在日常工作过程中，我们经常会遇到状态的变化场景，例如订单状态发生变化，商品状态的变化。这些状态的变化，我们称为有限状态机，缩写为FSM( F State Machine).。之所以称其为有限，是因为这些场景中的状态往往是可以枚举出来的有限个的，所以称其为有限状态机。下面我们来看一个具体的场景例子。

简单场景：

地铁进站闸口的状态有两个：已经关闭、已经开启两个状态。刷卡后闸口从已关闭变为已开启，人通过后闸口状态从已开启变为已关闭。

01 遇到这类问题，在编码时我们应该如何处理呢？

基于Switch
基于状态集合
基于State模式
基于枚举的实现

下面我们针对每一种实现方式进行分析。场景分解后会有一下2种状态4种情况出现：

Index	State	Event	NextState	Action
1	闸机口 LOCKED	投币	闸机口 UN_LOCKED	闸机口打开闸门
2	闸机口 LOCKED	通过	闸机口 LOCKED	闸机口警告
3	闸机口 UN_LOCKED	投币	闸机口 UN _LOCKED	闸机口退币
4	闸机口 UN_LOCKED	通过	闸机口 LOCKED	闸机口关闭闸门

针对以上4种请求，共拆分了5个Test Case

T01

Given：一个Locked的进站闸口

When: 投入硬币

Then：打开闸口

T02

Given：一个Locked的进站闸口

When: 通过闸口

Then：警告提示

T03

Given：一个Unocked的进站闸口

When: 通过闸口

Then：闸口关闭

T04

Given：一个Unlocked的进站闸口

When: 投入硬币

Then：退还硬币

T05

Given：一个闸机口

When: 非法操作

Then：操作失败

代码地址:https://gitlab.com/tengbai/fsm-java

项目中共有4中状态机的实现方式。

基于Switch语句实现的有限状态机，代码在master分支

基于State模式实现的有限状态机。代码在state-pattern分支

基于状态集合实现的有限状态机。代码在collection-state分支

基于枚举实现的状态机。代码在enum-state分支

01.01 使用Switch来实现有限状态机

这种方式只需要懂得Java语法及可以实现出来。先看代码，然后我们在讨论这种实现方式是否好。

EntranceMachineTest.java

package com.page.java.fsm;

import com.page.java.fsm.exception.InvalidActionException;
import org.junit.jupiter.api.Test;

import static org.assertj.core.api.Assertions.assertThatThrownBy;
import static org.assertj.core.api.BDDAssertions.then;

class EntranceMachineTest {

    @Test
    void should_be_unlocked_when_insert_coin_given_a_entrance_machine_with_locked_state() {
        EntranceMachine entranceMachine = new EntranceMachine(EntranceMachineState.LOCKED);

        String result = entranceMachine.execute(Action.INSERT_COIN);

        then(result).isEqualTo("opened");
        then(entranceMachine.getState()).isEqualTo(EntranceMachineState.UNLOCKED);
    }

    @Test
    void should_be_locked_and_alarm_when_pass_given_a_entrance_machine_with_locked_state() {
        EntranceMachine entranceMachine = new EntranceMachine(EntranceMachineState.LOCKED);

        String result = entranceMachine.execute(Action.PASS);

        then(result).isEqualTo("alarm");
        then(entranceMachine.getState()).isEqualTo(EntranceMachineState.LOCKED);
    }

    @Test
    void should_fail_when_execute_invalid_action_given_a_entrance_with_locked_state() {
        EntranceMachine entranceMachine = new EntranceMachine(EntranceMachineState.LOCKED);

        assertThatThrownBy(() -> entranceMachine.execute(null))
                .isInstanceOf(InvalidActionException.class);
    }

    @Test
    void should_locked_when_pass_given_a_entrance_machine_with_unlocked_state() {
        EntranceMachine entranceMachine = new EntranceMachine(EntranceMachineState.UNLOCKED);

        String result = entranceMachine.execute(Action.PASS);

        then(result).isEqualTo("closed");
        then(entranceMachine.getState()).isEqualTo(EntranceMachineState.LOCKED);
    }

