Spring Statemachine 1.0.0.M3 is built and tested with JDK 7 and Spring Framework 4.1.6.RELEASE and doesn’t require any other dependencies outside of Spring Framework. Samples require spring-shell and spring-boot which pulls other dependencies beyond framework itself.

State machines are powerful because behaviour is always guaranteed to be consistent and relatively easily debugged due to ways how operational rules are written in stone when machine is started. Idea is that your application is and may exist in a finite number of states and then something happens which takes your application from one state to the next. What will drive a state machine are triggers which are either based on events or timers.

It is much easier to design high level logic outside of your application and then interact with a state machine with a various different ways. You can simply interact with a state machine by sending event, listening what a state machine does or simply request a current state.

Traditionally state machines are added to a existing project when developer realizes that code base is starting to look like a plate full of spaghetti. Spaghetti code looks like never ending hierarchical structure of IFs, ELSEs and BREAK clauses and probably compiler should ask developer to go home when things are starting to look too complex.

Project is a good candidate to use state machine if:

You are already trying to implement a state machine if:

One of the common tasks when using a Statemachine is to design its runtime configuration. This chapter will focus on how Spring Statemachine is configured and how it leverages Spring’s lightweight IoC containers to simplify the application internals to make it more manageable.

	Note
	Configuration examples in this section are not feature complete, i.e. you always need to have definitions of both states and transitions, otherwise state machine configuration would be ill-formed. We have simply made code snippets less verbose by leaving other needed parts away.

4.1 Configuring States

We’ll get into more complex configuration examples a bit later but lets first start with a something simple. For most simple state machine you just use EnumStateMachineConfigurerAdapter and define possible states, choose initial and optional end state.

@Configuration
@EnableStateMachine
public static class Config1Enums
        extends EnumStateMachineConfigurerAdapter<States, Events> {

    @Override
    public void configure(StateMachineStateConfigurer<States, Events> states)
            throws Exception {
        states
            .withStates()
                .initial(States.S1)
                .end(States.SF)
                .states(EnumSet.allOf(States.class));
    }

}

It’s also possible to use strings instead of enums as states and events by using StateMachineConfigurerAdapter as shown below. Most of a configuration examples is using enums but generally speaking strings and enums can be just interchanged.

@Configuration
@EnableStateMachine
public static class Config1Strings
        extends StateMachineConfigurerAdapter<String, String> {

    @Override
    public void configure(StateMachineStateConfigurer<String, String> states)
            throws Exception {
        states
            .withStates()
                .initial("S1")
                .end("SF")
                .states(new HashSet<String>(Arrays.asList("S1","S2","S3","S4")));
    }

}

	Note
	Using enums will bring more safe set of states and event types but limits possible combinations to compile time. Strings don’t have this limitation and allows user to use more dynamic ways to build state machine configurations but doesn’t allow same level of safety.

@Configuration
@EnableStateMachine
public static class Config2
        extends EnumStateMachineConfigurerAdapter<States, Events> {

    @Override
    public void configure(StateMachineStateConfigurer<States, Events> states)
            throws Exception {
        states
            .withStates()
                .initial(States.S1)
                .state(States.S1)
                .and()
                .withStates()
                    .parent(States.S1)
                    .initial(States.S2)
                    .state(States.S2);
    }

}

@Configuration
@EnableStateMachine
public static class Config10
        extends EnumStateMachineConfigurerAdapter<States2, Events> {

    @Override
    public void configure(StateMachineStateConfigurer<States2, Events> states)
            throws Exception {
        states
            .withStates()
                .initial(States2.S1)
                .state(States2.S2)
                .and()
                .withStates()
                    .parent(States2.S2)
                    .initial(States2.S2I)
                    .state(States2.S21)
                    .end(States2.S2F)
                    .and()
                .withStates()
                    .parent(States2.S2)
                    .initial(States2.S3I)
                    .state(States2.S31)
                    .end(States2.S3F);
    }

}

@Configuration
@EnableStateMachine
public static class Config3
        extends EnumStateMachineConfigurerAdapter<States, Events> {

    @Override
    public void configure(StateMachineStateConfigurer<States, Events> states)
            throws Exception {
        states
            .withStates()
                .initial(States.S1)
                .states(EnumSet.allOf(States.class));
    }

    @Override
    public void configure(StateMachineTransitionConfigurer<States, Events> transitions)
            throws Exception {
        transitions
            .withExternal()
                .source(States.S1).target(States.S2)
                .event(Events.E1)
                .and()
            .withInternal()
                .source(States.S2)
                .event(Events.E2)
                .and()
            .withLocal()
                .source(States.S2).target(States.S3)
                .event(Events.E3);
    }

}

@Configuration
@EnableStateMachine
public static class Config4
        extends EnumStateMachineConfigurerAdapter<States, Events> {

    @Override
    public void configure(StateMachineTransitionConfigurer<States, Events> transitions)
            throws Exception {
        transitions
            .withExternal()
                .source(States.S1).target(States.S2)
                .event(Events.E1)
                .guard(guard())
                .and()
            .withExternal()
                .source(States.S2).target(States.S3)
                .event(Events.E2)
                .guardExpression("true");

    }

    @Bean
    public Guard<States, Events> guard() {
        return new Guard<States, Events>() {

            @Override
            public boolean evaluate(StateContext<States, Events> context) {
                return true;
            }
        };
    }

}

@Configuration
@EnableStateMachine
public static class Config5
        extends EnumStateMachineConfigurerAdapter<States, Events> {

    @Override
    public void configure(StateMachineTransitionConfigurer<States, Events> transitions)
            throws Exception {
        transitions
            .withExternal()
                .source(States.S1)
                .target(States.S2)
                .event(Events.E1)
                .action(action());
    }

    @Bean
    public Action<States, Events> action() {
        return new Action<States, Events>() {

            @Override
            public void execute(StateContext<States, Events> context) {
                // do something
            }
        };
    }

}

@Configuration
@EnableStateMachine
public static class Config11
        extends EnumStateMachineConfigurerAdapter<States, Events> {

    @Override
    public void configure(StateMachineStateConfigurer<States, Events> states)
            throws Exception {
        states
            .withStates()
                .initial(States.S1, initialAction())
                .end(States.SF)
                .states(EnumSet.allOf(States.class));
    }

    @Bean
    public Action<States, Events> initialAction() {
        return new Action<States, Events>() {

            @Override
            public void execute(StateContext<States, Events> context) {
                // do something initially
            }
        };
    }

}

@Configuration
@EnableStateMachine
public static class Config1Enums
        extends EnumStateMachineConfigurerAdapter<States, Events> {

    @Override
    public void configure(StateMachineStateConfigurer<States, Events> states)
            throws Exception {
        states
            .withStates()
                .initial(States.S1)
                .end(States.SF)
                .states(EnumSet.allOf(States.class));
    }

}

@Configuration
@EnableStateMachine
public static class Config12
        extends EnumStateMachineConfigurerAdapter<States3, Events> {

    @Override
    public void configure(StateMachineStateConfigurer<States3, Events> states)
            throws Exception {
        states
        .withStates()
            .initial(States3.S1)
            .state(States3.S2)
            .and()
            .withStates()
                .parent(States3.S2)
                .initial(States3.S2I)
                .state(States3.S21)
                .state(States3.S22)
                .history(States3.SH, History.SHALLOW);
    }

}

@Configuration
@EnableStateMachine
public static class Config13
        extends EnumStateMachineConfigurerAdapter<States, Events> {

    @Override
    public void configure(StateMachineStateConfigurer<States, Events> states)
            throws Exception {
        states
            .withStates()
                .initial(States.SI)
                .choice(States.S1)
                .end(States.SF)
                .states(EnumSet.allOf(States.class));
    }

    @Override
    public void configure(StateMachineTransitionConfigurer<States, Events> transitions)
            throws Exception {
        transitions
            .withChoice()
                .source(States.S1)
                .first(States.S2, s2Guard())
                .then(States.S3, s3Guard())
                .last(States.S4);
    }

    @Bean
    public Guard<States, Events> s2Guard() {
        return new Guard<States, Events>() {

            @Override
            public boolean evaluate(StateContext<States, Events> context) {
                return false;
            }
        };
    }

