Package org.axonframework.messaging.eventhandling

Class SimpleEventHandlingComponent

java.lang.Object
org.axonframework.messaging.eventhandling.SimpleEventHandlingComponent
All Implemented Interfaces:
DescribableComponent, MessageHandler, EventHandler, EventHandlerRegistry<SimpleEventHandlingComponent>, EventHandlingComponent
public class SimpleEventHandlingComponent extends Object implements EventHandlingComponent, EventHandlerRegistry<SimpleEventHandlingComponent>
Simple implementation of the EventHandlingComponent, containing a collection of EventHandlers to invoke on handle(EventMessage, ProcessingContext).
Since:
5.0.0
Author:
Steven van Beelen

  • Method Details

    • create

      public static SimpleEventHandlingComponent create(@Nonnull String name)
      Instantiates a simple EventHandlingComponent that is able to handle events and delegate them to subcomponents.

      Uses a default sequencing policy that will first try for the SequentialPerAggregatePolicy, falling back to the SequentialPolicy when the former returns no sequence value.

      Parameters:
      name - The name of the component, used for describing the component.
      Returns:
      A simple EventHandlingComponent instance with the given name.

    • create

      public static SimpleEventHandlingComponent create(@Nonnull String name, @Nonnull SequencingPolicy sequencingPolicy)
      Instantiates a simple EventHandlingComponent that is able to handle events and delegate them to subcomponents, using the given sequencingPolicy to decide how to sequence incoming events.
      Parameters:
      name - The name of the component, used for describing the component.
      sequencingPolicy - The SequencingPolicy to use for sequencing events through, for example, sequenceIdentifierFor(EventMessage, ProcessingContext).
      Returns:
      A simple EventHandlingComponent instance with the given name.

    • handle

      @Nonnull public MessageStream.Empty<Message> handle(@Nonnull EventMessage event, @Nonnull ProcessingContext context)
      Description copied from interface: EventHandler
      Handles the given event within the given context.

      The result of handling is an empty stream.

      Specified by:
      handle in interface EventHandler
      Parameters:
      event - The event to handle.
      context - The context to the given event is handled in.
      Returns:
      An empty stream containing nothing.

    • subscribe

      public SimpleEventHandlingComponent subscribe(@Nonnull Set<QualifiedName> names, @Nonnull EventHandler handler)
      Description copied from interface: EventHandlerRegistry
      Subscribe the given handler for events of the given names.

      If a subscription already exists for any name in the given set, the behavior is undefined. Implementations may throw an exception to refuse duplicate subscription or alternatively decide whether the existing or new handler gets the subscription.

      Specified by:
      subscribe in interface EventHandlerRegistry<SimpleEventHandlingComponent>
      Parameters:
      names - The names of the given commandHandler can handle.
      handler - The handler instance that handles events for the given names.
      Returns:
      This registry for fluent interfacing.

    • subscribe

      public SimpleEventHandlingComponent subscribe(@Nonnull QualifiedName name, @Nonnull EventHandler handler)
      Description copied from interface: EventHandlerRegistry
      Subscribe the given handler for events of the given name.

      If a subscription already exists for the name, the behavior is undefined. Implementations may throw an exception to refuse duplicate subscription or alternatively decide whether the existing or new handler gets the subscription.

      Specified by:
      subscribe in interface EventHandlerRegistry<SimpleEventHandlingComponent>
      Parameters:
      name - The name the given commandHandler can handle.
      handler - The handler instance that handles events for the given name.
      Returns:
      This registry for fluent interfacing.

    • supportedEvents

      public Set<QualifiedName> supportedEvents()
      Description copied from interface: EventHandlingComponent
      All supported events, referenced through a QualifiedName.
      Specified by:
      supportedEvents in interface EventHandlingComponent
      Returns:
      All supported events, referenced through a QualifiedName.

    • sequenceIdentifierFor

      @Nonnull public Object sequenceIdentifierFor(@Nonnull EventMessage event, @Nonnull ProcessingContext context)
      Returns the sequence identifier for the given event. When two events have the same sequence identifier (as defined by their equals method), they will be executed sequentially. Important: All EventHandlers for the same QualifiedName within a single EventHandlingComponent must return the same sequence identifier for a given event. Mixing different sequence identifiers within the scope of a single EventHandlingComponent is not supported and may lead to undefined behavior.

      The implementation automatically chooses the appropriate sequencing policy based on the event context:

      • If the aggregate identifier is present in the ProcessingContext (i.e., the event is an aggregate event), it uses SequentialPerAggregatePolicy to ensure events for the same aggregate are processed sequentially while allowing concurrent processing of events from different aggregates.
      • If no aggregate identifier is present (i.e., the event is not an aggregate event), it uses SequentialPolicy which ensures all events are processed sequentially (no concurrency) as the safest default option.

      Override this method to provide custom sequencing behavior. Or use a SequenceOverridingEventHandlingComponent if you cannot inherit from a certain EventHandlingComponent implementation.

      Specified by:
      sequenceIdentifierFor in interface EventHandlingComponent
      Parameters:
      event - The event for which to get the sequencing identifier.
      context - The processing context in which the event is being handled.
      Returns:
      A sequence identifier for the given event.

    • describeTo

      public void describeTo(@Nonnull ComponentDescriptor descriptor)
      Description copied from interface: DescribableComponent
      Describe the properties of this DescribableComponent with the given descriptor.

      Components should call the appropriate describeProperty methods on the descriptor to register their properties. The descriptor is responsible for determining how these properties are formatted and structured in the final output.

      Best Practices: As a general rule, all relevant fields of a DescribableComponent implementation should be described in this method. However, developers have discretion to include only the fields that make sense in the context. Not every field may be meaningful for description purposes, especially internal implementation details. Furthermore, components might want to expose different information based on their current state. The final decision on what properties to include lies with the person implementing the describeTo method, who should focus on providing information that is useful for understanding the component's configuration and state.

      Example implementation: 

       public void describeTo(ComponentDescriptor descriptor) {
     descriptor.describeProperty("name", this.name);
     descriptor.describeProperty("enabled", this.enabled);
     descriptor.describeProperty("configuration", this.configuration); // A nested component
     descriptor.describeProperty("handlers", this.eventHandlers);      // A collection
 }
      Specified by:
      describeTo in interface DescribableComponent
      Parameters:
      descriptor - The component descriptor to describe this DescribableComponentn its properties in.