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:

Spring Statemachine 1.1.0.M1 is built and tested with JDK 8(all artifacts have JDK 7 compatibility) and Spring Framework 4.2.4.RELEASE and doesn’t require any other dependencies outside of Spring Framework within its core system.

Other optional parts like Chapter 25, Using Distributed States has dependencies to a Zookeeper, while Part VI, “State Machine Examples” has dependencies to spring-shell and spring-boot which pulls other dependencies beyond framework itself.

The following modules are available for Spring Statemachine.

Here is a typical build.gradle file:

buildscript {
    repositories {
        maven { url "http://repo.spring.io/libs-release" }
    }
    dependencies {
        classpath("org.springframework.boot:spring-boot-gradle-plugin:1.2.5.RELEASE")
    }
}

apply plugin: 'base'
apply plugin: 'java'
apply plugin: 'eclipse'
apply plugin: 'idea'
apply plugin: 'spring-boot'
version =  '0.1.0'
archivesBaseName = 'gs-statemachine'

repositories {
    mavenCentral()
    maven { url "http://repo.spring.io/libs-release" }
    maven { url "http://repo.spring.io/libs-milestone" }
    maven { url "http://repo.spring.io/libs-snapshot" }
}

dependencies {
    compile("org.springframework.statemachine:spring-statemachine-core:1.0.0.BUILD-SNAPSHOT")
    compile("org.springframework.boot:spring-boot-starter:1.2.5.RELEASE")
    testCompile("org.springframework.statemachine:spring-statemachine-test:1.0.0.BUILD-SNAPSHOT")
}

task wrapper(type: Wrapper) {
    gradleVersion = '1.11'
}

	Note
	Replace 1.0.0.BUILD-SNAPSHOT with a version you want to use.

Having a normal project structure you’d build this with command:

# ./gradlew clean build

Expected Spring Boot packaged fat-jar would be build/libs/gs-statemachine-0.1.0.jar.

	Note
	You don’t need repos libs-milestone and libs-snapshot for production development.

Here is a typical pom.xml file:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>org.springframework</groupId>
    <artifactId>gs-statemachine</artifactId>
    <version>0.1.0</version>
    <packaging>jar</packaging>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>1.2.5.RELEASE</version>
    </parent>

    <dependencies>
        <dependency>
            <groupId>org.springframework.statemachine</groupId>
            <artifactId>spring-statemachine-core</artifactId>
            <version>1.0.0.BUILD-SNAPSHOT</version>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
            <version>1.0.0.BUILD-SNAPSHOT</version>
        </dependency>
        <dependency>
            <groupId>org.springframework.statemachine</groupId>
            <artifactId>spring-statemachine-test</artifactId>
            <version>1.0.0.BUILD-SNAPSHOT</version>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>2.3.2</version>
            </plugin>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
            <plugin>
                <artifactId>maven-failsafe-plugin</artifactId>
                <executions>
                    <execution>
                        <phase>package</phase>
                        <goals>
                            <goal>integration-test</goal>
                            <goal>verify</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>

    <repositories>
        <repository>
            <id>spring-release</id>
            <url>http://repo.spring.io/libs-release</url>
            <snapshots><enabled>false</enabled></snapshots>
        </repository>
        <repository>
            <id>spring-milestone</id>
            <url>http://repo.spring.io/libs-milestone</url>
            <snapshots><enabled>false</enabled></snapshots>
        </repository>
        <repository>
            <id>spring-snapshot</id>
            <url>http://repo.spring.io/libs-snapshot</url>
            <snapshots><enabled>true</enabled></snapshots>
        </repository>
    </repositories>

    <pluginRepositories>
        <pluginRepository>
            <id>spring-release</id>
            <url>http://repo.spring.io/libs-release</url>
            <snapshots><enabled>false</enabled></snapshots>
        </pluginRepository>
    </pluginRepositories>

</project>

	Note
	Replace 1.0.0.BUILD-SNAPSHOT with a version you want to use.

Having a normal project structure you’d build this with command:

# mvn clean package

Expected Spring Boot packaged fat-jar would be target/gs-statemachine-0.1.0.jar.

	Note
	You don’t need repos libs-milestone and libs-snapshot for production development.

Let’s start by creating a simple Spring Boot Application class implementing CommandLineRunner.

@SpringBootApplication
public class Application implements CommandLineRunner {

    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }

}

Add states and events:

public enum States {
    SI, S1, S2
}

public enum Events {
    E1, E2
}

Add state machine configuration:

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

    @Override
    public void configure(StateMachineConfigurationConfigurer<States, Events> config)
            throws Exception {
        config
            .withConfiguration()
                .autoStartup(true)
                .listener(listener());
    }

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

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

    @Bean
    public StateMachineListener<States, Events> listener() {
        return new StateMachineListenerAdapter<States, Events>() {
            @Override
            public void stateChanged(State<States, Events> from, State<States, Events> to) {
                System.out.println("State change to " + to.getId());
            }
        };
    }
}

Implement CommandLineRunner, autowire StateMachine:

@Autowired
private StateMachine<States, Events> stateMachine;

@Override
public void run(String... args) throws Exception {
    stateMachine.sendEvent(Events.E1);
    stateMachine.sendEvent(Events.E2);
}

Depending whether you build your application using Gradle or Maven it’s run java -jar build/libs/gs-statemachine-0.1.0.jar or java -jar target/gs-statemachine-0.1.0.jar respectively.

What is expected for running this command is a normal Spring Boot output but if you look closely you see lines:

State change to SI
State change to S1
State change to S2

Spring Statemachine 1.1 is focusing on security and a better interoperability with web applications.

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.

9.1 Using enable annotations

We use familiar spring enabler annotations to ease configuration. Two annotations exists, @EnableStateMachine and @EnableStateMachineFactory. These annontations if placed in a @Configuration class will enable some basic functionality needed by a state machines.