    @Bean
    public Guard<States, Events> s3Guard() {
        return new Guard<States, Events>() {

            @Override
            public boolean evaluate(StateContext<States, Events> context) {
                return true;
            }
        };
    }

}

@Configuration
@EnableStateMachine
public static class Config14
        extends EnumStateMachineConfigurerAdapter<States2, Events> {

    @Override
    public void configure(StateMachineStateConfigurer<States2, Events> states)
            throws Exception {
        states
            .withStates()
                .initial(States2.S1)
                .fork(States2.S2)
                .state(States2.S3)
                .and()
                .withStates()
                    .parent(States2.S3)
                    .initial(States2.S2I)
                    .state(States2.S21)
                    .state(States2.S22)
                    .end(States2.S2F)
                    .and()
                .withStates()
                    .parent(States2.S3)
                    .initial(States2.S3I)
                    .state(States2.S31)
                    .state(States2.S32)
                    .end(States2.S3F);
    }

    @Override
    public void configure(StateMachineTransitionConfigurer<States2, Events> transitions)
            throws Exception {
        transitions
            .withFork()
                .source(States2.S2)
                .target(States2.S22)
                .target(States2.S32);
    }

}

@Configuration
@EnableStateMachine
public static class Config15
        extends EnumStateMachineConfigurerAdapter<States2, Events> {

    @Override
    public void configure(StateMachineStateConfigurer<States2, Events> states)
            throws Exception {
        states
            .withStates()
                .initial(States2.S1)
                .state(States2.S3)
                .join(States2.S4)
                .and()
                .withStates()
                    .parent(States2.S3)
                    .initial(States2.S2I)
                    .state(States2.S21)
                    .state(States2.S22)
                    .end(States2.S2F)
                    .and()
                .withStates()
                    .parent(States2.S3)
                    .initial(States2.S3I)
                    .state(States2.S31)
                    .state(States2.S32)
                    .end(States2.S3F);
    }

    @Override
    public void configure(StateMachineTransitionConfigurer<States2, Events> transitions)
            throws Exception {
        transitions
            .withJoin()
                .source(States2.S2F)
                .source(States2.S3F)
                .target(States2.S5);
    }

}

@Configuration
@EnableStateMachine
public static class Config17
        extends EnumStateMachineConfigurerAdapter<States, Events> {

    @Override
    public void configure(StateMachineConfigurationConfigurer<States, Events> config)
            throws Exception {
        config
            .withConfiguration()
                .autoStartup(true)
                .beanFactory(new StaticListableBeanFactory())
                .taskExecutor(new SyncTaskExecutor())
                .taskScheduler(new ConcurrentTaskScheduler())
                .listener(new StateMachineListenerAdapter<States, Events>());
    }

}

@Configuration
@EnableStateMachine
public static class Config18
        extends EnumStateMachineConfigurerAdapter<States, Events> {

    @Override
    public void configure(StateMachineConfigurationConfigurer<States, Events> config)
            throws Exception {
        config
            .withDistributed()
                .ensemble(stateMachineEnsemble());
    }

    @Bean
    public StateMachineEnsemble<States, Events> stateMachineEnsemble()
            throws Exception {
        // naturally not null but should return ensemble instance
        return null;
    }

}

There are use cases when state machine needs to be created dynamically instead of defining static configuration at compile time. For example if there are custom components which are using its own state machines and these components are created dynamically it is impossible to have a static state machined build during the application start. Internally state machines are always build via a factory interfaces and this then gives user an option to use this feature programmatically. Configuration for state machine factory is exactly same as you’ve seen in various examples in this document where state machine configuration is hard coded.

@Configuration
@EnableStateMachineFactory
public static class Config6
        extends EnumStateMachineConfigurerAdapter<States, Events> {

    @Override
    public void configure(StateMachineStateConfigurer<States, Events> states)
            throws Exception {
        states
            .withStates()
                .initial(States.S1)
                .end(States.SF)
                .states(EnumSet.allOf(States.class));
    }

}

static class Bean3 {

    @Autowired
    StateMachineFactory<States, Events> factory;

    void method() {
        StateMachine<States,Events> stateMachine = factory.getStateMachine();
        stateMachine.start();
    }
}

5.2 State Machine via Builder

Using adapters shown above has a limitation imposed by its requirement to work via Spring @Configuration classes and application context. While this is a very clear model to configure a state machine instances it will limit configuration at a compile time which is not always what a user wants to do. If there is a requirement to build more dynamic state machines, a simple builder patter can be used to construct similar instances. Using strings as states and events this builder patter can be used to build fully dynamic state machines outside of a Spring application context as shown above.

StateMachine<String, String> buildMachine() throws Exception {
    Builder<String, String> builder = StateMachineBuilder.builder();
    builder.configureStates()
        .withStates()
            .initial("S1")
            .end("SF")
            .states(new HashSet<String>(Arrays.asList("S1","S2","S3","S4")));
    return builder.build();
}

Builder is using same configuration interfaces behind the scenes that the @Configuration model using adapter classes. Same model goes to configuring transitions, states and common configuration via builder’s methods. This simply means that whatever you can use with a normal EnumStateMachineConfigurerAdapter or StateMachineConfigurerAdapter can be used dynamically via a builder.

	Note
	Currently builder.configureStates(), builder.configureTransitions() and other interface methods cannot be chained together meaning builder methods needs to be called individually.

Actions are one of the most useful components from user perspective to interact and collaborate with a state machine. Actions can be executed in various places in a state machine and its states lifecycle like entering or exiting states or during a transitions.

@Override
public void configure(StateMachineStateConfigurer<States, Events> states)
        throws Exception {
    states
        .withStates()
            .initial(States.SI)
            .state(States.S1, action1(), action2())
            .state(States.S2, action1(), action2())
            .state(States.S3, action1(), action3());
}

Above action1 and action2 beans are attached to states entry and exit respectively.

@Bean
public Action<States, Events> action1() {
    return new Action<States, Events>() {

        @Override
        public void execute(StateContext<States, Events> context) {
        }
    };
}

@Bean
public BaseAction action2() {
    return new BaseAction();
}

@Bean
public SpelAction action3() {
    ExpressionParser parser = new SpelExpressionParser();
    return new SpelAction(
            parser.parseExpression(
                    "stateMachine.sendEvent(T(org.springframework.statemachine.docs.Events).E1)"));
}

static class BaseAction implements Action<States, Events> {

    @Override
    public void execute(StateContext<States, Events> context) {
    }
}

static class SpelAction extends SpelExpressionAction<States, Events> {

    public SpelAction(Expression expression) {
        super(expression);
    }
}

You can directly implement Action as an anonymous function or create a your own implementation and define appropriate implementation as a bean.

In action3 a SpEL expression is used to send event Events.E1 into a state machine.

	Note
	StateContext is described in section Chapter 8, Using StateContext.

Above guard1 and guard2 beans are attached to states entry and exit respectively.

@Override
public void configure(StateMachineTransitionConfigurer<States, Events> transitions)
        throws Exception {
    transitions
        .withExternal()
            .source(States.SI).target(States.S1)
            .event(Events.E1)
            .guard(guard1())
            .and()
        .withExternal()
            .source(States.S1).target(States.S2)
            .event(Events.E1)
            .guard(guard2())
            .and()
        .withExternal()
            .source(States.S2).target(States.S3)
            .event(Events.E2)
            .guardExpression("extendedState.variables.get('myvar')");
}

You can directly implement Guard as an anonymous function or create a your own implementation and define appropriate implementation as a bean. In above sample guardExpression is simply checking if extended state variable myvar evaluates to TRUE.

@Bean
public Guard<States, Events> guard1() {
    return new Guard<States, Events>() {

        @Override
        public boolean evaluate(StateContext<States, Events> context) {
            return true;
        }
    };
}

@Bean
public BaseGuard guard2() {
    return new BaseGuard();
}

static class BaseGuard implements Guard<States, Events> {

    @Override
    public boolean evaluate(StateContext<States, Events> context) {
        return false;
    }
}

	Note
	StateContext is described in section Chapter 8, Using StateContext.

StateContext is a domain object representing a current status of a state machine within a transition or an action. Context gives an access to a various information like event, message headers, extended state variables, current transition and a top-level state machine in case there is a need to send events to a further processing.