    @Test
    void should_refund_and_unlocked_when_insert_coin_given_a_entrance_machine_with_unlocked_state() {
        EntranceMachine entranceMachine = new EntranceMachine(EntranceMachineState.UNLOCKED);

        String result = entranceMachine.execute(Action.INSERT_COIN);

        then(result).isEqualTo("refund");
        then(entranceMachine.getState()).isEqualTo(EntranceMachineState.UNLOCKED);
    }
}

Action.java

public enum Action {
    INSERT_COIN,
    PASS
}

EntranceMachineState.java

public enum EntranceMachineState {
    UNLOCKED,
    LOCKED
}

InvalidActionException.java

package com.page.java.fsm.exception;

public class InvalidActionException extends RuntimeException {
}

EntranceMachine.java

package com.page.java.fsm;

import com.page.java.fsm.exception.InvalidActionException;
import lombok.Data;

import java.util.Objects;

@Data
public class EntranceMachine {

    private EntranceMachineState state;

    public EntranceMachine(EntranceMachineState state) {
        this.state = state;
    }

    public String execute(Action action) {
        if (Objects.isNull(action)) {
            throw new InvalidActionException();
        }

        if (EntranceMachineState.LOCKED.equals(state)) {
            switch (action) {
                case INSERT_COIN:
                    setState(EntranceMachineState.UNLOCKED);
                    return open();
                case PASS:
                    return alarm();
            }
        }

        if (EntranceMachineState.UNLOCKED.equals(state)) {
            switch (action) {
                case PASS:
                    setState(EntranceMachineState.LOCKED);
                    return close();
                case INSERT_COIN:
                    return refund();
            }
        }
        return null;
    }

    private String refund() {
        return "refund";
    }

    private String close() {
        return "closed";
    }

    private String alarm() {
        return "alarm";
    }

    private String open() {
        return "opened";
    }
}

if(), swich语句都是switch语句，但是Switch是一种Code Bad Smell，因为它本质上一种重复。当代码中有多处相同的switch时，会让系统变得晦涩难懂，脆弱，不易修改。

上面的代码虽然出现了多层嵌套但是还算是结构简单，不过想通过并不能很清楚闸机口的逻辑还是化点时间。如果闸机口的状态等多一些，那就阅读、理解起来也就更加困难。

所以在日常工作，我遵循**“事不过三，三则重构”**的原则：

事不过三：

当只有一两个状态（或者重复）时，那么先用最简单的实现实现。

一旦出现三种以及以上的状态（或者重复），立即重构。

01.02 State模式

EntranceMachineTest.java

package com.page.java.fsm;

import com.page.java.fsm.exception.InvalidActionException;
import org.junit.jupiter.api.Test;

import static org.assertj.core.api.Assertions.assertThatThrownBy;
import static org.assertj.core.api.BDDAssertions.then;

class EntranceMachineTest {

    @Test
    void should_be_unlocked_when_insert_coin_given_a_entrance_machine_with_locked_state() {
        EntranceMachine entranceMachine = new EntranceMachine(new LockedEntranceMachineState());

        String result = entranceMachine.execute(Action.INSERT_COIN);

        then(result).isEqualTo("opened");
        then(entranceMachine.isUnlocked()).isTrue();
    }

    @Test
    void should_be_locked_and_alarm_when_pass_given_a_entrance_machine_with_locked_state() {
        EntranceMachine entranceMachine = new EntranceMachine(new LockedEntranceMachineState());

        String result = entranceMachine.execute(Action.PASS);

        then(result).isEqualTo("alarm");
        then(entranceMachine.isLocked()).isTrue();
    }

    @Test
    void should_fail_when_execute_invalid_action_given_a_entrance_with_locked_state() {
        EntranceMachine entranceMachine = new EntranceMachine(new LockedEntranceMachineState());

        assertThatThrownBy(() -> entranceMachine.execute(null))
                .isInstanceOf(InvalidActionException.class);
    }

