Transition Configuration | Spring Statemachine

Introduction

In this tutorial, we will explore the concept of Transition Configuration in the Spring Statemachine framework. A state machine is a model of computation that consists of states, transitions, and events. Transition configuration is essential for defining how the state machine moves from one state to another based on specific events.

Basic Concepts

A transition in a state machine is a directed link between two states. It is triggered by an event, which causes the state machine to move from the source state to the target state. Transitions can be configured with various properties, such as actions, guards, and effects.

States: The distinct conditions or situations in which the state machine can be.
Events: The occurrences that can trigger transitions between states.
Actions: Operations that are executed when a transition occurs.
Guards: Conditions that must be satisfied for a transition to be executed.

Setting Up Spring Statemachine

Before configuring transitions, ensure you have the Spring Statemachine dependency in your project. You can add it via Maven or Gradle. Below is the Maven dependency:

Add the following dependency in your pom.xml:

                    <dependency>
                        <groupId>org.springframework.statemachine</groupId>
                        <artifactId>spring-statemachine-core</artifactId>
                        <version>2.2.1</version>
                    </dependency>

Configuring Transitions

To configure transitions, you will create a state machine configuration class that implements the StateMachineConfigurer interface. Within this class, you can define your states, events, and transitions.

Here is an example of a basic state machine configuration:

                    import org.springframework.statemachine.config.EnableStateMachine;
                    import org.springframework.statemachine.config.builders.StateMachineConfigurerAdapter;
                    import org.springframework.statemachine.config.builders.StateMachineBuilder;
                    import org.springframework.statemachine.state.State;
                    import org.springframework.statemachine.transition.Transition;

                    @EnableStateMachine
                    public class StateMachineConfig extends StateMachineConfigurerAdapter {

                        @Override
                        public void configure(StateMachineStateConfigurer<String, String> states) throws Exception {
                            states
                                .withStates()
                                    .initial("STATE1")
                                    .state("STATE2")
                                    .end("STATE3");
                        }

                        @Override
                        public void configure(StateMachineTransitionConfigurer<String, String> transitions) throws Exception {
                            transitions
                                .withExternal()
                                    .source("STATE1").target("STATE2").event("EVENT1")
                                .and()
                                .withExternal()
                                    .source("STATE2").target("STATE3").event("EVENT2");
                        }
                    }

In this configuration:

We define three states: STATE1, STATE2, and STATE3.
The state machine starts at STATE1.
EVENT1 transitions the machine from STATE1 to STATE2.
EVENT2 transitions the machine from STATE2 to STATE3.

Adding Actions and Guards

Actions can be added to transitions to perform specific tasks when a transition occurs. Guards can be used to restrict transitions based on certain conditions.

Here's how you can add actions and guards to transitions:

                    transitions
                        .withExternal()
                            .source("STATE1").target("STATE2").event("EVENT1")
                            .action(context -> System.out.println("Transitioning to STATE2"))
                            .guard(context -> someCondition())
                        .and()
                        .withExternal()
                            .source("STATE2").target("STATE3").event("EVENT2")
                            .action(context -> System.out.println("Transitioning to STATE3"));

In this example:

When transitioning from STATE1 to STATE2, it prints a message and checks a condition through a guard.
The transition to STATE3 also prints a message but does not have a guard.

Conclusion

Transition configuration is a crucial aspect of building effective state machines using Spring Statemachine. By defining states, events, transitions, actions, and guards, you can create complex flow control logic in your applications. This tutorial provided a foundational understanding of how to configure transitions, and you can build upon this knowledge to create more sophisticated state machines tailored to your use cases.