Driving a statemachine is done via transitions which are triggered by triggers. Currently supported triggers are EventTrigger and TimerTrigger.

@Autowired
StateMachine<States, Events> stateMachine;

void signalMachine() {
    stateMachine.sendEvent(Events.E1);

    Message<Events> message = MessageBuilder
            .withPayload(Events.E2)
            .setHeader("foo", "bar")
            .build();
    stateMachine.sendEvent(message);
}

There are use cases where you just want to know what is happening with a state machine, react to something or simply get logging for debugging purposes. SSM provides interfaces for adding listeners which then gives an option to get callback when various state changes, actions, etc are happening.

You basically have two options, either to listen Spring application context events or directly attach listener to a state machine. Both of these basically will provide same information where one is producing events as event classes and other producing callbacks via a listener interface. Both of these have pros and cons which will be discussed later.

static class StateMachineApplicationEventListener
        implements ApplicationListener<StateMachineEvent> {

    @Override
    public void onApplicationEvent(StateMachineEvent event) {
    }
}

static class StateMachineEventListener
        extends StateMachineListenerAdapter<States, Events> {

    @Override
    public void stateChanged(State<States, Events> from, State<States, Events> to) {
    }

    @Override
    public void stateEntered(State<States, Events> state) {
    }

    @Override
    public void stateExited(State<States, Events> state) {
    }

    @Override
    public void transition(Transition<States, Events> transition) {
    }

    @Override
    public void transitionStarted(Transition<States, Events> transition) {
    }

    @Override
    public void transitionEnded(Transition<States, Events> transition) {
    }

    @Override
    public void stateMachineStarted(StateMachine<States, Events> stateMachine) {
    }

    @Override
    public void stateMachineStopped(StateMachine<States, Events> stateMachine) {
    }
}

static class Config7 {

    @Autowired
    StateMachine<States, Events> stateMachine;

    @Bean
    public StateMachineEventListener stateMachineEventListener() {
        StateMachineEventListener listener = new StateMachineEventListener();
        stateMachine.addStateListener(listener);
        return listener;
    }

}

@Configuration
@EnableStateMachine(contextEvents = false)
public static class Config8
        extends EnumStateMachineConfigurerAdapter<States, Events> {
}

@Configuration
@EnableStateMachineFactory(contextEvents = false)
public static class Config9
        extends EnumStateMachineConfigurerAdapter<States, Events> {
}

It is a little limited to do interaction with a state machine by either listening its events or using actions with states and transitions. Time to time this approach would be too limited and verbose to create interaction with the application a state machine is working with. For this specific use case we have made a spring style context integration which easily attach state machine functionality into your beans.

@WithStateMachine
static class Bean1 {

    @OnTransition(source = "S1", target = "S2")
    public void fromS1ToS2() {
    }
}

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
@OnTransition
static @interface StatesOnTransition {

    States[] source() default {};

    States[] target() default {};
}

@WithStateMachine
static class Bean2 {

    @StatesOnTransition(source = States.S1, target = States.S2)
    public void fromS1ToS2() {
    }
}

StateMachine is a main interface to communicate with a state machine itself. Time to time there is a need to get more dynamical and programmatic access to internal structures of a state machine and its nested machines and regions. For these use cases a StateMachine is exposing a functional interface StateMachineAccessor which provides an interface to get access to individual StateMachine and Region instances.

StateMachineFunction is a simple functional interface which allows to apply StateMachineAccess interface into a state machine. With jdk7 these will create a little verbose code but with jdk8 lambdas things look relatively non-verbose.

Method doWithAllRegions gives access to all Region instances in a state machine.

stateMachine.getStateMachineAccessor().doWithAllRegions(new StateMachineFunction<StateMachineAccess<String,String>>() {

    @Override
    public void apply(StateMachineAccess<String, String> function) {
        function.setRelay(stateMachine);
    }
});

stateMachine.getStateMachineAccessor()
    .doWithAllRegions(access -> access.setRelay(stateMachine));

Method doWithRegion gives access to single Region instance in a state machine.

stateMachine.getStateMachineAccessor().doWithRegion(new StateMachineFunction<StateMachineAccess<String,String>>() {

    @Override
    public void apply(StateMachineAccess<String, String> function) {
        function.setRelay(stateMachine);
    }
});

stateMachine.getStateMachineAccessor()
    .doWithRegion(access -> access.setRelay(stateMachine));

Method withAllRegions gives access to all Region instances in a state machine.

for (StateMachineAccess<String, String> access : stateMachine.getStateMachineAccessor().withAllRegions()) {
    access.setRelay(stateMachine);
}

stateMachine.getStateMachineAccessor().withAllRegions()
    .stream().forEach(access -> access.setRelay(stateMachine));

Method withRegion gives access to single Region instance in a state machine.

stateMachine.getStateMachineAccessor()
    .withRegion().setRelay(stateMachine);

Instead of using a StateMachineListener interface one option is to use a StateMachineInterceptor. One conceptual difference is that an interceptor can be used to intercept and stop a current state change or transition logic. Instead of implementing full interface, adapter class StateMachineInterceptorAdapter can be used to override default no-op methods.

	Note
	There is one recipe Chapter 17, Persist and one sample Chapter 24, Persist which are related to use of an interceptor.

Interceptor can be registered via StateMachineAccessor. Concept of an interceptor is relatively deep internal feature and thus is not exposed directly via StateMachine interface.

stateMachine.getStateMachineAccessor()
    .withRegion().addStateMachineInterceptor(new StateMachineInterceptor<String, String>() {

        @Override
        public StateContext<String, String> preTransition(StateContext<String, String> stateContext) {
            return stateContext;
        }

        @Override
        public void preStateChange(State<String, String> state, Message<String> message,
                Transition<String, String> transition, StateMachine<String, String> stateMachine) {
        }

        @Override
        public StateContext<String, String> postTransition(StateContext<String, String> stateContext) {
            return stateContext;
        }

        @Override
        public void postStateChange(State<String, String> state, Message<String> message,
                Transition<String, String> transition, StateMachine<String, String> stateMachine) {
        }

        @Override
        public Exception stateMachineError(StateMachine<String, String> stateMachine,
                Exception exception) {
            return exception;
        }
    });

	Note
	More about error handling shown in above example, see section Chapter 14, State Machine Error Handling.

If state machine detects an internal error during a state transition logic it may throw an exception. Before this exception is processed internally, user is given a change to intercept.

Normal StateMachineInterceptor can be used to intercept errors and example of it is shown above.

StateMachine<String, String> stateMachine;

void addInterceptor() {
    stateMachine.getStateMachineAccessor()
        .doWithRegion(new StateMachineFunction<StateMachineAccess<String, String>>() {

        @Override
        public void apply(StateMachineAccess<String, String> function) {
            function.addStateMachineInterceptor(
                    new StateMachineInterceptorAdapter<String, String>() {
                @Override
                public Exception stateMachineError(StateMachine<String, String> stateMachine,
                        Exception exception) {
                    // return null indicating handled error
                    return exception;
                }
            });
        }
    });

}

When errors are detected, normal event notify mechanism is executed. This allows to use either StateMachineListener or Spring Application context event listener, more about these read section Chapter 10, Listening State Machine Events.

Having said that, a simple listener would look like:

public static class ErrorStateMachineListener
        extends StateMachineListenerAdapter<String, String> {

    @Override
    public void stateMachineError(StateMachine<String, String> stateMachine, Exception exception) {
        // do something with error
    }
}

Generic ApplicationListener checking StateMachineEvent would look like.

public static class GenericApplicationEventListener
        implements ApplicationListener<StateMachineEvent> {

    @Override
    public void onApplicationEvent(StateMachineEvent event) {
        if (event instanceof OnStateMachineError) {
            // do something with error
        }
    }
}

It’s also possible to define ApplicationListener directly to recognize only StateMachineEvent instances.

public static class ErrorApplicationEventListener
        implements ApplicationListener<OnStateMachineError> {

    @Override
    public void onApplicationEvent(OnStateMachineError event) {
        // do something with error
    }
}

Traditionally an instance of a state machine is used as is within a running program. More dynamic behaviour is possible to achieve via dynamic builders and factories which allows state machine instantiation on-demand. Building an instance of a state machine is relatively heavy operation so if there is a need to i.e. handle arbitrary state change in a database using a state machine we need to find a better and faster way to do it.

