定义在该挡位下的行为。

public class FirstGear implements GearState {
    @Override
    public void accelerate(Car car) {
        System.out.println("当前1档：加速...");
        car.setState(new SecondGear()); // 切换到2挡
    }

    @Override
    public void decelerate(Car car) {
        System.out.println("当前1档：减速...");
        car.setState(new NeutralGear()); // 切换到空挡
    }
}

public class SecondGear implements GearState {
    @Override
    public void accelerate(Car car) {
        System.out.println("当前2档：加速...");
        car.setState(new ThirdGear()); // 切换到3挡
    }

    @Override
    public void decelerate(Car car) {
        System.out.println("当前2档：减速...");
        car.setState(new FirstGear()); // 切换到1挡
    }
}

public class ThirdGear implements GearState {
    @Override
    public void accelerate(Car car) {
        System.out.println("当前3档：加速...");
        car.setState(new FourthGear()); // 切换到4挡
    }

    @Override
    public void decelerate(Car car) {
        System.out.println("当前3档：减速...");
        car.setState(new SecondGear()); // 切换到2挡
    }
}

public class FourthGear implements GearState {
    @Override
    public void accelerate(Car car) {
        System.out.println("当前4档：加速...");
        car.setState(new FifthGear()); // 切换到5挡
    }

    @Override
    public void decelerate(Car car) {
        System.out.println("当前1档：减速...");
        car.setState(new ThirdGear()); // 切换到3挡
    }
}

public class FifthGear implements GearState {
    @Override
    public void accelerate(Car car) {
        System.out.println("当前5档：加速...");
        // 保持在5挡
    }

    @Override
    public void decelerate(Car car) {
        System.out.println("当前5档：减速...");
        car.setState(new FourthGear()); // 切换到4挡
    }
}

public class NeutralGear implements GearState {
    @Override
    public void accelerate(Car car) {
        System.out.println("当前空档：加速...");
        car.setState(new FirstGear()); // 切换到1挡
    }

    @Override
    public void decelerate(Car car) {
        System.out.println("当前空档：减速...");
        // 保持在空挡
    }
}

3、Context（上下文）：（当前车辆，记录当前状态）

上下文可以被视为汽车的驾驶系统，它包含当前挡位的状态，并根据驾驶员的操作切换挡位

public class Car {
    private GearState currentState;

    public Car() {
        this.currentState = new NeutralGear(); // 初始状态为空挡
    }

    public void setState(GearState state) {
        this.currentState = state;
    }

    public void accelerate() {
        currentState.accelerate(this);
    }

    public void decelerate() {
        currentState.decelerate(this);
    }
}

4、场景（开车）

public class Main {
    public static void main(String[] args) {
        Car car = new Car();

        car.accelerate(); // 初始状态为空挡，加速后切换到1挡
        car.accelerate(); // 当前状态为1挡，加速后切换到2挡
        car.accelerate(); // 当前状态为2挡，加速后切换到3挡
        car.decelerate(); // 当前状态为3挡，减速后切换到2挡
        car.decelerate(); // 当前状态为2挡，减速后切换到1挡
        car.decelerate(); // 当前状态为1挡，减速后切换到空挡
    }
}


//输出
当前0档：加速...
当前1档：加速...
当前2档：加速...
当前3档：加速...
当前3档：减速...
当前2档：减速...
当前1档：减速...
当前0档：减速...

3.2 在JAVA开发中常用的Spring状态机

在Spring框架中，可以使用Spring State Machine来实现状态机模式。Spring State Machine是一个功能强大的状态机框架，支持定义状态、事件和状态转换，并且可以与Spring生态系统无缝集成。在实际开发中，可能使用Spring框架比较多，一般都是直接使用现成的Spring State Machine 来实现状态模式。

1、集成依赖包

<dependency>
	<groupId>org.springframework.statemachine</groupId>
	<artifactId>spring-statemachine-core</artifactId>
	<version>2.1.3.RELEASE</version>
</dependency>

2、定义状态和事件

//车辆档位枚举类
public enum CarState {
    NEUTRAL,     //空挡
    FIRST_GEAR,  //一档
    SECOND_GEAR, //二挡
    THIRD_GEAR,  //三挡
    FOURTH_GEAR, //四挡
    FIFTH_GEAR;   //五档
}


//车辆行为枚举类
public enum CarEvent {
    ACCELERATE,     //加速行为
    DECELERATE;     //减速行为
}