@EnableStateMachine is used when a configuration wants to create an instance of a StateMachine. Usually @Configuration class extends adapters EnumStateMachineConfigurerAdapter or StateMachineConfigurerAdapter which allows user to override configuration callback methods. We automatically detect if user is using these adapter classes and modify runtime configuration logic.

@EnableStateMachineFactory is used when a configuration wants to create an instance of a StateMachineFactory.

	Note
	Usage examples of these are shown in below sections.

9.2 Configuring States

We’ll get into more complex configuration examples a bit later but let’s 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 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 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 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 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 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 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;
            }
        };
    }

}

9.7 Configuring Actions

Actions can be defined to be executed with transitions and states itself. Action is always executed as a result of a transition which originates from a trigger.

@Configuration
@EnableStateMachine
public class Config51
        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
            }
        };
    }

}

In above a single Action is defined as bean action and associated with a transition from S1 to S2.

@Configuration
@EnableStateMachine
public class Config52
        extends EnumStateMachineConfigurerAdapter<States, Events> {

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

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

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

}

	Note
	Usually you would not define same Action instance for different stages but we did it here not to make too much noise in a code snippet.

In above a single Action is defined as bean action and associated with states S1, S2 and S3. There is more going on there which needs more clarification:

• We defined action for initial state S1.
• We defined entry action for state S1 and left exit action empty.
• We defined exit action for state S2 and left entry action empty.
• We defined entry action as well as exit action for state S3.
• Notice how state S1 is used twice with initial() and state() functions. This is only needed if you want to define entry or exit actions with initial state.

	Important
	Defining action with initial() function only executes particular action when state machine or sub state is started. Think this action to be initializing action which is only executed once. Action defined with state() is then executed if state machine is transitioning back and forward between initial and non-inital states.

@Configuration
@EnableStateMachine
public 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 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 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 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 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 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 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 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 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));
    }

}

public class Bean3 {

    @Autowired
    StateMachineFactory<States, Events> factory;

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

10.2 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 pattern can be used to construct similar instances. Using strings as states and events this builder pattern can be used to build fully dynamic state machines outside of a Spring application context as shown above.

StateMachine<String, String> buildMachine1() 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 builder.configureConfiguration() interface methods cannot be chained together meaning builder methods needs to be called individually.

StateMachine<String, String> buildMachine2() throws Exception {
    Builder<String, String> builder = StateMachineBuilder.builder();
    builder.configureConfiguration()
        .withConfiguration()
            .autoStartup(false)
            .beanFactory(null)
            .taskExecutor(null)
            .taskScheduler(null)
            .listener(null);
    return builder.build();
}

It is important to understand on what cases common configuration needs to be used with a machines instantiated from a builder. Configurer returned from a withConfiguration() can be used to setup autoStart, TaskScheduler, TaskExecutor, BeanFactory and additionally register a StateMachineListener. If StateMachine instance returned from a builder is registered as a bean via @Bean, i.e. BeanFactory is attached automatically and then a default TaskExecutor can be found from there. If instances are used outside of a spring application context these methods must be used to setup needed facilities.

When en event is sent it may fire an EventTrigger which then may cause a transition to happen if a state machine is in a state where trigger is evaluated successfully. Normally this may lead to a situation where an event is not accepted and is dropped. However it may be desirable to postpone this event until a state machine enters other state, in which it is possible to accept that event. In other words an event simply arrives at an inconvenient time.

Spring Statemachine provides a mechanism for deferring events for later processing. Every state can have a list of deferred events. If an event in the current state’s deferred event list occurs, the event will be saved (deferred) for future processing until a state is entered that does not list the event in its deferred event list. When such a state is entered, the state machine will automatically recall any saved events that are no longer deferred and will then either consume or discard these events. It is possible for a superstate to have a transition defined on an event that is deferred by a substate. Following same hierarchical state machines concepts, the substate takes precedence over the superstate, the event will be deferred and the transition for the superstate will not be executed. With orthogonal regions where one orthogonal region defers an event and another accepts the event, the accept takes precedence and the event is consumed and not deferred.

The most obvious use case for event deferring is when an event is causing a transition into a particular state and state machine is then returned back to its original state where second event should cause a same transition. Lets take this with a simple example.

@Configuration
@EnableStateMachine
static class Config5 extends StateMachineConfigurerAdapter<String, String> {

    @Override
    public void configure(StateMachineStateConfigurer<String, String> states)
            throws Exception {
        states
            .withStates()
                .initial("READY")
                .state("DEPLOYPREPARE", "DEPLOY")
                .state("DEPLOYEXECUTE", "DEPLOY");
    }

    @Override
    public void configure(StateMachineTransitionConfigurer<String, String> transitions)
            throws Exception {
        transitions
            .withExternal()
                .source("READY").target("DEPLOYPREPARE")
                .event("DEPLOY")
                .and()
            .withExternal()
                .source("DEPLOYPREPARE").target("DEPLOYEXECUTE")
                .and()
            .withExternal()
                .source("DEPLOYEXECUTE").target("READY");
    }
}

In above state machine has state READY which indicates that machine is ready to process events which would take it into a DEPLOY state where the actual deployment would happen. After deploy actions has been executed machine is then returned back into a READY state. Sending multiple events in a READY state is not causing any trouble if machine is using synchronous executor because event sending would block between event calls. However if executor is using threads then other events may get lost because machine is no longer in a state where event could be processed. Thus deferring some of these events allows machine to preserve these events.