Persist feature allows user to save a state of a state machine itself into an external repository and later reset a state machine based of serialized state. For example if you have a database table keeping orders it would be way too expensive to update order state via a state machine if a new instance would need to be build for every change. Persist feature allows you to reset a state machine state without instantiating a new state machine instance.

	Note
	There is one recipe Chapter 17, Persist and one sample Chapter 24, Persist which provides more info about persisting states.

While it is possible to build a custom persistence feature using a StateMachineListener it has one conceptual problem. When listener notifies a change of state, state change has already happened. If a custom persistent method within a listener fails to update serialized state in an external repository, state in a state machine and state in an external repository are then in inconsistent state.

State machine interceptor can be used instead of where attempt to save serialized state into an external storage is done during the a state change within a state machine. If this interceptor callback fails, state change attempt will be halted and instead of ending into an inconsistent state, user can then handle this error manually. Using the interceptors are discussed in Chapter 13, State Machine Interceptor.

Distributed state is probably one of a most compicated concepts of a Spring State Machine. What exactly is a distributed state? A state within a single state machine is naturally really simple to understand but when there is a need to introduce a shared distributed state thoughout a state machines, things will get a little complicated.

	Note
	Distributed state functionality is still a preview feature and is not yet considered to be stable in this particular release. We expect this feature to mature towards the first official release.

For configuration support see section Section 4.8, “Configuring Common Settings” and actual usage example see sample Chapter 25, Zookeeper.

Distributed State Machine is implemented via a DistributedStateMachine class which simply wraps an actual instance of a StateMachine. DistributedStateMachine intercepts communication with a StateMachine instance and works with distributed state abstractions handled via interface StateMachineEnsemble. Depending on an actual implementation StateMachinePersist interface may also be used to serialize a StateMachineContext which contains enought information to reset a StateMachine.

While Distributed State Machine is implemented via an abstraction, only one implementation currently exists based on Zookeeper.

Current technical documentation of a Zookeeker based distributed state machine can be found from an appendice Appendix C, Distributed State Machine with Zookeeper.

Persist recipe is a simple utility which allows to use a single state machine instance to persist and update a state of an arbitrary item in a repository.

Recipe’s main class is PersistStateMachineHandler which assumes user to do three different things:

There is a sample demonstrating usage of this recipe at Chapter 24, Persist.

Tasks recipe is a concept to execute DAG of Runnable instances using a state machine. This recipe has been developed from ideas introduced in sample Chapter 22, Tasks.

Generic concept of a state machine is shown below. In this state chart everything under TASKS just shows a generic concept of how a single task is executed. Because this recipe allows to register deep hierarcical DAG of tasks, meaning a real state chart would be deep nested collection of sub-states and regions, there’s no need to be more presise.

For example if you have only two registered tasks, below state chart would be correct with TASK_id replaced with TASK_1 and TASK_2 if registered tasks ids are 1 and 2.

Executing a Runnable may result an error and especially if a complex DAG of tasks is involved it is desirable that there is a way to handle tasks execution errors and then having a way to continue execution without executing already successfully executed tasks. Addition to this it would be nice if some execution errors can be handled automatically and as a last fallback, if error can’t be handled automatically, state maching is put into a state where user can handle errors manually.

TasksHandler contains a builder method to configure handler instance and follows a simple builder patter. This builder can be used to register Runnable tasks, TasksListener instances, define StateMachinePersist hook, and setup custom TaskExecutor instance.

Now lets take a simple Runnable just doing a simple sleep as shown below. This is a base of all examples in this chapter.

private static Runnable sleepRunnable() {
    return new Runnable() {

        @Override
        public void run() {
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
            }
        }
    };
}

To execute multiple sleepRunnable tasks just register tasks and execute runTasks() method from TasksHandler.

TasksHandler handler = TasksHandler.builder()
        .task("1", sleepRunnable())
        .task("2", sleepRunnable())
        .task("3", sleepRunnable())
        .build();

handler.runTasks();

Order to listen what is happening with a task execution an instance of a TasksListener can be registered with a TasksHandler. Recipe provides an adapter TasksListenerAdapter if you dont' want to implement a full interface. Listener provides a various hooks to listen tasks execution events.

private static class MyTasksListener extends TasksListenerAdapter {

    @Override
    public void onTasksStarted() {
    }

    @Override
    public void onTasksContinue() {
    }

    @Override
    public void onTaskPreExecute(Object id) {
    }

    @Override
    public void onTaskPostExecute(Object id) {
    }

    @Override
    public void onTaskFailed(Object id, Exception exception) {
    }

    @Override
    public void onTaskSuccess(Object id) {
    }

    @Override
    public void onTasksSuccess() {
    }

    @Override
    public void onTasksError() {
    }

    @Override
    public void onTasksAutomaticFix(TasksHandler handler, StateContext<String, String> context) {
    }
}

Listeners can be either registered via a builder or directly with a TasksHandler as shown above.

MyTasksListener listener1 = new MyTasksListener();
MyTasksListener listener2 = new MyTasksListener();

TasksHandler handler = TasksHandler.builder()
        .task("1", sleepRunnable())
        .task("2", sleepRunnable())
        .task("3", sleepRunnable())
        .listener(listener1)
        .build();

handler.addTasksListener(listener2);
handler.removeTasksListener(listener2);

handler.runTasks();

Above sample show how to create a deep nested DAG of tasks. Every task needs to have an unique identifier and optionally as task can be defined to be a sub-task. Effectively this will create a DAG of tasks.

TasksHandler handler = TasksHandler.builder()
        .task("1", sleepRunnable())
        .task("1", "12", sleepRunnable())
        .task("1", "13", sleepRunnable())
        .task("2", sleepRunnable())
        .task("2", "22", sleepRunnable())
        .task("2", "23", sleepRunnable())
        .task("3", sleepRunnable())
        .task("3", "32", sleepRunnable())
        .task("3", "33", sleepRunnable())
        .build();

handler.runTasks();

When error happens and a state machine running these tasks goes into a ERROR state, user can call handler methods fixCurrentProblems to reset current state of tasks kept in a state machine extended state variables. Handler method continueFromError can then be used to instruct state machine to transition from ERROR state back to READY state where tasks can be executed again.

TasksHandler handler = TasksHandler.builder()
        .task("1", sleepRunnable())
        .task("2", sleepRunnable())
        .task("3", sleepRunnable())
        .build();

        handler.runTasks();
        handler.fixCurrentProblems();
        handler.continueFromError();

Turnstile is a simple device which gives you an access if payment is made and is a very simple to model using a state machine. In its simplest form there are only two states, LOCKED and UNLOCKED. Two events, COIN and PUSH can happen if you try to go through it or you make a payment.

States. 

public static enum States {
    LOCKED, UNLOCKED
}

Events. 

public static enum Events {
    COIN, PUSH
}

Configuration. 

@Configuration
@EnableStateMachine
static class StateMachineConfig
        extends EnumStateMachineConfigurerAdapter<States, Events> {

    @Override
    public void configure(StateMachineStateConfigurer<States, Events> states)
            throws Exception {
        states
            .withStates()
                .initial(States.LOCKED)
                .states(EnumSet.allOf(States.class));
    }

    @Override
    public void configure(StateMachineTransitionConfigurer<States, Events> transitions)
            throws Exception {
        transitions
            .withExternal()
                .source(States.LOCKED)
                .target(States.UNLOCKED)
                .event(Events.COIN)
                .and()
            .withExternal()
                .source(States.UNLOCKED)
                .target(States.LOCKED)
                .event(Events.PUSH);
    }

}

You can see how this sample state machine interacts with event by running turnstile sample.