    @Test
    void should_locked_when_pass_given_a_entrance_machine_with_unlocked_state() {
        EntranceMachine entranceMachine = new EntranceMachine(new UnlockedEntranceMachineState());

        String result = entranceMachine.execute(Action.PASS);

        then(result).isEqualTo("closed");
        then(entranceMachine.isLocked()).isTrue();
    }

    @Test
    void should_refund_and_unlocked_when_insert_coin_given_a_entrance_machine_with_unlocked_state() {
        EntranceMachine entranceMachine = new EntranceMachine(new UnlockedEntranceMachineState());

        String result = entranceMachine.execute(Action.INSERT_COIN);

        then(result).isEqualTo("refund");
        then(entranceMachine.isUnlocked()).isTrue();
    }
}

EntranceMachineState.java

package com.page.java.fsm;

public interface EntranceMachineState {

    String insertCoin(EntranceMachine entranceMachine);

    String pass(EntranceMachine entranceMachine);
}

LockedEntranceMachineState.java

package com.page.java.fsm;

public class LockedEntranceMachineState implements EntranceMachineState {

    @Override
    public String insertCoin(EntranceMachine entranceMachine) {
        return entranceMachine.open();
    }

    @Override
    public String pass(EntranceMachine entranceMachine) {
        return entranceMachine.alarm();
    }
}

UnlockedEntranceMachineState.java

package com.page.java.fsm;

public class UnlockedEntranceMachineState implements EntranceMachineState {

    @Override
    public String insertCoin(EntranceMachine entranceMachine) {
        return entranceMachine.refund();
    }

    @Override
    public String pass(EntranceMachine entranceMachine) {
        return entranceMachine.close();
    }
}

Action.java

package com.page.java.fsm;

public enum Action {
    PASS,
    INSERT_COIN
}

EntranceMachine.java

package com.page.java.fsm;

import com.page.java.fsm.exception.InvalidActionException;

import java.util.Objects;

public class EntranceMachine {

    private EntranceMachineState locked = new LockedEntranceMachineState();

    private EntranceMachineState unlocked = new UnlockedEntranceMachineState();

    private EntranceMachineState state;

    public EntranceMachine(EntranceMachineState state) {
        this.state = state;
    }

    public String execute(Action action) {
        if (Objects.isNull(action)) {
            throw new InvalidActionException();
        }

        if (Action.PASS.equals(action)) {
            return state.pass(this);
        }

        return state.insertCoin(this);
    }

    public boolean isUnlocked() {
        return state == unlocked;
    }

    public boolean isLocked() {
        return state == locked;
    }

    public String open() {
        setState(unlocked);
        return "opened";
    }

    public String alarm() {
        setState(locked);
        return "alarm";
    }

    public String refund() {
        setState(unlocked);
        return "refund";
    }

    public String close() {
        setState(locked);
        return "closed";
    }

    private void setState(EntranceMachineState state) {
        this.state = state;
    }
}

State模式和Proxy模式类似，但是在State模式中EntranceMachineState持有EntranceMachine实例的引用。

我们发现EntranceMachine的execute()方法的逻辑变的简单，但是代码复杂度升高了。因为每个state实例都提供了两个动作实现insertCoin()和pass()。这个地方本人认为并不够表意，因为作出的动作被添加到两个状态上，虽然能够实现业务业务，但是并不利于理解清楚业务意思。

State模式，虽然能够将逻辑进行拆分，但是那些状态的顺序，以及有几种状态，都不是很直观的观察到。

不过在实际业务中，State模式也是一种很好的实现方式，毕竟他避免了switch的堆积问题。

01.03 使用状态集合

状态集合是将一组描述状态变化的事务元素组成的集合。

集合中的每一个元素包含4个属性：当前的状态，事件，下一个状态，触发的动作。

使用时遍历集合根据动作找到特定的元素，并更具元素上的属性和事件来完成业务逻辑。

具体代码如下：

EntranceMachineTest.java

package com.page.java.fsm;

import com.page.java.fsm.exception.InvalidActionException;
import org.junit.jupiter.api.Test;

import static org.assertj.core.api.Assertions.assertThatThrownBy;
import static org.assertj.core.api.BDDAssertions.then;

class EntranceMachineTest {

    @Test
    void should_be_unlocked_when_insert_coin_given_a_entrance_machine_with_locked_state() {
        EntranceMachine entranceMachine = new EntranceMachine(EntranceMachineState.LOCKED);