@Configuration
@EnableStateMachine
static class Config6 extends StateMachineConfigurerAdapter<String, String> {

    @Override
    public void configure(StateMachineStateConfigurer<String, String> states)
            throws Exception {
        states
            .withStates()
                .initial("READY")
                .state("DEPLOY", "DEPLOY")
                .state("DONE")
                .and()
                .withStates()
                    .parent("DEPLOY")
                    .initial("DEPLOYPREPARE")
                    .state("DEPLOYPREPARE", "DONE")
                    .state("DEPLOYEXECUTE");
    }

    @Override
    public void configure(StateMachineTransitionConfigurer<String, String> transitions)
            throws Exception {
        transitions
            .withExternal()
                .source("READY").target("DEPLOY")
                .event("DEPLOY")
                .and()
            .withExternal()
                .source("DEPLOYPREPARE").target("DEPLOYEXECUTE")
                .and()
            .withExternal()
                .source("DEPLOYEXECUTE").target("READY")
                .and()
            .withExternal()
                .source("READY").target("DONE")
                .event("DONE")
                .and()
            .withExternal()
                .source("DEPLOY").target("DONE")
                .event("DONE");
    }
}

In above state machine which is using nested states instead of a flat state model, event DEPLOY can be deferred directly in a substate. It is also showing concept of deferring event DONE in one of a sub-states which would then override anonymous transition between DEPLOY and DONE states if state machine happens to be in a DEPLOYPREPARE state when DONE event is dispatched. In DEPLOYEXECUTE state DONE event is not deferred, thus event would be handled in a super state.

Support for scopes in a state machine is very limited but it is possible to enable use of session scope using a normal spring @Scope annotation. Firstly if state machine is build manually via a builder and returned into context as @Bean, and secondly via an contifuration adapter. Both of these simply needs an a @Scope to be present where scopeName is set to session and proxyMode to ScopedProxyMode.TARGET_CLASS. Examples for both use cases are shown below.

	Tip
	See sample Chapter 37, Scope how to use session scoping.

@Configuration
public class Config3 {

    @Bean
    @Scope(scopeName="session", proxyMode=ScopedProxyMode.TARGET_CLASS)
    StateMachine<String, String> stateMachine() throws Exception {
        Builder<String, String> builder = StateMachineBuilder.builder();
        builder.configureConfiguration()
            .withConfiguration()
                .autoStartup(true)
                .taskExecutor(new SyncTaskExecutor());
        builder.configureStates()
            .withStates()
                .initial("S1")
                .state("S2");
        builder.configureTransitions()
            .withExternal()
                .source("S1")
                .target("S2")
                .event("E1");
        StateMachine<String, String> stateMachine = builder.build();
        return stateMachine;
    }

}

@Configuration
@EnableStateMachine
@Scope(scopeName="session", proxyMode=ScopedProxyMode.TARGET_CLASS)
public static class Config4 extends StateMachineConfigurerAdapter<String, String> {

    @Override
    public void configure(StateMachineConfigurationConfigurer<String, String> config) throws Exception {
        config
            .withConfiguration()
                .autoStartup(true);
    }

    @Override
    public void configure(StateMachineStateConfigurer<String, String> states) throws Exception {
        states
            .withStates()
                .initial("S1")
                .state("S2");
    }

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

}

Once you have scoped state machine into session, autowiring it into a @Controller will give new state machine instance per session. State machine is then destroyed when HttpSession is invalidated.

@Controller
public class StateMachineController {

    @Autowired
    StateMachine<String, String> stateMachine;

    @RequestMapping(path="/state", method=RequestMethod.POST)
    public HttpEntity<Void> setState(@RequestParam("event") String event) {
        stateMachine.sendEvent(event);
        return new ResponseEntity<Void>(HttpStatus.ACCEPTED);
    }

    @RequestMapping(path="/state", method=RequestMethod.GET)
    @ResponseBody
    public String getState() {
        return stateMachine.getState().getId();
    }
}

	Note
	Using state machines in a session scopes needs a careful planning mostly because it is a relatively heavy component.

	Note
	Spring Statemachine poms don’t have any dependencies to Spring MVC classes which you will need to work with session scope. But if you’re working with a web application, you’ve already pulled those deps directly from Spring MVC or Spring Boot.

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)"));
}

public class BaseAction implements Action<States, Events> {

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

public 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 16, 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();
}

public class BaseGuard implements Guard<States, Events> {

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

	Note
	StateContext is described in section Chapter 16, Using StateContext.

Let’s assume that we’d need to create a state machine tracking how many times a user is pressing a key on a keyboard and then terminate when keys are pressed 1000 times. Possible but a really naive solution would be to create a new state for each 1000 key presses. Going even worse combinations you might suddenly have astronomical number of states which naturally is not very practical.

This is where extended state variables comes into rescue by not having a necessity to add more states to drive state machine changes, instead a simple variable change can be done during a transition.

StateMachine has a method getExtendedState() which returns an interface ExtendedState which gives an access to extended state variables. You can access variables directly via a state machine or StateContext during a callback from actions or transitions.

public Action<String, String> myVariableAction() {
    return new Action<String, String>() {

        @Override
        public void execute(StateContext<String, String> context) {
            context.getExtendedState()
                .getVariables().put("mykey", "myvalue");
        }
    };
}

If there is a need to get notified for extended state variable changes, there are two options; either use StateMachineListener and listen extendedStateChanged(key, value) callbacks:

public class ExtendedStateVariableListener
        extends StateMachineListenerAdapter<String, String> {

    @Override
    public void extendedStateChanged(Object key, Object value) {
        // do something with changed variable
    }
}

Or implement a Spring Application context listeners for OnExtendedStateChanged. Naturally as mentioned in Chapter 18, Listening State Machine Events you can also listen all StateMachineEvent events.

public class ExtendedStateVariableEventListener
        implements ApplicationListener<OnExtendedStateChanged> {

    @Override
    public void onApplicationEvent(OnExtendedStateChanged event) {
        // do something with changed variable
    }
}

StateContext is a one of a most important objects when working with a state machine as it is passed into various methods and callbacks to give status of a current state of a state machine and where it is possibly going. If simplifying things a little it can be considered to be a snapshot of a current state machine stage where it is at a time StateContext is passed on.