$ java -jar spring-statemachine-samples-turnstile-1.0.0.BUILD-SNAPSHOT.jar

sm>sm print
+----------------------------------------------------------------+
|                              SM                                |
+----------------------------------------------------------------+
|                                                                |
|         +----------------+          +----------------+         |
|     *-->|     LOCKED     |          |    UNLOCKED    |         |
|         +----------------+          +----------------+         |
|     +---| entry/         |          | entry/         |---+     |
|     |   | exit/          |          | exit/          |   |     |
|     |   |                |          |                |   |     |
| PUSH|   |                |---COIN-->|                |   |COIN |
|     |   |                |          |                |   |     |
|     |   |                |          |                |   |     |
|     |   |                |<--PUSH---|                |   |     |
|     +-->|                |          |                |<--+     |
|         |                |          |                |         |
|         +----------------+          +----------------+         |
|                                                                |
+----------------------------------------------------------------+

sm>sm start
State changed to LOCKED
State machine started

sm>sm event COIN
State changed to UNLOCKED
Event COIN send

sm>sm event PUSH
State changed to LOCKED
Event PUSH send

Showcase is a complex state machine showing all possible transition topologies up to four levels of state nesting.

States. 

public static enum States {
    S0, S1, S11, S12, S2, S21, S211, S212
}

Events. 

public static enum Events {
    A, B, C, D, E, F, G, H, I
}

Configuration - states. 

@Override
public void configure(StateMachineStateConfigurer<States, Events> states)
        throws Exception {
    states
        .withStates()
            .initial(States.S0, fooAction())
            .state(States.S0)
            .and()
            .withStates()
                .parent(States.S0)
                .initial(States.S1)
                .state(States.S1)
                .and()
                .withStates()
                    .parent(States.S1)
                    .initial(States.S11)
                    .state(States.S11)
                    .state(States.S12)
                    .and()
            .withStates()
                .parent(States.S0)
                .state(States.S2)
                .and()
                .withStates()
                    .parent(States.S2)
                    .initial(States.S21)
                    .state(States.S21)
                    .and()
                    .withStates()
                        .parent(States.S21)
                        .initial(States.S211)
                        .state(States.S211)
                        .state(States.S212);
}

Configuration - transitions. 

@Override
public void configure(StateMachineTransitionConfigurer<States, Events> transitions)
        throws Exception {
    transitions
        .withExternal()
            .source(States.S1).target(States.S1).event(Events.A)
            .guard(foo1Guard())
            .and()
        .withExternal()
            .source(States.S1).target(States.S11).event(Events.B)
            .and()
        .withExternal()
            .source(States.S21).target(States.S211).event(Events.B)
            .and()
        .withExternal()
            .source(States.S1).target(States.S2).event(Events.C)
            .and()
        .withExternal()
            .source(States.S2).target(States.S1).event(Events.C)
            .and()
        .withExternal()
            .source(States.S1).target(States.S0).event(Events.D)
            .and()
        .withExternal()
            .source(States.S211).target(States.S21).event(Events.D)
            .and()
        .withExternal()
            .source(States.S0).target(States.S211).event(Events.E)
            .and()
        .withExternal()
            .source(States.S1).target(States.S211).event(Events.F)
            .and()
        .withExternal()
            .source(States.S2).target(States.S11).event(Events.F)
            .and()
        .withExternal()
            .source(States.S11).target(States.S211).event(Events.G)
            .and()
        .withExternal()
            .source(States.S211).target(States.S0).event(Events.G)
            .and()
        .withInternal()
            .source(States.S0).event(Events.H)
            .guard(foo0Guard())
            .action(fooAction())
            .and()
        .withInternal()
            .source(States.S2).event(Events.H)
            .guard(foo1Guard())
            .action(fooAction())
            .and()
        .withInternal()
            .source(States.S1).event(Events.H)
            .and()
        .withExternal()
            .source(States.S11).target(States.S12).event(Events.I)
            .and()
        .withExternal()
            .source(States.S211).target(States.S212).event(Events.I)
            .and()
        .withExternal()
            .source(States.S12).target(States.S212).event(Events.I);

}

Configuration - actions and guard. 

@Bean
public FooGuard foo0Guard() {
    return new FooGuard(0);
}

@Bean
public FooGuard foo1Guard() {
    return new FooGuard(1);
}

@Bean
public FooAction fooAction() {
    return new FooAction();
}

Action. 

private static class FooAction implements Action<States, Events> {

    @Override
    public void execute(StateContext<States, Events> context) {
        Map<Object, Object> variables = context.getExtendedState().getVariables();
        Integer foo = context.getExtendedState().get("foo", Integer.class);
        if (foo == null) {
            log.info("Init foo to 0");
            variables.put("foo", 0);
        } else if (foo == 0) {
            log.info("Switch foo to 1");
            variables.put("foo", 1);
        } else if (foo == 1) {
            log.info("Switch foo to 0");
            variables.put("foo", 0);
        }
    }
}

Guard. 

private static class FooGuard implements Guard<States, Events> {

    private final int match;

    public FooGuard(int match) {
        this.match = match;
    }

    @Override
    public boolean evaluate(StateContext<States, Events> context) {
        Object foo = context.getExtendedState().getVariables().get("foo");
        return !(foo == null || !foo.equals(match));
    }
}

Lets go through what this state machine do when it’s executed and we send various event to it.

sm>sm start
Entry state S0
Entry state S1
Entry state S11
Init foo to 0
State machine started

sm>sm event A
Event A send

sm>sm event C
Exit state S11
Exit state S1
Entry state S2
Entry state S21
Entry state S211
Event C send

sm>sm event H
Switch foo to 1
Event H send

sm>sm event C
Exit state S211
Exit state S21
Exit state S2
Entry state S1
Entry state S11
Event C send

sm>sm event A
Exit state S11
Exit state S1
Entry state S1
Entry state S11
Event A send

What happens in above sample:

CD Player is a sample which resembles better use case of most of use have used in a real world. CD Player itself is a really simple entity where user can open a deck, insert or change a disk, then drive player functionality by pressing various buttons like eject, play, stop, pause, rewind and backward.

How many of use have really given a thought of what it will take to make a code for a CD Player which interacts with a hardware. Yes, concept of a player is overly simple but if you look behind a scenes things actually get a bit convoluted.

You’ve probably noticed that if your deck is open and you press play, deck will close and a song will start to play if CD was inserted in a first place. In a sense when deck is open you first need to close it and then try to start playing if cd is actually inserted. Hopefully you have now realised that a simple CD Player is not anymore so simple. Sure you can wrap all this with a simple class with few boolean variables and probably few nested if/else clauses, that will do the job, but what about if you need to make all this behaviour much more complex, do you really want to keep adding more flags and if/else clauses.

Lets go through how this sample and its state machine is designed and how those two interacts with each other. Below three config sections are used withing a EnumStateMachineConfigurerAdapter.

@Override
public void configure(StateMachineStateConfigurer<States, Events> states)
        throws Exception {
    states
        .withStates()
            .initial(States.IDLE)
            .state(States.IDLE)
            .and()
            .withStates()
                .parent(States.IDLE)
                .initial(States.CLOSED)
                .state(States.CLOSED, closedEntryAction(), null)
                .state(States.OPEN)
                .and()
        .withStates()
            .state(States.BUSY)
            .and()
            .withStates()
                .parent(States.BUSY)
                .initial(States.PLAYING)
                .state(States.PLAYING)
                .state(States.PAUSED);

}

@Override
public void configure(StateMachineTransitionConfigurer<States, Events> transitions)
        throws Exception {
    transitions
        .withExternal()
            .source(States.CLOSED).target(States.OPEN).event(Events.EJECT)
            .and()
        .withExternal()
            .source(States.OPEN).target(States.CLOSED).event(Events.EJECT)
            .and()
        .withExternal()
            .source(States.OPEN).target(States.CLOSED).event(Events.PLAY)
            .and()
        .withExternal()
            .source(States.PLAYING).target(States.PAUSED).event(Events.PAUSE)
            .and()
        .withInternal()
            .source(States.PLAYING)
            .action(playingAction())
            .timer(1000)
            .and()
        .withInternal()
            .source(States.PLAYING).event(Events.BACK)
            .action(trackAction())
            .and()
        .withInternal()
            .source(States.PLAYING).event(Events.FORWARD)
            .action(trackAction())
            .and()
        .withExternal()
            .source(States.PAUSED).target(States.PLAYING).event(Events.PAUSE)
            .and()
        .withExternal()
            .source(States.BUSY).target(States.IDLE).event(Events.STOP)
            .and()
        .withExternal()
            .source(States.IDLE).target(States.BUSY).event(Events.PLAY)
            .action(playAction())
            .guard(playGuard())
            .and()
        .withInternal()
            .source(States.OPEN).event(Events.LOAD).action(loadAction());
}

