摘要：狀態機的使用細節上一篇文章介紹了以及三款狀態機引擎的實現原理，以及我為何選擇作為解決方案。被代理的默認實現為，內部描述了狀態機實例創建細節包括類型信息等，同時也包含了的一些全局共享資源包括等。

date: 2017-06-19 15:50:18

上一篇文章介紹了stateless4j、spring-statemachine以及squirrel-foundation三款狀態機引擎的實現原理，以及我為何選擇squirrel-foundation作為解決方案。本文主要介紹一下項目中如何使用squirrel-foundation的一些細節以及如何與spring進行集成。在閱讀本文前，建議先閱讀官方的使用手冊。

StateMachine: StateMachine實例由StateMachineBuilder創建不被共享，對于使用annotation方式(或fluent api)定義的StateMachine，StateMachine實例即根據此定義創建，相應的action也由本實例執行，與spring的集成最終要的就是講spring的bean實例注入給由builder創建的狀態機實例；

StateMachineBuilder: 本質上是由StateMachineBuilderFactory創建的動態代理。被代理的StateMachineBuilder默認實現為StateMachineBuilderImpl，內部描述了狀態機實例創建細節包括State、Event、Context類型信息、constructor等，同時也包含了StateMachine的一些全局共享資源包括StateConverter、EventConverter、MvelScriptManager等。StateMachineBuilder可被復用，使用中可被實現為singleton；

StateMachineBuilderFactory: 為StateMachineBuilder創建的動態代理實例；

狀態正常遷移
TransitionBegin--(exit->transition->entry)-->TransitionComplete-->TransitionEnd

狀態遷移異常
TransitionBegin--(exit->transition->entry)-->TransitionException-->TransitionEnd

狀態遷移事件拒絕
TransitionBegin-->TransitionDeclined-->TransitionEnd

從statemachine的生命流程上可以看到，StateMachineBuilder可以單例方式由spring container管理，而stateMachine的instance的生命周期伴隨著請求(或業務)。
從這兩點出發，集成spring需要完成兩件事：

(1).通過Spring創建StateMachineBuilder實例；

(2).業務函數中通過(1)的StateMachineBuilder實例創建StateMachine實例，并向StateMachine暴露SpringApplicationContext；

泛型參數+覆蓋默認構造函數隱藏StateMachineBuilder創建細節，實現ApplicationContextAware接口，接受applicationContext注入，并注入給stateMachine實例。

public abstract class AbstractStateMachineEngine implements ApplicationContextAware {
    protected UntypedStateMachineBuilder stateMachineBuilder = null;
    @SuppressWarnings("unchecked")
    public AbstractStateMachineEngine() {
        //識別泛型參數
        Class genericType = (Class)GenericTypeResolver.resolveTypeArgument(getClass(),
            AbstractStateMachineEngine.class);
        stateMachineBuilder = StateMachineBuilderFactory.create(genericType, ApplicationContext.class);
    }
    //注入applicationContext，并在創建StateMachine實例時注入
    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        this.applicationContext = applicationContext;
    }
    //delegate fire
    public void fire(int rmaId, State initialState, Trigger trigger, StateMachineContext context) {
        T stateMachine = stateMachineBuilder.newUntypedStateMachine(
                            initialState
                            //暫時開啟debug進行日志trace
                            StateMachineConfiguration.create().enableDebugMode(true).enableAutoStart(true),
                            //注入applicationContext
                            applicationContext);
        stateMachine.fire(trigger, context);
    }
    ...
}
@Service
class DiscountRefundStateMachineEngine extends AbstractStateMachineEngine {
}
@Service
public class ReturnGoodsStateMachineEngine extends AbstractStateMachineEngine {
}