	Note
	In Spring Statemachine 1.0.x StateContext usage were relatively naive in terms of how it was used to just pass stuff around as a simple POJO. Starting from Spring Statemachine 1.1.x its role has been greatly improved by making it a first class citizen in a state machine.

In overall StateContext can be used as.

StateContext is passed into various components interacting with user like Action and Guard.

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);
}

@Configuration
@EnableStateMachine
public class Config2 extends StateMachineConfigurerAdapter<String, String> {

    @Override
    public void configure(StateMachineStateConfigurer<String, String> states)
            throws Exception {
        states
            .withStates()
                .initial("S1")
                .state("S2");
    }

    @Override
    public void configure(StateMachineTransitionConfigurer<String, String> transitions)
            throws Exception {
        transitions
            .withExternal()
                .source("S1")
                .target("S2")
                .event("E1")
                .and()
            .withInternal()
                .source("S2")
                .action(timerAction())
                .timer(1000);
    }

    @Bean
    public TimerAction timerAction() {
        return new TimerAction();
    }
}

public class TimerAction implements Action<String, String> {

    @Override
    public void execute(StateContext<String, String> context) {
        // do something in every 1 sec
    }
}

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.

public class StateMachineApplicationEventListener
        implements ApplicationListener<StateMachineEvent> {

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

@Configuration
public class ListenerConfig {

    @Bean
    public StateMachineApplicationEventListener contextListener() {
        return new StateMachineApplicationEventListener();
    }
}

@Configuration
@EnableStateMachine
public class ManualBuilderConfig {

    @Bean
    public StateMachine<String, String> stateMachine() throws Exception {

        Builder<String, String> builder = StateMachineBuilder.builder();
        builder.configureStates()
            .withStates()
                .initial("S1")
                .state("S2");
        builder.configureTransitions()
            .withExternal()
                .source("S1")
                .target("S2")
                .event("E1");
        return builder.build();
    }
}

public 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) {
    }

    @Override
    public void eventNotAccepted(Message<Events> event) {
    }

    @Override
    public void extendedStateChanged(Object key, Object value) {
    }

    @Override
    public void stateMachineError(StateMachine<States, Events> stateMachine, Exception exception) {
    }

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

public 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 class Config8
        extends EnumStateMachineConfigurerAdapter<States, Events> {
}

@Configuration
@EnableStateMachineFactory(contextEvents = false)
public 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.

Available annotations has been harmonised to enable access to same state machine execution points than what is available from Chapter 18, Listening State Machine Events.

@WithStateMachine annotation can be used to associate a state machine with an existing bean. Then it is possible to start adding supported annotations to methods of that bean.

@WithStateMachine
public class Bean1 {

    @OnTransition
    public void anyTransition() {
    }
}

It is also possible to attach to any other state machine from an application context by using annotation name field.

@WithStateMachine(name = "myMachineBeanName")
public class Bean2 {

    @OnTransition
    public void anyTransition() {
    }
}

	Note
	Return type of these methods doesn’t matter and is effectively discard.

19.1 Method Parameters

Every annotation is supporting exactly same set of possible method parameters but runtime behaviour is different depending on an annotation itself and a stage where annotated method is called. To better understand how context works see Chapter 16, Using StateContext.

	Note
	For differences between method parameters, see individual annotation docs below.

Effectively all annotated methods are called using Spring SPel expressions which are build dynamically during the process. As to make this work these expressions needs to have a root object it evaluates against. This root object is a StateContext and we have also made some tweaks internally so that it is possible to access StateContext methods directly without going through the context handle.

Simplest method parameter would naturally be a StateContext itself.

@WithStateMachine
public class Bean3 {

    @OnTransition
    public void anyTransition(StateContext<String, String> stateContext) {
    }
}

Rest of the StateContext content can be accessed as shown below. Number of parameters or order of those doesn’t matter.

@WithStateMachine
public class Bean4 {

    @OnTransition
    public void anyTransition(
            @EventHeaders Map<String, Object> headers,
            ExtendedState extendedState,
            StateMachine<String, String> stateMachine,
            Message<String> message,
            Exception e) {
    }
}

@WithStateMachine
public class Bean5 {

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

    @OnTransition
    public void anyTransition() {
    }
}

@WithStateMachine
public class Bean6 {

    @StatesOnTransition(source = States.S1, target = States.S2)
    public void fromS1ToS2(@EventHeaders Map<String, Object> headers, ExtendedState extendedState) {
    }
}

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

    States[] source() default {};

    States[] target() default {};
}

@WithStateMachine
public class Bean7 {

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

@WithStateMachine
public class Bean8 {

    @OnStateChanged
    public void anyStateChange() {
    }
}

@WithStateMachine
public class Bean9 {

    @OnStateChanged(source = "S1", target = "S2")
    public void stateChangeFromS1toS2() {
    }
}

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
@OnStateChanged
public @interface StatesOnStates {

    States[] source() default {};

    States[] target() default {};
}

@WithStateMachine
public class Bean10 {

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

@WithStateMachine
public class Bean11 {

    @OnStateEntry
    public void anyStateEntry() {
    }

    @OnStateExit
    public void anyStateExit() {
    }
}

@WithStateMachine
public class Bean12 {

    @OnEventNotAccepted
    public void anyEventNotAccepted() {
    }

    @OnEventNotAccepted(event = "E1")
    public void e1EventNotAccepted() {
    }
}

@WithStateMachine
public class Bean13 {

    @OnStateMachineStart
    public void onStateMachineStart() {
    }

    @OnStateMachineStop
    public void onStateMachineStop() {
    }
}

@WithStateMachine
public class Bean14 {

    @OnStateMachineError
    public void onStateMachineError() {
    }
}

@WithStateMachine
public class Bean15 {

    @OnExtendedStateChanged
    public void anyStateChange() {
    }

    @OnExtendedStateChanged(key = "key1")
    public void key1Changed() {
    }
}

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 27, Persist and one sample Chapter 34, 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 Message<String> preEvent(Message<String> message, StateMachine<String, String> stateMachine) {
            return message;
        }

        @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 23, State Machine Error Handling.

