Customizing Reads: Triggering Events on GET Requests with Spring Data REST

While Spring Data REST excels at generating CRUD endpoints, the standard life cycle events we’ve discussed primarily revolve around data modification (Create, Update, Delete). You might encounter scenarios where you need to trigger specific actions or logic when an entity is read via a GET request.

Out of the box, Spring Data REST doesn’t directly publish the standard BeforeLoadEvent or AfterLoadEvent for GET requests in the same way it does for persistence operations. However, you can achieve this by leveraging Spring Data REST’s customization capabilities and Spring’s event publishing mechanism.

This article will explore different approaches to trigger custom events when entities are retrieved through Spring Data REST GET endpoints.

Understanding the Challenge

The core reason why standard Spring Data Commons persistence events aren’t triggered on GET requests is that these events are tied to the entity management lifecycle (persisting, updating, deleting). Reading data is a separate operation.

Therefore, we need to find alternative ways to intercept the read operation and publish our custom events. Here are a few strategies:

1. Using @RepositoryEventHandler

Spring Data REST provides the @RepositoryEventHandler annotation, which allows you to intercept repository method invocations. We can use this to trigger an event after a successful retrieval.

• Create your custom event:

  import org.springframework.context.ApplicationEvent;
  
  public class ProductReadEvent extends ApplicationEvent {
      private final Product product;
  
      public ProductReadEvent(Object source, Product product) {
          super(source);
          this.product = product;
      }
  
      public Product getProduct() {
          return product;
      }
  }
  

• Create a Repository Event Handler:

  import org.springframework.beans.factory.annotation.Autowired;
  import org.springframework.context.ApplicationEventPublisher;
  import org.springframework.data.rest.core.annotation.HandleAfterGet;
  import org.springframework.data.rest.core.annotation.RepositoryEventHandler;
  
  @RepositoryEventHandler(Product.class)
  public class ProductReadEventHandler {
  
      @Autowired
      private ApplicationEventPublisher publisher;
  
      @HandleAfterGet
      public void handleAfterGet(Product product) {
          publisher.publishEvent(new ProductReadEvent(this, product));
          System.out.println("Product read event triggered for product ID: " + product.getId());
          // Perform any other actions after a product is read
      }
  }
  

  • @RepositoryEventHandler(Product.class): This annotation indicates that this handler is specific to the Product entity.
  • @Autowired private ApplicationEventPublisher publisher;: We inject the ApplicationEventPublisher to publish our custom event.
  • @HandleAfterGet: This annotation marks the handleAfterGet method to be invoked after an entity of type Product is successfully retrieved by a GET request to /products/{id}. The retrieved Product instance is passed as an argument.
  • Inside the method, we create and publish our ProductReadEvent.

• Create an Event Listener:

  import org.springframework.context.event.EventListener;
  import org.springframework.stereotype.Component;
  
  @Component
  public class ProductReadEventListener {
  
      @EventListener
      public void handleProductReadEvent(ProductReadEvent event) {
          Product readProduct = event.getProduct();
          System.out.println("Listener received ProductReadEvent for product: " + readProduct.getName());
          // Perform actions based on the read event (e.g., logging access)
      }
  }
  

2. Using a Custom Controller

For more control over the read operation and the event triggering, you can create a custom Spring MVC controller that handles the retrieval of your entity. This bypasses the default Spring Data REST handling for that specific endpoint.

• Create a custom controller:

  import org.springframework.beans.factory.annotation.Autowired;
  import org.springframework.context.ApplicationEventPublisher;
  import org.springframework.data.rest.webmvc.RepositoryRestController;
  import org.springframework.http.ResponseEntity;
  import org.springframework.web.bind.annotation.GetMapping;
  import org.springframework.web.bind.annotation.PathVariable;
  import org.springframework.web.bind.annotation.ResponseBody;
  
  @RepositoryRestController
  public class CustomProductController {
  
      @Autowired
      private ProductRepository productRepository;
  
      @Autowired
      private ApplicationEventPublisher publisher;
  