3、配置状态机

@Configuration
@EnableStateMachine(name="carStateMachine")
public class CarStateMachineConfig extends StateMachineConfigurerAdapter<CarState, CarEvent> {
    public void configure(StateMachineStateConfigurer<CarState, CarEvent> states) throws Exception {
        states.withStates().initial(CarState.NEUTRAL).states(EnumSet.allOf(CarState.class));
    }

    @Override
    public void configure(StateMachineTransitionConfigurer<CarState, CarEvent> states) throws Exception {
        states.withExternal().source(CarState.NEUTRAL).target(CarState.FIRST_GEAR).event(CarEvent.ACCELERATE)
                .and().withExternal().source(CarState.FIRST_GEAR).target(CarState.SECOND_GEAR).event(CarEvent.ACCELERATE)
                .and().withExternal().source(CarState.SECOND_GEAR).target(CarState.THIRD_GEAR).event(CarEvent.ACCELERATE)
                .and().withExternal().source(CarState.THIRD_GEAR).target(CarState.FOURTH_GEAR).event(CarEvent.ACCELERATE)
                .and().withExternal().source(CarState.FOURTH_GEAR).target(CarState.FIFTH_GEAR).event(CarEvent.ACCELERATE)
                .and().withExternal().source(CarState.FIFTH_GEAR).event(CarEvent.ACCELERATE) // No change
                .and().withExternal().source(CarState.FIFTH_GEAR).target(CarState.FOURTH_GEAR).event(CarEvent.DECELERATE)
                .and().withExternal().source(CarState.FOURTH_GEAR).target(CarState.THIRD_GEAR).event(CarEvent.DECELERATE)
                .and().withExternal().source(CarState.THIRD_GEAR).target(CarState.SECOND_GEAR).event(CarEvent.DECELERATE)
                .and().withExternal().source(CarState.SECOND_GEAR).target(CarState.FIRST_GEAR).event(CarEvent.DECELERATE)
                .and().withExternal().source(CarState.FIRST_GEAR).target(CarState.NEUTRAL).event(CarEvent.DECELERATE);
    }

    // 配置状态机持久化
    @Bean
    public DefaultStateMachinePersister machinePersister() {
        return new DefaultStateMachinePersister<>(new StateMachinePersist<Object, Object, Car>() {

            @Override
            public void write(StateMachineContext<Object, Object> stateMachineContext, Car car) throws Exception {
                //持久化操作。可以通过任何形式进行持久化。redis 、 mongodb、mysql，ecache
            }

            @Override
            public StateMachineContext<Object, Object> read(Car car) throws Exception {
                // 从持久化组件里进行读取
                return new DefaultStateMachineContext(car.getCarState(), null, null, null);
            }
        });
    }
}

4、定义状态监听器

在Spring State Machine中，可以通过@WithStateMachine注解和@OnTransition注解来定义状态监听器。这些监听器会在状态转换时被触发。

// 监听器是监听到 action 后进行状态的一个变更。
@Slf4j
@Component("carStateListener")
@WithStateMachine(name="carStateMachine")
public class CarStateListener {
    @OnTransition(source = "NEUTRAL", target = "FIRST_GEAR")
    public boolean onAccelerateFromNeutral(Message<CarEvent> message) {
        log.info("Accelerating from NEUTRAL to FIRST_GEAR");
        Car car = (Car) message.getHeaders().get("car");
        car.setCarState(CarState.FIRST_GEAR);
        return true;
    }

    @OnTransition(source = "FIRST_GEAR", target = "SECOND_GEAR")
    public boolean onAccelerateFromFirstGear(Message<CarEvent> message) {
        log.info("Accelerating from FIRST_GEAR to SECOND_GEAR");
        Car car = (Car) message.getHeaders().get("car");
        car.setCarState(CarState.SECOND_GEAR);
        return true;
    }

    @OnTransition(source = "SECOND_GEAR", target = "THIRD_GEAR")
    public boolean onAccelerateFromSecondGear(Message<CarEvent> message) {
        log.info("Accelerating from SECOND_GEAR to THIRD_GEAR");
        Car car = (Car) message.getHeaders().get("car");
        car.setCarState(CarState.THIRD_GEAR);
        return true;
    }

    @OnTransition(source = "THIRD_GEAR", target = "FOURTH_GEAR")
    public boolean onAccelerateFromThirdGear(Message<CarEvent> message) {
        log.info("Accelerating from THIRD_GEAR to FOURTH_GEAR");
        Car car = (Car) message.getHeaders().get("car");
        car.setCarState(CarState.FOURTH_GEAR);
        return true;
    }