Security features are build atop of functionality from a Spring Security. Security features are handy when it is required to protect part of a state machine execution and interaction with it.

	Important
	We expect user to be fairly familiar with a Spring Security meaning we don’t go into details of how overall security framework works. For this read Spring Security reference documentation.

First level of defence with a security is naturally protecting events which really are a driver from user point of view what is going to happen in a state machine. More fine grained security settings can then be defined for transitions and actions. This can be think of like allowing an employee to access a building, walk around it and then giving more detailed access rights to enter different rooms and allow to switch lights on and off while being on those rooms. If you trust your users then event security may be all you need, if you don’t, then more detailed security needs to be applied.

More detailed info can be found from section Section 22.6, “Understanding Security”.

	Tip
	For complete example, see sample Chapter 38, Security.

@Configuration
@EnableStateMachine
static class Config4 extends StateMachineConfigurerAdapter<String, String> {

    @Override
    public void configure(StateMachineConfigurationConfigurer<String, String> config)
            throws Exception {
        config
            .withSecurity()
                .enabled(true)
                .transitionAccessDecisionManager(null)
                .eventAccessDecisionManager(null);
    }
}

@Configuration
@EnableStateMachine
static class Config1 extends StateMachineConfigurerAdapter<String, String> {

    @Override
    public void configure(StateMachineConfigurationConfigurer<String, String> config)
            throws Exception {
        config
            .withSecurity()
                .enabled(true)
                .event("true")
                .event("ROLE_ANONYMOUS", ComparisonType.ANY);
    }
}

@Configuration
@EnableStateMachine
static class Config6 extends StateMachineConfigurerAdapter<String, String> {

    @Override
    public void configure(StateMachineConfigurationConfigurer<String, String> config)
            throws Exception {
        config
            .withSecurity()
                .enabled(true)
                .transition("true")
                .transition("ROLE_ANONYMOUS", ComparisonType.ANY);
    }
}

@Configuration
@EnableStateMachine
static class Config2 extends StateMachineConfigurerAdapter<String, String> {

    @Override
    public void configure(StateMachineTransitionConfigurer<String, String> transitions)
            throws Exception {
        transitions
            .withExternal()
                .source("S0")
                .target("S1")
                .event("A")
                .secured("ROLE_ANONYMOUS", ComparisonType.ANY)
                .secured("hasTarget('S1')");
    }
}

@Configuration
@EnableStateMachine
static class Config3 extends StateMachineConfigurerAdapter<String, String> {

    @Override
    public void configure(StateMachineConfigurationConfigurer<String, String> config)
            throws Exception {
        config
            .withSecurity()
                .enabled(true);
    }

    @Override
    public void configure(StateMachineStateConfigurer<String, String> states)
            throws Exception {
        states
            .withStates()
                .initial("S0")
                .state("S1");
    }

    @Override
    public void configure(StateMachineTransitionConfigurer<String, String> transitions)
            throws Exception {
        transitions
            .withExternal()
                .source("S0")
                .target("S1")
                .action(securedAction())
                .event("A");
    }

    @Scope(proxyMode = ScopedProxyMode.TARGET_CLASS)
    @Bean
    public Action<String, String> securedAction() {
        return new Action<String, String>() {

            @Secured("ROLE_ANONYMOUS")
            @Override
            public void execute(StateContext<String, String> context) {
            }
        };
    }

}

@Configuration
@EnableGlobalMethodSecurity(securedEnabled = true)
public static class Config5 extends WebSecurityConfigurerAdapter {

    @Autowired
    public void configureGlobal(AuthenticationManagerBuilder auth) throws Exception {
        auth
            .inMemoryAuthentication()
                .withUser("user").password("password").roles("USER");
    }
}

22.6 Understanding Security

This section provides more detailed info how security works within a state machine. Not really something you’d need to know but it is always better to be transparent instead of hiding all the magic what happens behind a scenes.

	Note
	Security only makes sense if State Machine is executed in a wallet garden where user don’t have direct access to the application thus could modify Spring Security’s SecurityContext hold in a thread local. If user controls the jvm, then effectively there is no security at all.

Integration point for security is done with a StateMachineInterceptor which is then added automatically into a state machine if security is enabled. Specific class is a StateMachineSecurityInterceptor which intercepts events and transitions. This interceptor then consults Spring Security’s AccessDecisionManager if event can be send or if transition can be executed. Effectively if decision or vote with a AccessDecisionManager will result an exception, event or transition is denied.

Due to way how AccessDecisionManager from Spring Security works, we need one instance of it per secured object. This is a reason why there is a different manager for events and transitions. In this case events and transitions are different class objects we’re securing.

On default for events, voters EventExpressionVoter, EventVoter and RoleVoter are added into a AccessDecisionManager.

On default for transitions, voters TransitionExpressionVoter, TransitionVoter and RoleVoter are added into a AccessDecisionManager.

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 chance 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 18, Listening State Machine Events.

Having said that, a simple listener would look like:

public 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 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 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 27, Persist and one sample Chapter 34, 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 21, State Machine Interceptor.