      @GetMapping("/products/{id}/read-event") // Custom endpoint
      public @ResponseBody ResponseEntity<?> getProductAndTriggerEvent(@PathVariable Long id) {
          return productRepository.findById(id)
                  .map(product -> {
                      publisher.publishEvent(new ProductReadEvent(this, product));
                      System.out.println("Product read event triggered via custom controller for ID: " + id);
                      return ResponseEntity.ok(product);
                  })
                  .orElse(ResponseEntity.notFound().build());
      }
  }
  

  • @RepositoryRestController: This annotation is used for controllers that handle operations on Spring Data REST managed repositories.
  • We inject the ProductRepository to fetch the entity.
  • We define a custom endpoint /products/{id}/read-event.
  • Inside the method, we retrieve the product, publish our ProductReadEvent, and then return the product in the response.

  Note: This approach requires you to define a custom endpoint. The default /products/{id} endpoint will still function without triggering this event. If you want to override the default behavior, you would need to adjust the @GetMapping on your custom controller to /products/{id}. However, this might lead to unexpected behavior with other Spring Data REST features for that endpoint.

3. Intercepting Repository Methods with AOP (Aspect-Oriented Programming)

You could use AOP to intercept the execution of your repository’s findById() method (or other read methods) and publish an event after it returns successfully.

• Create an Aspect:

  import org.aspectj.lang.annotation.AfterReturning;
  import org.aspectj.lang.annotation.Aspect;
  import org.springframework.beans.factory.annotation.Autowired;
  import org.springframework.context.ApplicationEventPublisher;
  import org.springframework.stereotype.Component;
  
  @Aspect
  @Component
  public class ReadEventAspect {
  
      @Autowired
      private ApplicationEventPublisher publisher;
  
      @AfterReturning(pointcut = "execution(* com.example.repository.ProductRepository.findById(..))", returning = "result")
      public void afterProductRead(Object result) {
          if (result instanceof java.util.Optional) {
              ((java.util.Optional<?>) result).ifPresent(product -> {
                  if (product instanceof Product) {
                      publisher.publishEvent(new ProductReadEvent(this, (Product) product));
                      System.out.println("Product read event triggered via AOP for product: " + ((Product) product).getId());
                  }
              });
          }
      }
  }
  

  • @Aspect and @Component: Mark this class as an Aspect and a Spring-managed component.
  • @AfterReturning: This advice runs after the findById() method of ProductRepository returns successfully.
  • pointcut: Specifies the method to intercept. Adjust the package and class name accordingly.
  • returning = "result": Binds the return value of the intercepted method to the result parameter.
  • We check if the result is an Optional<Product> and, if present, publish the ProductReadEvent.

Choosing the Right Approach

• @RepositoryEventHandler (@HandleAfterGet): This is the most straightforward and idiomatic way to trigger events specifically after a Spring Data REST GET request for a single item. It’s tightly coupled with the REST layer.
• Custom Controller: Provides the most flexibility if you need to perform additional logic or customize the endpoint for triggering the read event. However, it involves more manual controller code.
• AOP: Offers a more decoupled way to intercept repository method calls, regardless of how they are invoked (via REST or other services). This can be useful if you want to trigger read events in other parts of your application as well. However, AOP can be more complex to understand and debug.

Considerations

• Performance: Be mindful of the operations you perform in your event listeners. Long-running tasks could impact the response time of your GET requests. Consider asynchronous processing for time-consuming operations.
• Context: Ensure your event contains enough context (e.g., the retrieved entity, user information if available) for your listeners to perform their tasks effectively.
• Idempotency: If the actions triggered by your read event are not idempotent, be cautious about scenarios where a GET request might be retried.

Conclusion

While Spring Data REST doesn’t provide built-in events for GET requests, you can effectively trigger custom events using @RepositoryEventHandler, custom controllers, or AOP. The @RepositoryEventHandler with @HandleAfterGet is generally the most aligned with Spring Data REST’s philosophy for handling REST-specific events. Choose the approach that best suits your needs in terms of coupling, control, and the scope of where you want to trigger these read-related actions. Remember to design your events and listeners carefully to ensure performance and maintainability.