@Bean
public ClosedEntryAction closedEntryAction() {
    return new ClosedEntryAction();
}

@Bean
public LoadAction loadAction() {
    return new LoadAction();
}

@Bean
public TrackAction trackAction() {
    return new TrackAction();
}

@Bean
public PlayAction playAction() {
    return new PlayAction();
}

@Bean
public PlayingAction playingAction() {
    return new PlayingAction();
}

@Bean
public PlayGuard playGuard() {
    return new PlayGuard();
}

What we did in above configuration:

This machine only have six states which are introduced as an enum.

public static enum States {
    // super state of PLAYING and PAUSED
    BUSY,
    PLAYING,
    PAUSED,
    // super state of CLOSED and OPEN
    IDLE,
    CLOSED,
    OPEN
}

Events represent, in a sense in this example, what buttons user would press and if user loads a cd disc into a deck.

public static enum Events {
    PLAY, STOP, PAUSE, EJECT, LOAD, FORWARD, BACK
}

Beans cdPlayer and library are just used with a sample to drive the application.

@Bean
public CdPlayer cdPlayer() {
    return new CdPlayer();
}

@Bean
public Library library() {
    return Library.buildSampleLibrary();
}

We can define extended state variable key as simple enums.

public static enum Variables {
    CD, TRACK, ELAPSEDTIME
}

public static enum Headers {
    TRACKSHIFT
}

We wanted to make this samply type safe so we’re defining our own annotation @StatesOnTransition which have a mandatory meta annotation @OnTransition.

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
@OnTransition
public static @interface StatesOnTransition {

    States[] source() default {};

    States[] target() default {};

}

ClosedEntryAction is a entry action for state CLOSED to simply send and PLAY event to a statemachine if cd disc is present.

public static class ClosedEntryAction implements Action<States, Events> {

    @Override
    public void execute(StateContext<States, Events> context) {
        if (context.getTransition() != null
                && context.getEvent() == Events.PLAY
                && context.getTransition().getTarget().getId() == States.CLOSED
                && context.getExtendedState().getVariables().get(Variables.CD) != null) {
            context.getStateMachine().sendEvent(Events.PLAY);
        }
    }
}

LoadAction is simply updating extended state variable if event headers contained information about a cd disc to load.

public static class LoadAction implements Action<States, Events> {

    @Override
    public void execute(StateContext<States, Events> context) {
        Object cd = context.getMessageHeader(Variables.CD);
        context.getExtendedState().getVariables().put(Variables.CD, cd);
    }
}

PlayAction is simply resetting player elapsed time which is kept as an extended state variable.

public static class PlayAction implements Action<States, Events> {

    @Override
    public void execute(StateContext<States, Events> context) {
        context.getExtendedState().getVariables().put(Variables.ELAPSEDTIME, 0l);
        context.getExtendedState().getVariables().put(Variables.TRACK, 0);
    }
}

PlayGuard is used to guard transition from IDLE to BUSY with event PLAY if extended state variable CD doesn’t indicate that cd disc has been loaded.

public static class PlayGuard implements Guard<States, Events> {

    @Override
    public boolean evaluate(StateContext<States, Events> context) {
        ExtendedState extendedState = context.getExtendedState();
        return extendedState.getVariables().get(Variables.CD) != null;
    }
}

PlayingAction is updating extended state variable ELAPSEDTIME which cd player itself can read and update lcd status. Action also handles track shift if user is going back or forward in tracks.

public static class PlayingAction implements Action<States, Events> {

    @Override
    public void execute(StateContext<States, Events> context) {
        Map<Object, Object> variables = context.getExtendedState().getVariables();
        Object elapsed = variables.get(Variables.ELAPSEDTIME);
        Object cd = variables.get(Variables.CD);
        Object track = variables.get(Variables.TRACK);
        if (elapsed instanceof Long) {
            long e = ((Long)elapsed) + 1000l;
            if (e > ((Cd) cd).getTracks()[((Integer) track)].getLength()*1000) {
                context.getStateMachine().sendEvent(MessageBuilder
                        .withPayload(Events.FORWARD)
                        .setHeader(Headers.TRACKSHIFT.toString(), 1).build());
            } else {
                variables.put(Variables.ELAPSEDTIME, e);
            }
        }
    }
}

TrackAction handles track shift action if user is going back or forward in tracks. If it is a last track of a cd, playing is stopped and STOP event sent to a state machine.

public static class TrackAction implements Action<States, Events> {

    @Override
    public void execute(StateContext<States, Events> context) {
        Map<Object, Object> variables = context.getExtendedState().getVariables();
        Object trackshift = context.getMessageHeader(Headers.TRACKSHIFT.toString());
        Object track = variables.get(Variables.TRACK);
        Object cd = variables.get(Variables.CD);
        if (trackshift instanceof Integer && track instanceof Integer && cd instanceof Cd) {
            int next = ((Integer)track) + ((Integer)trackshift);
            if (next >= 0 &&  ((Cd)cd).getTracks().length > next) {
                variables.put(Variables.ELAPSEDTIME, 0l);
                variables.put(Variables.TRACK, next);
            } else if (((Cd)cd).getTracks().length <= next) {
                context.getStateMachine().sendEvent(Events.STOP);
            }
        }
    }
}

One other important aspect of a state machines is that they have their own responsibilies mostly around handling states and all application level logic should be kept outside. This means that application needs to have a ways to interact with a state machine and below sample is how cdplayer does it order to update lcd status. Also pay attention that we annotated CdPlayer with @WithStateMachine which instructs state machine to find methods from your pojo which are then called with various transitions.

@OnTransition(target = "BUSY")
public void busy(ExtendedState extendedState) {
    Object cd = extendedState.getVariables().get(Variables.CD);
    if (cd != null) {
        cdStatus = ((Cd)cd).getName();
    }
}

In above example we use @OnTransition annotation to hook a callback when transition happens with a target state BUSY.

@StatesOnTransition(target = {States.CLOSED, States.IDLE})
public void closed(ExtendedState extendedState) {
    Object cd = extendedState.getVariables().get(Variables.CD);
    if (cd != null) {
        cdStatus = ((Cd)cd).getName();
    } else {
        cdStatus = "No CD";
    }
    trackStatus = "";
}

@OnTransition we used above can only be used with strings which are matched from enums. @StatesOnTransition is then something what user can create into his own application to get a type safe annotation where a real enums can be used.

Lets see an example how this state machine actually works.

sm>sm start
Entry state IDLE
Entry state CLOSED
State machine started

sm>cd lcd
No CD

sm>cd library
0: Greatest Hits
  0: Bohemian Rhapsody  05:56
  1: Another One Bites the Dust  03:36
1: Greatest Hits II
  0: A Kind of Magic  04:22
  1: Under Pressure  04:08

sm>cd eject
Exit state CLOSED
Entry state OPEN

sm>cd load 0
Loading cd Greatest Hits

sm>cd play
Exit state OPEN
Entry state CLOSED
Exit state CLOSED
Exit state IDLE
Entry state BUSY
Entry state PLAYING

sm>cd lcd
Greatest Hits Bohemian Rhapsody 00:03

sm>cd forward

sm>cd lcd
Greatest Hits Another One Bites the Dust 00:04

sm>cd stop
Exit state PLAYING
Exit state BUSY
Entry state IDLE
Entry state CLOSED

sm>cd lcd
Greatest Hits

What happened in above run:

Tasks is a sample demonstrating a parallel task handling within a regions and additionally adds an error handling to either automatically or manually fixing task problems before continuing back to a state where tasks can be run again.

On a high level what happens in this state machine is:

States. 

public static enum States {
    READY,
    FORK, JOIN, CHOICE,
    TASKS, T1, T1E, T2, T2E, T3, T3E,
    ERROR, AUTOMATIC, MANUAL
}

Events. 

public static enum Events {
    RUN, FALLBACK, CONTINUE, FIX;
}

Configuration - states. 

