有限状态机的4种Java实现对比

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

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); } } 复制代码

Action.java

public enum Action { INSERT_COIN, PASS } 复制代码

EntranceMachineState.java

public enum EntranceMachineState { UNLOCKED, LOCKED } 复制代码

InvalidActionException.java

package com.page.java.fsm.exception; public class InvalidActionException extends RuntimeException { } 复制代码

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"; } } 复制代码

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：

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(); } } 复制代码

EntranceMachineState.java

package com.page.java.fsm; public interface EntranceMachineState { String insertCoin(EntranceMachine entranceMachine); String pass(EntranceMachine entranceMachine); } 复制代码

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(); } } 复制代码

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(); } } 复制代码

Action.java

package com.page.java.fsm; public enum Action { PASS, INSERT_COIN } 复制代码

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; } } 复制代码

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); } } 复制代码

Action.java

package com.page.java.fsm; public enum Action { PASS, INSERT_COIN } 复制代码

EntranceMachineState.java

package com.page.java.fsm; public enum EntranceMachineState { LOCKED, UNLOCKED } 复制代码

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(); } } 复制代码

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; } 复制代码

Event.java

package com.page.java.fsm.events; public interface Event { String execute(); } 复制代码

OpenEvent.java

package com.page.java.fsm.events; public class OpenEvent implements Event { @Override public String execute() { return "opened"; } } 复制代码

AlarmEvent.java

package com.page.java.fsm.events; public class AlarmEvent implements Event { @Override public String execute() { return "alarm"; } } 复制代码

CloseEvent.java

package com.page.java.fsm.events; public class CloseEvent implements Event { @Override public String execute() { return "closed"; } } 复制代码

RefundEvent.java

package com.page.java.fsm.events; public class RefundEvent implements Event { @Override public String execute() { return "refund"; } } 复制代码

InvalidActionException.java

package com.page.java.fsm.exception; public class InvalidActionException extends RuntimeException { } 复制代码

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); } } 复制代码

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"; } } 复制代码

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); } 复制代码

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); } 复制代码

InvalidActionException.java

package com.page.java.fsm.exception; public class InvalidActionException extends RuntimeException { } 复制代码

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：

1. 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。

2. 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。