        String result = entranceMachine.execute(Action.INSERT_COIN);

        then(result).isEqualTo("opened");
        then(entranceMachine.getState()).isEqualTo(EntranceMachineState.UNLOCKED);
    }

    @Test
    void should_be_alarm_when_pass_given_a_entrance_machine_with_locked_state() {
        EntranceMachine entranceMachine = new EntranceMachine(EntranceMachineState.LOCKED);

        String result = entranceMachine.execute(Action.PASS);

        then(result).isEqualTo("alarm");
        then(entranceMachine.getState()).isEqualTo(EntranceMachineState.LOCKED);
    }

    @Test
    void should_fail_when_execute_invalid_action_given_a_entrance_machine() {
        EntranceMachine entranceMachine = new EntranceMachine(EntranceMachineState.LOCKED);

        assertThatThrownBy(() -> entranceMachine.execute(null))
                .isInstanceOf(InvalidActionException.class);

    }

    @Test
    void should_closed_when_pass_given_a_entrance_machine_with_unlocked() {
        EntranceMachine entranceMachine = new EntranceMachine(EntranceMachineState.UNLOCKED);

        String result = entranceMachine.execute(Action.PASS);

        then(result).isEqualTo("closed");
        then(entranceMachine.getState()).isEqualTo(EntranceMachineState.LOCKED);
    }

    @Test
    void should_refund_when_insert_coin_given_a_entrance_machine_with_unlocked() {
        EntranceMachine entranceMachine = new EntranceMachine(EntranceMachineState.UNLOCKED);

        String result = entranceMachine.execute(Action.INSERT_COIN);

        then(result).isEqualTo("refund");
        then(entranceMachine.getState()).isEqualTo(EntranceMachineState.UNLOCKED);
    }
}

Action.java

package com.page.java.fsm;

public enum Action {
    PASS,
    INSERT_COIN
}

EntranceMachineState.java

package com.page.java.fsm;

public enum EntranceMachineState {
    LOCKED,
    UNLOCKED
}

EntranceMachine.java

package com.page.java.fsm;

import com.page.java.fsm.events.AlarmEvent;
import com.page.java.fsm.events.CloseEvent;
import com.page.java.fsm.events.OpenEvent;
import com.page.java.fsm.events.RefundEvent;
import com.page.java.fsm.exception.InvalidActionException;
import lombok.Data;

import java.util.Arrays;
import java.util.List;
import java.util.Optional;

@Data
public class EntranceMachine {

    List<EntranceMachineTransaction> entranceMachineTransactionList = Arrays.asList(
            EntranceMachineTransaction.builder()
                    .currentState(EntranceMachineState.LOCKED)
                    .action(Action.INSERT_COIN)
                    .nextState(EntranceMachineState.UNLOCKED)
                    .event(new OpenEvent())
                    .build(),
            EntranceMachineTransaction.builder()
                    .currentState(EntranceMachineState.LOCKED)
                    .action(Action.PASS)
                    .nextState(EntranceMachineState.LOCKED)
                    .event(new AlarmEvent())
                    .build(),
            EntranceMachineTransaction.builder()
                    .currentState(EntranceMachineState.UNLOCKED)
                    .action(Action.PASS)
                    .nextState(EntranceMachineState.LOCKED)
                    .event(new CloseEvent())
                    .build(),
            EntranceMachineTransaction.builder()
                    .currentState(EntranceMachineState.UNLOCKED)
                    .action(Action.INSERT_COIN)
                    .nextState(EntranceMachineState.UNLOCKED)
                    .event(new RefundEvent())
                    .build()
    );

    private EntranceMachineState state;

    public EntranceMachine(EntranceMachineState state) {
        setState(state);
    }

    public String execute(Action action) {
        Optional<EntranceMachineTransaction> transactionOptional = entranceMachineTransactionList
                .stream()
                .filter(transaction ->
                        transaction.getAction().equals(action) && transaction.getCurrentState().equals(state))
                .findFirst();

        if (!transactionOptional.isPresent()) {
            throw new InvalidActionException();
        }

