[FIXED] Spring boot @RequestBody default POJO mapping behavior?

Issue

I have a java class with uppercase field names and some of them with under scroll, like this:

public class DATADto {
  private String UPPERCASE;
  private String UNDER_SCROLL;

  public String getUPPERCASE() { return UPPERCASE; }
  public void setUPPERCASE(String s) { UPPERCASE = s; }
  ...//setters and getters
}

and I used this in a rest endpoint that accepts json in a spring rest controller:

@RestController
@RequestMapping({"/api/path"})
public class MyRestController {
   @PostMapping(path = {"/Data"}, consumes = MediaType.APPLICATION_JSON_VALUE,
            produces = MediaType.APPLICATION_JSON_VALUE)

    public ResponseEntity<?> useDATADto(@RequestBody DATADto aDATADto ) {
     //do something
    }
}

what JSON fields do I need to send by default and why?

Solution

The story goes like this..

Spring Boot by default uses Jackson ObjectMapper to serialize and deserialize Java objects.

In this context, by serialization we mean the conversion of java objects into json, deserialization is the reverse process.

Regarding the @RequestBody annotation, the following is written in the documentation:

Annotation indicating a method parameter should be bound to the body of the web request. The body of the request is passed through an HttpMessageConverter to resolve the method argument depending on the content type of the request. Optionally, automatic validation can be applied by annotating the argument with @Valid.

In short, @RequestBody annotation tells Spring to deserialize an incoming request body into an object passed as a parameter to the handler method. Spring achieves this using MessageConverter

Since Spring Boot uses Jackson by default for serializing and deserializing request and response objects in your REST APIs, and Jackson uses MappingJackson2HttpMessageConverter, so that will be message converter implementation that spring will use. You can read more about that here.

The important thing is that Jackson uses Java Bean naming conventions to figure out the json properties in a Java class. Acutally it uses default PropertyNamingStrategy . Here is what is written in documentation:

In absence of a registered custom strategy, default Java property naming strategy is used, which leaves field names as is, and removes set/get/is prefix from methods (as well as lower-cases initial sequence of capitalized characters).

So, since you didn't set any naming strategy, it will use default one.

Beacause of that, if you send payload like this :

{
    "uppercase": "YOUR_VALUE",
    "under_scroll": "YOUR_VALUE"
}

That won't work, you will get exception, since there jackson won't find under_scroll property in your class, it will look for under_SCROLL , therefore this payload:

{
    "uppercase": "YOUR_VALUE",
    "under_SCROLL": "YOUR_VALUE"
}

will work.

To change default PropertyNamingStrategy check this article.

Answered By - Nemanja
Answer Checked By - Mary Flores (JavaFixing Volunteer)