24.1 Using Redis

Support for persisting State Machine into Redis is done via RepositoryStateMachinePersist which implements StateMachinePersist. Specific implementation is a RedisStateMachineContextRepository whic uses kryo serialization to persist a StateMachineContext into Redis.

	Tip
	Check sample Chapter 39, Event Service for detailed usage.

RedisStateMachineContextRepository will need a RedisConnectionFactory for it to work and we recommend a JedisConnectionFactory for it as seeing from above example.

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 through 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 generic configuration support see section Section 9.9, “Configuring Common Settings” and actual usage example see sample Chapter 35, 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 enough information to reset a StateMachine.

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

Here is a generic example of how Zookeeper based Distributed State Machine would be configured.

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

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

    @Override
    public void configure(StateMachineStateConfigurer<String, String> states)
            throws Exception {
        // config states
    }

    @Override
    public void configure(StateMachineTransitionConfigurer<String, String> transitions)
            throws Exception {
        // config transitions
    }

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

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

}

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

We have also added a set of utility classes to easy testing of a state machine instances. These are used in a framework itself but are also very useful for end users.

StateMachineTestPlanBuilder is used to build a StateMachineTestPlan which then have one method test() which runs a plan. StateMachineTestPlanBuilder contains a fluent builder api to add steps into a plan and during these steps you can send events and check various conditions like state changes, transitions and extended state variables.

Let’s take a simple StateMachine build using below example:

private StateMachine<String, String> buildMachine() throws Exception {
    StateMachineBuilder.Builder<String, String> builder = StateMachineBuilder.builder();

    builder.configureConfiguration()
        .withConfiguration()
            .taskExecutor(new SyncTaskExecutor())
            .autoStartup(true);

    builder.configureStates()
            .withStates()
                .initial("SI")
                .state("S1");

    builder.configureTransitions()
            .withExternal()
                .source("SI").target("S1")
                .event("E1");

    return builder.build();
}

In below test plan we have two steps, first we check that initial state S1 is indeed set, secondly we send an event E1 and expect one state change to happen and machine to end up into a state S1.

StateMachine<String, String> machine = buildMachine();
StateMachineTestPlan<String, String> plan =
        StateMachineTestPlanBuilder.<String, String>builder()
            .defaultAwaitTime(2)
            .stateMachine(machine)
            .step()
                .expectStates("SI")
                .and()
            .step()
                .sendEvent("E1")
                .expectStateChanged(1)
                .expectStates("S1")
                .and()
            .build();
plan.test();

These utilities are also used within a framework to test distributed state machine features and multiple machines can be added to a plan. If multiple machines are added then it is also possible to choose if event is sent to particular, random or all machines.

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 34, 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 32, 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 machine 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 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 don’t want to implement a full interface. Listener provides a various hooks to listen tasks execution events.

private 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 enum States {
    LOCKED, UNLOCKED
}

Events. 

public 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-1.1.0.M1.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 enum States {
    S0, S1, S11, S12, S2, S21, S211, S212
}

Events. 

public 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));
    }
}

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

sm>sm start
Init foo to 0
Entry state S0
Entry state S1
Entry state S11
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
Internal transition source=S0
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:

Let’s take closer look of how hierarchical states and their event handling works with a below example.

sm>sm variables
No variables

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

sm>sm variables
foo=0

sm>sm event H
Internal transition source=S1
Event H send

sm>sm variables
foo=0

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

sm>sm variables
foo=0

sm>sm event H
Switch foo to 1
Internal transition source=S0
Event H send

sm>sm variables
foo=1

sm>sm event H
Switch foo to 0
Internal transition source=S2
Event H send

sm>sm variables
foo=0

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 us 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 the 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.

Let’s 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 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 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 enum Variables {
    CD, TRACK, ELAPSEDTIME
}

public 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 @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 responsibilities 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.

Let’s 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 enum States {
    READY,
    FORK, JOIN, CHOICE,
    TASKS, T1, T1E, T2, T2E, T3, T3E,
    ERROR, AUTOMATIC, MANUAL
}

Events. 

public 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(name = StateMachineSystemConstants.TASK_EXECUTOR_BEAN_NAME)
public TaskExecutor taskExecutor() {
    ThreadPoolTaskExecutor taskExecutor = new ThreadPoolTaskExecutor();
    taskExecutor.setCorePoolSize(5);
    return taskExecutor;
}

Let’s 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 enum States {
    RUNNING, HISTORY, END,
    WASHING, RINSING, DRYING,
    POWEROFF
}

Events. 

public 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);
}

Let’s 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 27, Persist to demonstrate 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 29, 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;
}

Let’s go through a simple example where two different shell instances are started with command

@n1:~# java -jar spring-statemachine-samples-zookeeper-1.1.0.M1.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 30, Showcase to work with a distributed state machine. The state machine logic is shown above:

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

Let’s go through a simple example where three different sample instances are started. 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-1.1.0.M1.jar
@n2:~# java -jar spring-statemachine-samples-web-1.1.0.M1.jar
@n3:~# java -jar spring-statemachine-samples-web-1.1.0.M1.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 let’s press button Event H and what is supposed to happen is that internal transition is executed on all state machines changing extended state 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 K which is supposed to take state machine state back to state S11 and you should see this happening in all browser sessions.

Scope is a state machine example using a session scope to provide individual instance for every user.

This is a simple state machine having states S0, S1 and S2. Transitions between those are controlled via events A, B and C as shown in a state chart.

@n1:~# java -jar spring-statemachine-samples-scope-1.1.0.M1.jar

When instance is running you can open a browser and play with a state machine. If you open same page using a different browser, i.e one in Chrome and one in Firefox, you should get a new state machine instance per user session.

Security is a state machine example using most of a compinations of securing a state machine. It is securing sending events, transitions and actions.