        EntranceMachineTransaction transaction = transactionOptional.get();
        setState(transaction.getNextState());
        return transaction.getEvent().execute();
    }
}

EntranceMachineTransaction.java

package com.page.java.fsm;

import com.page.java.fsm.events.Event;
import lombok.AllArgsConstructor;
import lombok.Builder;
import lombok.Data;
import lombok.NoArgsConstructor;

@Data
@Builder
@NoArgsConstructor
@AllArgsConstructor
public class EntranceMachineTransaction {

    private EntranceMachineState currentState;

    private Action action;

    private EntranceMachineState nextState;

    private Event event;
}

Event.java

package com.page.java.fsm.events;

public interface Event {

    String execute();
}

OpenEvent.java

package com.page.java.fsm.events;

public class OpenEvent implements Event {
    @Override
    public String execute() {
        return "opened";
    }
}

AlarmEvent.java

package com.page.java.fsm.events;

public class AlarmEvent implements Event {
    @Override
    public String execute() {
        return "alarm";
    }
}

CloseEvent.java

package com.page.java.fsm.events;

public class CloseEvent implements Event {
    @Override
    public String execute() {
        return "closed";
    }
}

RefundEvent.java

package com.page.java.fsm.events;

public class RefundEvent implements Event {
    @Override
    public String execute() {
        return "refund";
    }
}

InvalidActionException.java

package com.page.java.fsm.exception;

public class InvalidActionException extends RuntimeException {
}

相比于Switch的实现方式，状态集合的实现方式对状态规则的描述更加直观。且扩展性更强，不需求修改实现路基，只需要添加相关的状态描述即可。

我们知道日常工作中读代码和写代码比例在10:1，有些场景下甚至到了20:1。Switch需要我们每次在脑子中组织一次状态的顺序和规则，而集合能够很直观的表达出这个规则。

01.04 使用Enum的来实现状态机

EntranceMachineTest.java

package com.page.java.fsm;

import com.page.java.fsm.exception.InvalidActionException;
import org.junit.jupiter.api.Test;

import static org.assertj.core.api.Assertions.assertThatThrownBy;
import static org.assertj.core.api.BDDAssertions.then;

class EntranceMachineTest {

    @Test
    void should_unlocked_when_insert_coin_given_a_entrance_machine_with_locked_state() {
        EntranceMachine entranceMachine = new EntranceMachine(EntranceMachineState.LOCKED);

        String result = entranceMachine.execute(Action.INSERT_COIN);

        then(result).isEqualTo("opened");
        then(entranceMachine.getState()).isEqualTo(EntranceMachineState.UNLOCKED);
    }

    @Test
    void should_alarm_when_pass_given_a_entrance_machine_with_locked_state() {
        EntranceMachine entranceMachine = new EntranceMachine(EntranceMachineState.LOCKED);

        String result = entranceMachine.execute(Action.PASS);

        then(result).isEqualTo("alarm");
        then(entranceMachine.getState()).isEqualTo(EntranceMachineState.LOCKED);
    }

    @Test
    void should_fail_when_execute_invalid_action_given_a_entrance_machine() {
        EntranceMachine entranceMachine = new EntranceMachine(EntranceMachineState.LOCKED);

        assertThatThrownBy(() -> entranceMachine.execute(null))
                .isInstanceOf(InvalidActionException.class);
    }

    @Test
    void should_refund_when_insert_coin_given_a_entrance_machine_with_unlocked_state() {
        EntranceMachine entranceMachine = new EntranceMachine(EntranceMachineState.UNLOCKED);

        String result = entranceMachine.execute(Action.INSERT_COIN);

        then(result).isEqualTo("refund");
        then(entranceMachine.getState()).isEqualTo(EntranceMachineState.UNLOCKED);
    }