    @OnTransition(source = "FOURTH_GEAR", target = "FIFTH_GEAR")
    public boolean onAccelerateFromFourthGear(Message<CarEvent> message) {
        log.info("Accelerating from FOURTH_GEAR to FIFTH_GEAR");
        Car car = (Car) message.getHeaders().get("car");
        car.setCarState(CarState.FIFTH_GEAR);
        return true;
    }

    @OnTransition(source = "FIFTH_GEAR", target = "FOURTH_GEAR")
    public boolean onDecelerateFromFifthGear(Message<CarEvent> message) {
        log.info("Decelerating from FIFTH_GEAR to FOURTH_GEAR");
        Car car = (Car) message.getHeaders().get("car");
        car.setCarState(CarState.FOURTH_GEAR);
        return true;
    }

    @OnTransition(source = "FOURTH_GEAR", target = "THIRD_GEAR")
    public boolean onDecelerateFromFourthGear(Message<CarEvent> message) {
        log.info("Decelerating from FOURTH_GEAR to THIRD_GEAR");
        Car car = (Car) message.getHeaders().get("car");
        car.setCarState(CarState.THIRD_GEAR);
        return true;
    }

    @OnTransition(source = "THIRD_GEAR", target = "SECOND_GEAR")
    public boolean onDecelerateFromThirdGear(Message<CarEvent> message) {
        log.info("Decelerating from THIRD_GEAR to SECOND_GEAR");
        Car car = (Car) message.getHeaders().get("car");
        car.setCarState(CarState.SECOND_GEAR);
        return true;
    }

    @OnTransition(source = "SECOND_GEAR", target = "FIRST_GEAR")
    public boolean onDecelerateFromSecondGear(Message<CarEvent> message) {
        log.info("Decelerating from SECOND_GEAR to FIRST_GEAR");
        Car car = (Car) message.getHeaders().get("car");
        car.setCarState(CarState.FIRST_GEAR);
        return true;
    }

    @OnTransition(source = "FIRST_GEAR", target = "NEUTRAL")
    public boolean onDecelerateFromFirstGear(Message<CarEvent> message) {
        log.info("Decelerating from FIRST_GEAR to NEUTRAL");
        Car car = (Car) message.getHeaders().get("car");
        car.setCarState(CarState.NEUTRAL);
        return true;
    }

}

4、使用状态机

@Service
public class CarService {

    @Resource
    private StateMachine<CarState, CarEvent> carStateMachine;

    @Resource
    private StateMachinePersister<CarState, CarEvent, Car> carMachinePersister;

    //模拟一个存储
    private Car car = new Car();


    //加速方法
    public Car acc() {
        if (car.getCarState() == null){
            car.setCarState(CarState.NEUTRAL);
        }
        // 书写逻辑
        Message message = MessageBuilder
                    .withPayload(CarEvent.ACCELERATE)
                .setHeader("car", car).build();
            if(changeStateAction(message,car)) {
                return car;
            }
        return car;
    }

    //减速方法
    public Car dece() {
        if (car.getCarState() == null){
            car.setCarState(CarState.NEUTRAL);
        }
        // 书写逻辑
        Message message = MessageBuilder
                .withPayload(CarEvent.DECELERATE)
                .setHeader("car", car).build();
        if(changeStateAction(message,car)) {
            return car;
        }
        return car;
    }

    private boolean changeStateAction(Message<CarEvent> message, Car car) {
        try {
            carStateMachine.start();
            //尝试恢复状态机状态
            carMachinePersister.restore(carStateMachine, car); // 待议
            boolean res = carStateMachine.sendEvent(message);
            //持久化状态机状态
            carMachinePersister.persist(carStateMachine, car); // 持久
            return res;
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            carStateMachine.stop();
        }
        return false;
    }
}

4、场景（开车）

@SpringBootApplication
public class CarApplication {

    public static void main(String[] args) {
        ConfigurableApplicationContext context = SpringApplication.run(DesignApplication.class, args);
		CarService carService = context.getBean(CarService.class);

		Car car;
		car = carService.acc(); // 初始状态为空挡，加速后切换到1挡
		System.out.println("After accelerate: " + car.getCarState());
		car = carService.acc(); // 当前状态为1挡，加速后切换到2挡
		System.out.println("After accelerate: " + car.getCarState());
		car = carService.dece(); // 当前状态为2挡，减速后切换到1挡
		System.out.println("After decelerate: " + car.getCarState());
		car = carService.dece(); // 当前状态为1挡，减速后切换到空挡
		System.out.println("After decelerate: " + car.getCarState());
    }
}


//输出
After accelerate: FIRST_GEAR
After accelerate: SECOND_GEAR
After decelerate: FIRST_GEAR
After decelerate: NEUTRAL