Persiting a state machine context in HttpSession java.io.NotSerializableException: org.springframework.statemachine.support.DefaultStateMachineContext

I have created a StateMachinePersist implementation to persist the StateMachineContext y HttpSession:

@Suppress("UNCHECKED_CAST")
class SessionStateMachinePersist(private val session: HttpSession) : StateMachinePersist<States, String, String> {

    private val logger = LoggerFactory.getLogger(javaClass)

    override fun read(contextObj: String): StateMachineContext<States, String>?  {
        val stateMachineContext = session.getAttribute("${session.id}-$contextObj") as StateMachineContext<States, String>?
        logger.debug("Session {} has state machine context {}?", session.id, contextObj, stateMachineContext != null)
        return stateMachineContext
    }

    override fun write(context: StateMachineContext<States, String>, contextObj: String) {
        session.setAttribute("${session.id}-$contextObj", context)
    }
}

When it is going to persist the DefaultStateMachineContext it throws the following exception since the DefaultStateMachineContext isn't serializable: Caused by: java.io.NotSerializableException: org.springframework.statemachine.support.DefaultStateMachineContext

How can I use a StateMachineContext implementation that is serializable?

This is how I managed to make the State Machine Context serializable and persist it in the Http Session:

SerializableStateMachineContext

class SerializableStateMachineContext<S, E>(var id: String?, var childs: List<StateMachineContext<S, E>>?, var state: S?,
                                            var historyStates: Map<S, S>?, var event: E?, var eventHeaders: Map<String, Any>?,
                                            var extendedState: SerializableExtendedState?) : Serializable {

    constructor() : this(null, null, null, null, null, null, null)

}

SerializableExtendedState

class SerializableExtendedState(var variables: Map<Any, Any>) : Serializable {

    constructor() : this(mutableMapOf())

}

SessionStateMachinePersist

@Suppress("UNCHECKED_CAST")
class SessionStateMachinePersist(private val session: HttpSession) : StateMachinePersist<States, String, String> {

    private val logger = LoggerFactory.getLogger(javaClass)

    override fun read(contextObj: String): StateMachineContext<States, String>? {
        val stateMachineContext = session.getAttribute("${session.id}-$contextObj") as SerializableStateMachineContext<States, String>?
        logger.debug("Session {} has state machine context {}?", session.id, contextObj, stateMachineContext != null)
        return DefaultStateMachineContext(stateMachineContext?.childs ?: mutableListOf(), stateMachineContext?.state, stateMachineContext?.event,
                stateMachineContext?.eventHeaders, DefaultExtendedState(stateMachineContext?.extendedState?.variables ?: mutableMapOf()), stateMachineContext?.historyStates)
    }

    override fun write(context: StateMachineContext<States, String>, contextObj: String) {
        val serializableExtendedState = SerializableExtendedState(context.extendedState.variables.toMap())
        val serializableStateMachineContext = SerializableStateMachineContext(null, context.childs, context.state, context.historyStates, context.event, context.eventHeaders, serializableExtendedState)
        session.setAttribute("${session.id}-$contextObj", serializableStateMachineContext)
    }
}

Then, wherever you have to operate with the state machine you just have to restore it, execute the trigger operation and persist it, e.g.:

    @PostMapping(value = ["/process-input"], consumes = [MediaType.APPLICATION_JSON_VALUE], produces = [MediaType.APPLICATION_JSON_VALUE])
    @ResponseBody
    fun processInput(session: HttpSession, @RequestBody projectCreationInput: ProjectCreationInput, model: ModelMap, status: SessionStatus) {

        val stateMachinePersister = buildStateMachinePersister(session)
        stateMachinePersister.restore(stateMachine, "myid")

        ...

        triggerStateMachineExecution(projectCreationInput, event, status)

        ...

        stateMachinePersister.persist(stateMachine, "myid")

    }

    private fun buildStateMachinePersister(session: HttpSession): DefaultStateMachinePersister<States, String, String> {
        val stateMachinePersist = SessionStateMachinePersist(session)
        return DefaultStateMachinePersister(stateMachinePersist)
    }