@Override
public void configure(StateMachineStateConfigurer<States, Events> states)
        throws Exception {
    states
        .withStates()
            .initial(States.READY)
            .fork(States.FORK)
            .state(States.TASKS)
            .join(States.JOIN)
            .choice(States.CHOICE)
            .state(States.ERROR)
            .and()
            .withStates()
                .parent(States.TASKS)
                .initial(States.T1)
                .end(States.T1E)
                .and()
            .withStates()
                .parent(States.TASKS)
                .initial(States.T2)
                .end(States.T2E)
                .and()
            .withStates()
                .parent(States.TASKS)
                .initial(States.T3)
                .end(States.T3E)
                .and()
            .withStates()
                .parent(States.ERROR)
                .initial(States.AUTOMATIC)
                .state(States.AUTOMATIC, automaticAction(), null)
                .state(States.MANUAL);
}

Configuration - transitions. 

@Override
public void configure(StateMachineTransitionConfigurer<States, Events> transitions)
        throws Exception {
    transitions
        .withExternal()
            .source(States.READY).target(States.FORK)
            .event(Events.RUN)
            .and()
        .withFork()
            .source(States.FORK).target(States.TASKS)
            .and()
        .withExternal()
            .source(States.T1).target(States.T1E)
            .and()
        .withExternal()
            .source(States.T2).target(States.T2E)
            .and()
        .withExternal()
            .source(States.T3).target(States.T3E)
            .and()
        .withJoin()
            .source(States.TASKS).target(States.JOIN)
            .and()
        .withExternal()
            .source(States.JOIN).target(States.CHOICE)
            .and()
        .withChoice()
            .source(States.CHOICE)
            .first(States.ERROR, tasksChoiceGuard())
            .last(States.READY)
            .and()
        .withExternal()
            .source(States.ERROR).target(States.READY)
            .event(Events.CONTINUE)
            .and()
        .withExternal()
            .source(States.AUTOMATIC).target(States.MANUAL)
            .event(Events.FALLBACK)
            .and()
        .withInternal()
            .source(States.MANUAL)
            .action(fixAction())
            .event(Events.FIX);
}

Guard below is guarding choice entry into a ERROR state and needs to return TRUE if error has happened. For this guard simply checks that all extended state variables(T1, T2 and T3) are TRUE.

@Bean
public Guard<States, Events> tasksChoiceGuard() {
    return new Guard<States, Events>() {

        @Override
        public boolean evaluate(StateContext<States, Events> context) {
            Map<Object, Object> variables = context.getExtendedState().getVariables();
            return !(ObjectUtils.nullSafeEquals(variables.get("T1"), true)
                    && ObjectUtils.nullSafeEquals(variables.get("T2"), true)
                    && ObjectUtils.nullSafeEquals(variables.get("T3"), true));
        }
    };
}

Actions below will simply send event to a state machine to request next step which would be either fallback or continue back to ready.

@Bean
public Action<States, Events> automaticAction() {
    return new Action<States, Events>() {

        @Override
        public void execute(StateContext<States, Events> context) {
            Map<Object, Object> variables = context.getExtendedState().getVariables();
            if (ObjectUtils.nullSafeEquals(variables.get("T1"), true)
                    && ObjectUtils.nullSafeEquals(variables.get("T2"), true)
                    && ObjectUtils.nullSafeEquals(variables.get("T3"), true)) {
                context.getStateMachine().sendEvent(Events.CONTINUE);
            } else {
                context.getStateMachine().sendEvent(Events.FALLBACK);
            }
        }
    };
}

@Bean
public Action<States, Events> fixAction() {
    return new Action<States, Events>() {

        @Override
        public void execute(StateContext<States, Events> context) {
            Map<Object, Object> variables = context.getExtendedState().getVariables();
            variables.put("T1", true);
            variables.put("T2", true);
            variables.put("T3", true);
            context.getStateMachine().sendEvent(Events.CONTINUE);
        }
    };
}

Currently default region execution is synchronous but it can be changed to asynchronous by changing TaskExecutor. Task will simulate work by sleeping 2 seconds so you’ll able to see how actions in regions are executed parallel.

@Bean
public TaskExecutor taskExecutor() {
    ThreadPoolTaskExecutor taskExecutor = new ThreadPoolTaskExecutor();
    taskExecutor.setCorePoolSize(5);
    return taskExecutor;
}

Lets see an examples how this state machine actually works.

sm>sm start
State machine started
Entry state READY

sm>tasks run
Entry state TASKS
run task on T3
run task on T2
run task on T1
run task on T2 done
run task on T1 done
run task on T3 done
Entry state T2
Entry state T3
Entry state T1
Entry state T1E
Entry state T2E
Entry state T3E
Exit state TASKS
Entry state JOIN
Exit state JOIN
Entry state READY

In above we can execute tasks multiple times.

sm>tasks list
Tasks {T1=true, T3=true, T2=true}

sm>tasks fail T1

sm>tasks list
Tasks {T1=false, T3=true, T2=true}

sm>tasks run
Entry state TASKS
run task on T1
run task on T3
run task on T2
run task on T1 done
run task on T3 done
run task on T2 done
Entry state T1
Entry state T3
Entry state T2
Entry state T1E
Entry state T2E
Entry state T3E
Exit state TASKS
Entry state JOIN
Exit state JOIN
Entry state ERROR
Entry state AUTOMATIC
Exit state AUTOMATIC
Exit state ERROR
Entry state READY

In above, if we simulate failure for task T1, it is fixed automatically.

sm>tasks list
Tasks {T1=true, T3=true, T2=true}

sm>tasks fail T2

sm>tasks run
Entry state TASKS
run task on T2
run task on T1
run task on T3
run task on T2 done
run task on T1 done
run task on T3 done
Entry state T2
Entry state T1
Entry state T3
Entry state T1E
Entry state T2E
Entry state T3E
Exit state TASKS
Entry state JOIN
Exit state JOIN
Entry state ERROR
Entry state AUTOMATIC
Exit state AUTOMATIC
Entry state MANUAL

sm>tasks fix
Exit state MANUAL
Exit state ERROR
Entry state READY

In above if we simulate failure for either task T2 or T3, state machine goes to MANUAL state where problem needs to be fixed manually before we’re able to go back to READY state.

Washer is a sample demonstrating a use of a history state to recover a running state configuration with a simulated power off situation.

Anyone ever used a washing machine knows that if you can somehow pause the program it will continue from a same state when lid is closed. This kind of behaviour can be implemented in a state machine by using a history pseudo state.

States. 

public static enum States {
    RUNNING, HISTORY, END,
    WASHING, RINSING, DRYING,
    POWEROFF
}

Events. 

public static enum Events {
    RINSE, DRY, STOP,
    RESTOREPOWER, CUTPOWER
}

Configuration - states. 

@Override
public void configure(StateMachineStateConfigurer<States, Events> states)
        throws Exception {
    states
        .withStates()
            .initial(States.RUNNING)
            .state(States.POWEROFF)
            .end(States.END)
            .and()
            .withStates()
                .parent(States.RUNNING)
                .initial(States.WASHING)
                .state(States.RINSING)
                .state(States.DRYING)
                .history(States.HISTORY, History.SHALLOW);
}

Configuration - transitions. 

@Override
public void configure(StateMachineTransitionConfigurer<States, Events> transitions)
        throws Exception {
    transitions
        .withExternal()
            .source(States.WASHING).target(States.RINSING)
            .event(Events.RINSE)
            .and()
        .withExternal()
            .source(States.RINSING).target(States.DRYING)
            .event(Events.DRY)
            .and()
        .withExternal()
            .source(States.RUNNING)    .target(States.POWEROFF)
            .event(Events.CUTPOWER)
            .and()
        .withExternal()
            .source(States.POWEROFF).target(States.HISTORY)
            .event(Events.RESTOREPOWER)
            .and()
        .withExternal()
            .source(States.RUNNING).target(States.END)
            .event(Events.STOP);
}

Lets see an example how this state machine actually works.

sm>sm start
Entry state RUNNING
Entry state WASHING
State machine started

sm>sm event RINSE
Exit state WASHING
Entry state RINSING
Event RINSE send

sm>sm event DRY
Exit state RINSING
Entry state DRYING
Event DRY send

sm>sm event CUTPOWER
Exit state DRYING
Exit state RUNNING
Entry state POWEROFF
Event CUTPOWER send

sm>sm event RESTOREPOWER
Exit state POWEROFF
Entry state RUNNING
Entry state WASHING
Entry state DRYING
Event RESTOREPOWER send

What happened in above run:

Persist is a sample using recipe Chapter 17, Persist to demonstate how a database entry update logic can be controlled by a state machine.

