Why is the Spring REST Response Different in a Custom Controller?

What causes the REST response to vary in a custom controller within a Spring application?

When working with Spring Framework to create RESTful web services, developers might encounter situations where the response from a custom controller deviates from expectations. This variation can stem from several factors, including misconfiguration, handler method return types, and serialization issues. Understanding these causes is crucial to delivering consistent and predictable response data.

@RestController
@RequestMapping("/api/example")
public class ExampleController {

    @GetMapping
    public ResponseEntity<ExampleResponse> getExample() {
        ExampleResponse response = new ExampleResponse();
        response.setData("Sample data");
        return ResponseEntity.ok(response);
    }
}

Causes

• Incorrect mapping of request paths or methods leading to unexpected controllers being triggered.
• Mismatch between Media Types specified by the controller and those requested by the client, leading to content negotiation failures.
• Serialization problems due to entities lacking proper annotations such as @JsonIgnore, resulting in incomplete or omitted data in the response.
• Usage of ResponseEntity or misconfigured response types can alter the structure of the response unexpectedly.

Solutions

• Ensure that the request mapping in the controller correctly corresponds to the intended endpoint paths and methods by using @RequestMapping, @GetMapping, or similar annotations appropriately.
• Use the @ResponseBody annotation or return ResponseEntity instances to control the response directly and ensure the desired format is maintained.
• Validate the data model and confirm that all required fields are annotated correctly for serialization with JSON libraries like Jackson or Gson, ensuring visibility during the object transformation.
• Perform thorough exception handling with @ControllerAdvice to manage errors gracefully and maintain consistent response structures across your application.

Mistake: Forgetting to annotate response classes with @JsonProperty, leading to missing fields in the output.

Solution: Ensure all fields you want to serialize are properly annotated and that your model class adheres to JavaBean conventions.

Mistake: Not properly handling content negotiations, which can lead to HTML responses instead of JSON.

Solution: Specify media types explicitly using the @RequestMapping or similar annotations to enforce the desired response format.