    @Test
    void should_closed_when_pass_given_a_entrance_machine_with_unlocked_state() {
        EntranceMachine entranceMachine = new EntranceMachine(EntranceMachineState.UNLOCKED);

        String result = entranceMachine.execute(Action.PASS);

        then(result).isEqualTo("closed");
        then(entranceMachine.getState()).isEqualTo(EntranceMachineState.LOCKED);

    }

}

EntraceMachine.java

package com.page.java.fsm;


import com.page.java.fsm.exception.InvalidActionException;
import lombok.Data;

import java.util.Objects;

@Data
public class EntranceMachine {

    private EntranceMachineState state;

    public EntranceMachine(EntranceMachineState state) {
        setState(state);
    }

    public String execute(Action action) {
        if (Objects.isNull(action)) {
            throw new InvalidActionException();
        }

        return action.execute(this, state);
    }

    public String open() {
        return "opened";
    }

    public String alarm() {
        return "alarm";
    }

    public String refund() {
        return "refund";
    }

    public String close() {
        return "closed";
    }
}

Action.java

package com.page.java.fsm;

public enum Action {
    PASS {
        @Override
        public String execute(EntranceMachine entranceMachine, EntranceMachineState state) {
            return state.pass(entranceMachine);
        }
    },
    INSERT_COIN {
        @Override
        public String execute(EntranceMachine entranceMachine, EntranceMachineState state) {
            return state.insertCoin(entranceMachine);
        }
    };

    public abstract String execute(EntranceMachine entranceMachine, EntranceMachineState state);
}

EntranceMachineState.java

package com.page.java.fsm;

public enum EntranceMachineState {
    LOCKED {
        @Override
        public String insertCoin(EntranceMachine entranceMachine) {
            entranceMachine.setState(UNLOCKED);
            return entranceMachine.open();
        }

        @Override
        public String pass(EntranceMachine entranceMachine) {
            entranceMachine.setState(this);
            return entranceMachine.alarm();
        }
    },
    UNLOCKED {
        @Override
        public String insertCoin(EntranceMachine entranceMachine) {
            entranceMachine.setState(this);
            return entranceMachine.refund();
        }

        @Override
        public String pass(EntranceMachine entranceMachine) {
            entranceMachine.setState(LOCKED);
            return entranceMachine.close();
        }
    };

    public abstract String insertCoin(EntranceMachine entranceMachine);

    public abstract String pass(EntranceMachine entranceMachine);
}

InvalidActionException.java

package com.page.java.fsm.exception;

public class InvalidActionException extends RuntimeException {
}

通过上面的代码，可以发现Action、EntranceMachineState两个枚举的复杂度都提升了。不单单是定义了常量那么简单。还提供了相应的逻辑处理。

在EntranceMachineState.java的提交记录中，对进行了一次重构，将具体业务逻辑执行移动到EntranceMachine中，EntranceMachineState内每种状态的方法中只负责调度。这样能够通过EntranceMachineState相对直观的看清楚做了什么，状态变成了什么。

缺陷就是，EntranceMachine 对外提供了public的setState方法，这也就意味着调用者在将来维护是，很有可能滥用setState方法。

02 总结

通过上面4中对FSM的实现，我们看到每一种是实现都有优点和它的不足。那么在日常工作中，如何选择呢，我个人认为可以遵循一下两个建议：

遵循Simple Design。如果没有一个外部参考，那么用哪一种都不为过。所以引入一个原则作为参考，可以更好的帮助我们做决定。这里日常工作中我们经常使用Simple Design：通过测试、揭示意图、消除重复、最少元素。并在实现过程中不断重构，代码是重构出来的，而不是一次性的设计出来的。
在状态机的实现上多做尝试。例子只是一个简单的场景，所以只能看到简单场景下的实现效果，实际业务线上的状态会非常丰富，而且每种状态中可真行的动作也是不同的。所以针对特定场景遇到的问题，多尝试练习思考，练习思考后的经验才是最重要的。

参考

《敏捷开发实践》