The state machine logic and configuration is shown above:

StateMachine Config. 

@Configuration
@EnableStateMachine
static class StateMachineConfig
        extends StateMachineConfigurerAdapter<String, String> {

    @Override
    public void configure(StateMachineStateConfigurer<String, String> states)
            throws Exception {
        states
            .withStates()
                .initial("PLACED")
                .state("PROCESSING")
                .state("SENT")
                .state("DELIVERED");
    }

    @Override
    public void configure(StateMachineTransitionConfigurer<String, String> transitions)
            throws Exception {
        transitions
            .withExternal()
                .source("PLACED").target("PROCESSING")
                .event("PROCESS")
                .and()
            .withExternal()
                .source("PROCESSING").target("SENT")
                .event("SEND")
                .and()
            .withExternal()
                .source("SENT").target("DELIVERED")
                .event("DELIVER");
    }

}

PersistStateMachineHandler can be created using a below config:

Handler Config. 

@Configuration
static class PersistHandlerConfig {

    @Autowired
    private StateMachine<String, String> stateMachine;

    @Bean
    public Persist persist() {
        return new Persist(persistStateMachineHandler());
    }

    @Bean
    public PersistStateMachineHandler persistStateMachineHandler() {
        return new PersistStateMachineHandler(stateMachine);
    }

}

Order class used with this sample is shown below:

Order Class. 

public static class Order {
    int id;
    String state;

    public Order(int id, String state) {
        this.id = id;
        this.state = state;
    }

    @Override
    public String toString() {
        return "Order [id=" + id + ", state=" + state + "]";
    }

}

Now let’s see how this example works.

sm>persist db
Order [id=1, state=PLACED]
Order [id=2, state=PROCESSING]
Order [id=3, state=SENT]
Order [id=4, state=DELIVERED]

sm>persist process 1
Exit state PLACED
Entry state PROCESSING

sm>persist db
Order [id=2, state=PROCESSING]
Order [id=3, state=SENT]
Order [id=4, state=DELIVERED]
Order [id=1, state=PROCESSING]

sm>persist deliver 3
Exit state SENT
Entry state DELIVERED

sm>persist db
Order [id=2, state=PROCESSING]
Order [id=4, state=DELIVERED]
Order [id=1, state=PROCESSING]
Order [id=3, state=DELIVERED]

What happened in above run:

	Note
	If you’re wondering where is the database because there are literally no signs of it in a sample code. Sample is based on Spring Boot and because necessary classes are in a classpath, embedded HSQL instance is created automatically. Spring Boot will even create an instance of JdbcTemplate which you can just autowire like how it’s done in Persist.java. @Autowired private JdbcTemplate jdbcTemplate;

Finally we need to handle state changes:

public void change(int order, String event) {
    Order o = jdbcTemplate.queryForObject("select id, state from orders where id = ?", new Object[]{order}, new RowMapper<Order>() {
        public Order mapRow(ResultSet rs, int rowNum) throws SQLException {
            return new Order(rs.getInt("id"), rs.getString("state"));
        }
    });
    handler.handleEventWithState(MessageBuilder.withPayload(event).setHeader("order", order).build(), o.state);
}

And use a PersistStateChangeListener to update database:

private class LocalPersistStateChangeListener implements PersistStateChangeListener {

    @Override
    public void onPersist(State<String, String> state, Message<String> message,
            Transition<String, String> transition, StateMachine<String, String> stateMachine) {
        if (message != null && message.getHeaders().containsKey("order")) {
            Integer order = message.getHeaders().get("order", Integer.class);
            jdbcTemplate.update("update orders set state = ? where id = ?", state.getId(), order);
        }
    }
}

Zookeeper is a distributed version from sample Chapter 19, Turnstile.

	Note
	This sample needs and external Zookeeper instance accessible from localhost with default port and settings.

Configuration of this sample is almost same as turnstile sample. We only add configuration for distributed state machine where we configure StateMachineEnsemble.

@Override
public void configure(StateMachineConfigurationConfigurer<String, String> config) throws Exception {
    config
        .withDistributed()
            .ensemble(stateMachineEnsemble());
}

Actual StateMachineEnsemble needs to be created as bean together with CuratorFramework client.

@Bean
public StateMachineEnsemble<String, String> stateMachineEnsemble() throws Exception {
    return new ZookeeperStateMachineEnsemble<String, String>(curatorClient(), "/foo");
}

@Bean
public CuratorFramework curatorClient() throws Exception {
    CuratorFramework client = CuratorFrameworkFactory.builder().defaultData(new byte[0])
            .retryPolicy(new ExponentialBackoffRetry(1000, 3))
            .connectString("localhost:2181").build();
    client.start();
    return client;
}

Lets go through a simple example where two different shell instances are started with command java -jar spring-statemachine-samples-zookeeper-1.0.0.BUILD-SNAPSHOT.jar.

First open first shell instance(do not start second instance yet). When state machine is started it will end up into its initial state LOCKED. Then send event COIN to transit into UNLOCKED state.

Shell1. 

sm>sm start
Entry state LOCKED
State machine started

sm>sm event COIN
Exit state LOCKED
Entry state UNLOCKED
Event COIN send

sm>sm state
UNLOCKED

Open second shell instance and start a state machine. You should see that distributed state UNLOCKED is entered instead of default initial state LOCKED.

Shell2. 

sm>sm start
State machine started

sm>sm state
UNLOCKED

Then from either of a shells(we use second instance here) send event PUSH to transit from UNLOCKED into LOCKED state.

Shell2. 

sm>sm event PUSH
Exit state UNLOCKED
Entry state LOCKED
Event PUSH send

In other shell you should see state getting changed automatically based on distributed state kept in Zookeeper.

Shell1. 

sm>Exit state UNLOCKED
Entry state LOCKED

Web is a distributed state machine example using a zookeeper to handle distributed state. This example is meant to be run on a multiple browser sessions against a multiple different hosts.

This sample is using a modified state machine structure from a Chapter 20, Showcase to work with a distributed state machine.

	Note
	Due to nature of this sample an instanse of a Zookeeper is expected to be available from a localhost for every individual sample instance.

Lets go through a simple example where three different sample instances are started with command java -jar spring-statemachine-samples-web-1.0.0.BUILD-SNAPSHOT.jar. If you are running different instances on a same host you need to distinguish used port by adding --server.port=<myport> to the command. Otherwise default port for each host will be 8080.

In this sample run we have three hosts, n1, n2 and n3 which all have a local zookeeper instance running and a state machine sample running on a port 8080.

@n1:~# java -jar spring-statemachine-samples-web-1.0.0.BUILD-SNAPSHOT.jar
@n2:~# java -jar spring-statemachine-samples-web-1.0.0.BUILD-SNAPSHOT.jar
@n3:~# java -jar spring-statemachine-samples-web-1.0.0.BUILD-SNAPSHOT.jar

When all instances are running you should see all showing similar information via a browser where states are S0, S1 and S11, and extended state variable foo=0. Main state is S11.

When you press button Event C in any of a browser window, distributed state is changed to S211 which is the target state denoted by transition associated with an event C.

Then lets press button Event H and what is supposed to happen is that internal transition is executed on all state machines changing extended stare variable foo from value 0 to 1. This change is first done on a state machine receiving the event and then propagated to other state machines. You should only see variable foo to change from 0 to 1.

Last we simply send an event Event C which is supposed to take state machine state back to state S11 and you should see this happening in all browser sessions.

I want to transit to next state automatically.  ​

There are few choices a state machine developer can choose.

How I can initialise variables on state machine start.  ​

Important concept in a state machine is that nothing really happens unless there is a trigger which is causing a state transition which then can fire actions. However, having said that, Spring Statemachine always have an initial transition when state machine is started. With this initial transition user can execute a simple action which within a StateContext can do whatever it likes with an extended state variables.