Yet Another auTomata (finite state machine implementation in java).

There are a lot of FSM implementations, but typically they are very sophisticated and it's hard to incorporate them in your project. Yatomata tries to be very simple and nice. It follows the declarative way and uses the convention over configuration paradigm.

Just add the following dependency to your pom.xml:

    <dependency>
        <groupId>ru.yandex.qatools</groupId>
        <artifactId>yatomata</artifactId>
        <version>1.11</version>
    </dependency>

First create the FSM class:

    @FSM(start = Stopped.class)
    @Transitions({
            @Transit(from = Stopped.class, on = Run.class, to = Running.class),
            @Transit(from = Running.class, on = Stop.class, to = Stopped.class, stop = true),
    })
    public class MyFSM {

    }

It should have the @FSM annotation defining the initial (start) state class. It also should have the @Transitions definition, containing the list of the available transitions. Each transition should define the from state(s), the to state and the list of the events in on. Each transition may also have the stop value, defining the final transition (the end state).

• The from, to and stop attributes are optional, while on is a mandatory attribute.
• If no from is present, then it would be considered as a current state.
• If no to is specified, then there would be no state changing (the state would remain the same as before).

FSM usage example follows:

    Yatomata<MyFSM> fsm = new FSMBuilder(MyFSM.class).build();

    fsm.getCurrentState();   // returns instance of Stopped
    fsm.isStopped();         // returns false
    fsm.fire(new Run());     // returns instance of Running
    fsm.fire(new Stop());    // returns instance of Stopped
    fsm.getFSM();            // returns instance of MyFSM
    fsm.isStopped();         // returns true

You can declare the three types of the hook methods within the FSM class.

The main hook method should have the @OnTransit annotation and will be called during any of the transitions. It may have the three options of the arguments:

• Single argument: incoming event
• Two arguments: current state/next state, incoming event
• Three arguments: current state, next state, incoming event

Each transition initiates the call of the single hook method which meets the signature of the transition.

Important! If there are several hook methods with @OnTransit annotation, that meet the transition signature, only the first of them (which is found first) will be called.

Besides the @OnTransit annotation you can declare the @BeforeTransit and @AfterTransit methods. The main difference from the main transit methods is that these methods will be invoked before and after the transition accordingly. Moreover all of the methods annotated with these annotations that meet the transition arguments, will be called one time before and one time after the transition. This means that you can declare the several @BeforeTransit methods with the different signatures and if all of them match the transition, they will be invoked.

Example:

    @FSM(start = Idle.class)
    @Transitions({
            @Transit(from = Idle.class, on = Run.class, to = Running.class),
            @Transit(from = Running.class, on = Stop.class, to = Stopped.class, stop = true),
    })
    public class MyFSM {
        @OnTransit
        public void onRun(Idle from, Running to, Run event){}

        @OnTransit
        public void onRun(Idle from, Run event){}

        @BeforeTransit
        public void onBeforeRun(Idle state, Run event){}

        @BeforeTransit
        public void onBeforeRun(Run event){}
    }

In the example above when Run event is caught, there will be the call to the both of the onBeforeRun methods. But only the first onRun method will be invoked.

Sometimes you may want to explicitly specify arguments injection order. To do this you can use parameter annotations: @FromState, @ToState and @Event. For example:

    @FSM(start = Idle.class)
    @Transitions({
            @Transit(from = Idle.class, on = Run.class, to = Running.class)
    })
    public class MyFSM {
        @OnTransit
        public void onRun(@ToState Running to, @Event Run event, @FromState Idle from){}

        @OnException
        public void onRun(Exception e, @Event Run event){}

    }

There's no need to place annotations on exception parameter - it's always injected automatically.

You can define the methods with @NewState annotation that can be used as state initializers for your states depending on the types of the incoming messages. Each method should have two arguments: class of the new state and the incoming event. FSM class must have only single @NewState method with the only argument, which will be used to initialize the initial state. Example:

    @FSM(start = Undefined.class)
    @Transitions({
            @Transit(from = Undefined.class, to = Started.class, on = Start.class),
            @Transit(from = Started.class, to = Stopped.class, on = Stop.class),
    })
    public class MyFSM {

        @NewState
        public Started initState(Class<Started> stateClass, Start event) {
            return new Started();
        }

        @NewState
        public Stopped initState(Class<Stopped> stateClass, Stop event) {
            return new Stopped();
        }

        @NewState
        public Undefined initState(Class<Undefined> stateClass) {
            return new Undefined();
        }
    }

You can implement your own class hierarchy according to your preference. All the annotated methods and class annotations within superclasses will be inherited by their derived classes.

Yatomata allows you to implement the methods annotated with @OnException annotation. Such methods will be used as the exception handlers for your FSM. This ability allows you to skip the try-catch blocks declaration within every transition hook method. Example:

    @FSM(start = Quotient.class)
    @Transitions({
            @Transit(on = Denominator.class),
    })
    public class MyFSM {

        @OnTransit
        public void divide(Quotient quotient, Denominator denominator) {
            quotient.setValue(quotient.getValue() / denominator.getValue());
        }

        @OnException(preserve = true)
        public void onArithmeticException(ArithmeticException e, Quotient from, Quotient to, Denominator den) {
            logger.info("Failed to perform the division", e);
        }
    }

The preserve attribute (defaults to false) indicates if the transition still must be performed even if the exception is thrown.

Important! All the declared @OnException methods, whose signature matches the occurred exception, will be called when exception is thrown. Thus if you declare the method accepting the Throwable as an argument, it will be called upon each occurring error.

If you want to stop your FSM by a custom condition, you can implement the StopConditionAware interface with your class and then implement the isStopRequired method, which accepts a new state object and an incoming event.

    @FSM(start = Idle.class)
    @Transitions({
            @Transit(from = Idle.class, on = Run.class, to = Running.class),
            @Transit(from = Running.class, on = Stop.class, to = Stopped.class, stop = true),
    })
    public class MyFSM implements StopConditionAware<State, Event> {

        boolean isStopRequired(State state, Event event){
            return event instanceof Stop;
        }
    }

Important! isStopRequired cannot override the true stop condition which is defined by a transition. Thus if your transition forces FSM to stop, this condition will be ignored.

    @FSM(start = Idle.class)
    @Transitions({
            @Transit(from = Idle.class, on = ProcessStarted.class, to = Running.class),
            @Transit(from = Idle.class, on = TerminateProcess.class, to = Cancelling.class),
            @Transit(from = {Idle.class, Cancelling.class}, on = ProcessTerminated.class, stop = true),
            @Transit(from = Cancelling.class, on = ProcessStarted.class, stop = true),
            @Transit(from = Running.class, on = {ProcessCompleted.class, ProcessFailed.class, ProcessTerminated.class}, stop = true),
            @Transit(from = Running.class, on = TerminateProcess.class)
    })
    public class ExecuteStateMachine {
        @OnTransit
        public void onProcessStarted(Idle from, Running to, ProcessStarted event){}

        @OnTransit
        public void onProcessTerminate(Running from, TerminateProcess event){}

        @OnTransit
        public void onProcessStartedAtCancelling(Cancelling from, ProcessStarted event){}

        @OnTransit
        public void onProcessTerminatedAtCancelling(ExecuteState from, ProcessTerminated event){}
    }

    @FSM(start = CompletedCounterState.class)
    @Transitions( @Transit(on = {TestCompletionInfo.class}) )
    public class CompletedCounterStateMachine {
        @OnTransit
        public void aggregate(CompletedCounterState state, TestCompletionInfo event){}
    }