@n1:~# java -jar spring-statemachine-samples-secure-1.1.0.M1.jar

We secure event sending with a users having a role USER. None of a other users imposed by a Spring Security can’t send events into a state machine.

@Override
public void configure(StateMachineConfigurationConfigurer<States, Events> config)
        throws Exception {
    config
        .withConfiguration()
            .autoStartup(true)
            .and()
        .withSecurity()
            .enabled(true)
            .event("hasRole('USER')");
}

In this sample we define two users, user having a role USER and admin having both roles USER and ADMIN. Authentication for both user for password is password.

@EnableWebSecurity
@EnableGlobalMethodSecurity(securedEnabled = true)
static class SecurityConfig extends WebSecurityConfigurerAdapter {

    @Autowired
    public void configureGlobal(AuthenticationManagerBuilder auth) throws Exception {
        auth
            .inMemoryAuthentication()
                .withUser("user")
                    .password("password")
                    .roles("USER")
                    .and()
                .withUser("admin")
                    .password("password")
                    .roles("USER", "ADMIN");
    }
}

We define various transitions between states according to a statechart seen above. Only a user with active ADMIN role can execute external transitions between S2 and S3. Similarly ADMIN can only execute internal transition in a state S1.

@Override
public void configure(StateMachineTransitionConfigurer<States, Events> transitions)
        throws Exception {
    transitions
        .withExternal()
            .source(States.S0).target(States.S1).event(Events.A)
            .and()
        .withExternal()
            .source(States.S1).target(States.S2).event(Events.B)
            .and()
        .withExternal()
            .source(States.S2).target(States.S0).event(Events.C)
            .and()
        .withExternal()
            .source(States.S2).target(States.S3).event(Events.E)
            .secured("ROLE_ADMIN", ComparisonType.ANY)
            .and()
        .withExternal()
            .source(States.S3).target(States.S0).event(Events.C)
            .and()
        .withInternal()
            .source(States.S0).event(Events.D)
            .action(adminAction())
            .and()
        .withInternal()
            .source(States.S1).event(Events.F)
            .action(transitionAction())
            .secured("ROLE_ADMIN", ComparisonType.ANY);
}

Action adminAction is secured with a role ADMIN.

@Scope(proxyMode = ScopedProxyMode.TARGET_CLASS)
@Bean
public Action<States, Events> adminAction() {
    return new Action<States, Events>() {

        @Secured("ROLE_ADMIN")
        @Override
        public void execute(StateContext<States, Events> context) {
            log.info("Executed only for admin role");
        }
    };
}

Below Action would only be executed with internal transition in a state S1 when event F is send. Transition itself is secured with a role ADMIN so this transition will not be executed if current user does not hate that role.

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

        @Override
        public void execute(StateContext<States, Events> context) {
            log.info("Executed only for admin role");
        }
    };
}

Event Service is an example how state machine concepts can be used as a processing engine for events. Sample was born out from a question:

	Note
	Can Spring Statemachine be used as a microservice to feed events to it with millions different state machine instances.

In this example we will use a Redis to persist a state machine instances.

Obviously a million state machine instances in a jvm would be a relatively bad idea due to memory constraints. This simply leads to other available features from a Spring Statemachine to persist a StateMachineContext and re-use existing instances.

We assume few things like there is a shopping application which is sending different types of PageView events into a separate microservice which is then tracking user behaviour using a state machine. State model is shown below which simply have few states representing user navigating on product items list, add and remote items from a cart and going to a payment page and initiating a pay operation. Actual shopping application would send these events into this service for example using a simple rest calls. More about this later.

	Note
	Remember that focus here is to have an application which is exposing a REST api user can use to send events which would be processed by a state machine per request.

In below state machine configuration we simply model what we have in a state chart. Various actions are updating state machine Extended State to track number of entry’s into various states and also how many times internal transition for ADD and DEL are called and if PAY has been executed. Don’t focus on stateMachineTarget or @Scope for now, as we’ll explain those in a bit.

@Bean(name = "stateMachineTarget")
@Scope(scopeName="prototype")
public StateMachine<States, Events> stateMachineTarget() throws Exception {
    Builder<States, Events> builder = StateMachineBuilder.<States, Events>builder();

    builder.configureConfiguration()
        .withConfiguration()
            .autoStartup(true);

    builder.configureStates()
        .withStates()
            .initial(States.HOME)
            .states(EnumSet.allOf(States.class));

    builder.configureTransitions()
        .withInternal()
            .source(States.ITEMS).event(Events.ADD)
            .action(addAction())
            .and()
        .withInternal()
            .source(States.CART).event(Events.DEL)
            .action(delAction())
            .and()
        .withInternal()
            .source(States.PAYMENT).event(Events.PAY)
            .action(payAction())
            .and()
        .withExternal()
            .source(States.HOME).target(States.ITEMS)
            .action(pageviewAction())
            .event(Events.VIEW_I)
            .and()
        .withExternal()
            .source(States.CART).target(States.ITEMS)
            .action(pageviewAction())
            .event(Events.VIEW_I)
            .and()
        .withExternal()
            .source(States.ITEMS).target(States.CART)
            .action(pageviewAction())
            .event(Events.VIEW_C)
            .and()
        .withExternal()
            .source(States.PAYMENT).target(States.CART)
            .action(pageviewAction())
            .event(Events.VIEW_C)
            .and()
        .withExternal()
            .source(States.CART).target(States.PAYMENT)
            .action(pageviewAction())
            .event(Events.VIEW_P)
            .and()
        .withExternal()
            .source(States.ITEMS).target(States.HOME)
            .action(resetAction())
            .event(Events.RESET)
            .and()
        .withExternal()
            .source(States.CART).target(States.HOME)
            .action(resetAction())
            .event(Events.RESET)
            .and()
        .withExternal()
            .source(States.PAYMENT).target(States.HOME)
            .action(resetAction())
            .event(Events.RESET);

    return builder.build();
}

In below config we setup a RedisConnectionFactory which defaults to localhost and default ports. We also use StateMachinePersist with RepositoryStateMachinePersist implementation. These are used in a Controller handling REST calls.

