Event Handling | Spring Statemachine | Spring Framework Tutorial

Introduction to Event Handling

Event handling is a core concept in the Spring Statemachine framework that allows developers to define how the application responds to different events. In the context of state machines, events trigger transitions between states.

Understanding Events

In Spring Statemachine, an event is an occurrence that can trigger a state transition. Events can be user actions, system-generated signals, or any other identifiable actions that your application recognizes.

Events are defined in the state machine configuration, and each event is associated with a specific state transition.

Defining Events

To define events in a Spring Statemachine, you typically create an enum that represents the various events your application will handle. For example:

Example: Defining Events


                        public enum Events { START, STOP, PAUSE, RESUME }

Configuring State Machine with Events

Once the events are defined, you configure the state machine to respond to these events. This is done in the state machine configuration class.

Example: Configuring State Machine


                        @EnableStateMachine
                        public class StateMachineConfig extends StateMachineConfigurerAdapter {
                            @Override
                            public void configure(StateMachineStateConfigurer states) throws Exception {
                                states.withStates()
                                    .initial(States.IDLE)
                                    .state(States.RUNNING)
                                    .end(States.FINISHED);
                            }

                            @Override
                            public void configure(StateMachineTransitionConfigurer transitions) throws Exception {
                                transitions.withExternal()
                                    .source(States.IDLE).target(States.RUNNING).event(Events.START)
                                    .and()
                                    .withExternal()
                                    .source(States.RUNNING).target(States.IDLE).event(Events.STOP);
                            }
                        }

Handling Events

To handle events, you can use event listeners. Listeners can react to events as they are fired, allowing you to execute custom logic based on the state machine's current state or the event received.

Example: Adding an Event Listener


                        @Component
                        public class StateMachineEventListener implements StateMachineEventListener {
                            @Override
                            public void stateChanged(State from, State to) {
                                System.out.println("State changed from " + from.getId() + " to " + to.getId());
                            }

                            @Override
                            public void eventNotAccepted(StateMachineEvent event) {
                                System.out.println("Event not accepted: " + event);
                            }
                        }

Testing Event Handling

After configuring your state machine and event listeners, it's essential to test the event handling behavior. You can use unit tests to simulate events and verify that the state transitions occur as expected.

Example: Testing Event Handling


                        @SpringBootTest
                        public class StateMachineTests {

                            @Autowired
                            private StateMachine stateMachine;

                            @Test
                            public void testStartEvent() {
                                stateMachine.start();
                                stateMachine.sendEvent(Events.START);
                                assertEquals(States.RUNNING, stateMachine.getState().getId());
                            }
                        }

Conclusion

Event handling in Spring Statemachine is a powerful way to manage state transitions in response to various events. By defining events, configuring transitions, and adding listeners, you can create dynamic and responsive state machines tailored to your application's needs.

Event Handling in Spring Statemachine