StateMachine定義，接受SpringContext注入

@StateMachineParameters(stateType = State.class, eventType = Trigger.class,
    //StateMachineContext 自定義上下文，用來傳遞數據
    contextType = StateMachineContext.class)
@States({
    @State(name = "PENDING", initialState = true),
    @State(name = "CONFIRMING"),
    @State(name = "REJECTED"),
    @State(name = "REFUND_APPROVING"),
    @State(name = "REFUND_APPROVED"),
    @State(name = "REFUND_FINISHED")
})
@Transitions({
    @Transit(from = "PENDING", to = "CONFIRMING", on = "APPLY_CONFIRM",
        callMethod = "doSomething"),
    @Transit(from = "CONFIRMING", to = "REJECTED", on = "REJECT"),
    @Transit(from = "CONFIRMING", to = "REFUND_APPROVING", on = "APPLY_APPROVED"),
    @Transit(from = "REFUND_APPROVING", to = "REFUND_APPROVED", on = "REFUND_APPROVED"),
    @Transit(from = "REFUND_APPROVED", to = "REFUND_FINISHED", on = "REFUND_FINISH_CONFIRM")
})
public class DiscountRefundStateMachine extends AbstractUntypedStateMachine {
    protected ApplicationContext applicationContext;
    //定義構造函數接受ApplicationContext注入([參看New State Machine Instance](http://hekailiang.github.io/squirrel/))
    public DiscountRefundStateMachine(ApplicationContext applicationContext) {
        this.applicationContext = applicationContext;
    }
    public void doSomething(State fromState, State toState, Trigger event,
                         StateMachineContext stateMachineContext) {
         DemoBean demoBean = this.applicationContext.get("demoBean");
         //do something
    }
    ...
}

從StateMachine的事件響應流程中可以看到，TransitionBegin--(exit->transition->entry)-->TransitionComplete-->TransitionEnd，在TransitionComplete發生一個狀態已從source遷移到了target狀態，所以我選擇了在這個時間點進行了狀態的持久化（沒有選擇TransitionEnd做持久化，因為某些場景在持久化完成后還會存在一些外部動作的觸發，例如通知第三方系統當前狀態已完成變更）。

public class DiscountRefundStateMachine extends AbstractUntypedStateMachine {
    ..
    @Override
    protected void afterTransitionCompleted(Object fromState, Object toState, Object event, Object context) {
        if (context instanceof StateMachineContext && toState instanceof State) {
            StateMachineContext stateMachineContext = (StateMachineContext)context;
            //從上下文中獲取需要持久化的數據，例如訂單ID等
            Rma rma = stateMachineContext.get(MessageKeyEnum.RMA);
            //持久化
            rma.setStatus((State)toState);
            this.applicationContext.get("rmaRepository").updateRma(rma);
        } else {
            throw new Exception("type not support, context expect " + StateMachineContext.class.getSimpleName() + ", actually "
                    + context.getClass().getSimpleName() + ", state expect " + State.class.getSimpleName()
                    + ", actually "
                    + toState.getClass().getSimpleName());
        }
    }
}

由于StateMachine實例不是由Spring容器創建，所以這個過程中無法通過注解方式開啟事務(Spring沒有機會去創建事務代理)，我采用了編程式事務，在AbstractStateMachineEngine的fire函數中隱式的實現。
AbstractStateMachineEngine#fire

public abstract class AbstractStateMachineEngine implements ApplicationContextAware {
    ...
    public void fire(int rmaId, State initialState, Trigger trigger, StateMachineContext context) {
        JedisLock jedisLock = jedisLockFactory.buildLock(rmaId);
        //爭用分布式鎖
        if (jedisLock.tryLock()) {
            try {
                T stateMachine = stateMachineBuilder.newUntypedStateMachine(
                                    initialState
                                    //暫時開啟debug進行日志trace
                                    StateMachineConfiguration.create().enableDebugMode(true).enableAutoStart(true),
                                    //注入applicationContext
                                    applicationContext);
                DataSourceTransactionManager transactionManager = applicationContext.get("transactionManager")
                DefaultTransactionDefinition def = new DefaultTransactionDefinition();
                def.setPropagationBehavior(TransactionDefinition.PROPAGATION_REQUIRED);
                TransactionStatus status = transactionManager.getTransaction(def);
                try {
                    stateMachine.fire(trigger, context)
                    transactionManager.commit(status);
                } catch (Exception ex) {
                    transactionManager.rollback(status);
                    throw ex;
                }
            } finally {
                jedisLock.release();
            }
        } 
        ...
    }
}

squirrel statemachine提供了DotVisitor、SCXMLVisitor兩種實現方式用于生成狀態機描述文件，項目里我選擇了graphviz用來做狀態拓撲
graphviz gui工具下載
PS:由于squirrel默認的DotVisitorImpl對帶中文描述屬性的State/Event枚舉不友好，我在原有代碼上做了一些調整，有類似需求的可以看這里

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