Manipulate annotated property values as the DTO hits the Spring controller?

Question

Can you manipulate properties (e.g. set null ones to Hello world) annotated with a custom annotation as they are handed over to methods in your controller?

For example, let's assume we have a nested DTO:

public class MyDto {
  @MyAnnotation
  private String myProperty;

  private String unannotatedPropety;

  private InnerEntity innerEntity;

  // Getters and setters omitted for brevity
}

public class InnerEntity {
  
  @MyAnnotation
  private String anotherProperty;
  
  // Getters and setters omitted for brevity
}

@RestController(...)
public class MyController {
  
  @PostMapping(...)
  public Mono<ResponseEntity<MyResponse>> myRequestHandler(@RequestBody Mono<MyEntity> json) {
    // ..
  }
}

I initially thought a ConditionalGenericConverter could do the trick (its signature allows for null values to be converted, and it provides TypeDescriptors for its source and target properties, making introspection a breeze), but for controllers, Spring actually uses HttpMessageConverter for payloads (kudos), it seems, and I didn't want to reinvent the entire Jackson deserializer.

On the other hand, Spring + Hibernate Validator manage to introspect payloads and check all properties for specific annotations, so getting to annotated properties should be possible, I hope...

I could probably use AspectJ, but I want this to work in general, and not rely on the payload being of a specific type (like MyDto)... I'm basically hoping there exists a hook I can use, just like the converters API, that does the heavy lifting of reflection for me...

Is there an (easy) approach to do what I want to do?

Answer 1

The intl package is what you are looking for. It will format numbers in any locale for you. The format you use in your examples is the same as German, for example:

import 'package:intl/intl.dart';


void main () {
  final fmt = NumberFormat("#,##0.00", "de_DE");
  print(fmt.format(10525.25));
  print(fmt.format(40000.25));
}

Output:

10.525,25
40.000,25

You may need to tweak the first parameter to NumberFormat to suit your formatting requirement.