@Bean
public RedisConnectionFactory redisConnectionFactory() {
    return new JedisConnectionFactory();
}

@Bean
public StateMachinePersist<States, Events, String> stateMachinePersist(RedisConnectionFactory connectionFactory) {
    RedisStateMachineContextRepository<States, Events> repository =
            new RedisStateMachineContextRepository<States, Events>(connectionFactory);
    return new RepositoryStateMachinePersist<States, Events>(repository);
}

We now get into why StateMachine was created as stateMachineTarget and a prototype bean. State machine instantiation is a relatively expensive operation so it is better to try to pool instances instead of instantiating a new instance with every request. For this we first create a poolTargetSource which wraps stateMachineTarget and pools it with max size of 3. This poolTargetSource is then proxied with ProxyFactoryBean using a request scope. Effectively this means that every REST request will get pooled state machine instance from a bean factory. It’s shown later how these are used.

@Bean
@Scope(value = "request", proxyMode = ScopedProxyMode.TARGET_CLASS)
public ProxyFactoryBean stateMachine() {
    ProxyFactoryBean pfb = new ProxyFactoryBean();
    pfb.setTargetSource(poolTargetSource());
    return pfb;
}

@Bean
public CommonsPool2TargetSource poolTargetSource() {
    CommonsPool2TargetSource pool = new CommonsPool2TargetSource();
    pool.setMaxSize(3);
    pool.setTargetBeanName("stateMachineTarget");
    return pool;
}

Lets get into actual demo. You need to have a redis running on a localhost with a default settings. Then run the boot based sample application:

@n1:~# java -jar spring-statemachine-samples-eventservice-1.1.0.M1.jar

In a browser you see something like:

In this UI you have three users you can use, joe, bob and dave. Clicking button will show current state and extended state. Enabling a radio button before clicking users will send particular event for that user. This is a way you can play with this using an UI.

In our StateMachineController we autowire StateMachine and StateMachinePersist. StateMachine is a request scoped so you’ll get new instance per request while StateMachinePersist is normal singleton bean.

@Autowired
private StateMachine<States, Events> stateMachine;

@Autowired
private StateMachinePersist<States, Events, String> stateMachinePersist;

Below feedAndGetState is just used with an UI to do same things what actual REST api will do.

@RequestMapping("/state")
public String feedAndGetState(@RequestParam(value = "user", required = false) String user,
        @RequestParam(value = "id", required = false) Events id, Model model) throws Exception {
    model.addAttribute("user", user);
    model.addAttribute("allTypes", Events.values());
    model.addAttribute("stateChartModel", stateChartModel);
    // we may get into this page without a user so
    // do nothing with a state machine
    if (StringUtils.hasText(user)) {
        resetStateMachineFromStore(user);
        if (id != null) {
            feedMachine(user, id);
        }
        model.addAttribute("states", stateMachine.getState().getIds());
        model.addAttribute("extendedState", stateMachine.getExtendedState().getVariables());
    }
    return "states";
}

Below feedPageview is a REST method which accepts a post with a json content.

@RequestMapping(value = "/feed",method= RequestMethod.POST)
@ResponseStatus(HttpStatus.OK)
public void feedPageview(@RequestBody(required = true) Pageview event) throws Exception {
    Assert.notNull(event.getUser(), "User must be set");
    Assert.notNull(event.getId(), "Id must be set");
    resetStateMachineFromStore(event.getUser());
    feedMachine(event.getUser(), event.getId());
}

Below feedMachine will send event into a StateMachine and persists its state using a StateMachinePersist.

private void feedMachine(String user, Events id) throws Exception {
    stateMachine.sendEvent(id);
    stateMachinePersist.write(new DefaultStateMachineContext<States, Events>(stateMachine.getState().getId(), null, null,
            stateMachine.getExtendedState()), "testprefix:" + user);
}

Below resetStateMachineFromStore is used to reset a state machine for a particular user.

private StateMachine<States, Events> resetStateMachineFromStore(String user) throws Exception {
    final StateMachineContext<States, Events> context = stateMachinePersist.read("testprefix:" + user);
    stateMachine.stop();
    stateMachine.getStateMachineAccessor()
        .doWithAllRegions(new StateMachineFunction<StateMachineAccess<States, Events>>() {

        @Override
        public void apply(StateMachineAccess<States, Events> function) {
            function.resetStateMachine(context);
        }
    });
    stateMachine.start();
    return stateMachine;
}

As you’d send event using UI, same can be done using a REST calls:

# curl http://localhost:8080/feed -H "Content-Type: application/json" --data '{"user":"joe","id":"VIEW_I"}'

At this point you should have a content in Redis with a testprefix:joe key.

$ ./redis-cli
127.0.0.1:6379> KEYS *
1) "testprefix:joe"

Below is a three images when state for joe has been changed from HOME to ITEMS and when ADD action has been executed.

Send evend ADD:

Now your are still on state ITEMS and internal transition caused extended state variable COUNT to increase to 1.

Execute below curl rest call few times or do it via UI and you should see COUNT variable to increase with every call.

# curl http://localhost:8080/feed -H "Content-Type: application/json" # --data '{"user":"joe","id":"ADD"}'

Order to implement actual logic to track user behaviour, you’d then define these in a various Actions in a state machine. You can associate different Actions with transitions, state entry’s or state exits' and with various listener methods.

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.