Automatic deserialization of String to Object with Jackson

Context

Say you have:

public class Dto {
private String name;
private String List<Custom> customs;
// getters and setters...
}

and

public class Custom {
private String something;
private String else;
// getters and setters...
}

Your Spring MVC RestController receives a list of Dto:

@PostMapping
public String create(@RequestBody @Valid List<Dto> dtos) {
return myService.process(features);
}

Input

However, you know that the client-side service which will send data to your controller will send something like this:

[
{
"name": "Bob",
"customs": [
"{\n        \"something\": \"yes\",\n        \"else\": \"no\"\n      }"
]
}
]

Notice how the List<Custom> actually ends up being received as a List<String>. Please assume this cannot be changed on the client-side and we have to deal with it on the server-side.

Question

Is there a Jackson annotation which would automagically take the input String and try to serialize it into a Custom class?

Attempts

A few things that didn't work, including:

@JsonSerialize(using = ToStringSerializer.class)
private List<Custom> customs;

along with

public Custom(String json) {
try {
new ObjectMapper().readerFor(Custom.class).readValue(json);
} catch (JsonProcessingException e) {
e.printStackTrace();
}
}

As it is, we have had to change the customs type to List<String> and add a utility method which converts a String into a Custom using an ObjectMapper. This is rather dissatisfying.

You need to implement custom deserialiser or converter which would be used to convert given payload to required type. One trick, you could use is to create new ObjectMapper and use it for internal deserialisation.

Example usage:

class CustomConverter extends StdConverter<String, Custom> {
private final ObjectMapper mapper = new ObjectMapper();
@Override
public Custom convert(String value) {
try {
return mapper.readValue(value, Custom.class);
} catch (JsonProcessingException e) {
throw new IllegalArgumentException(value);
}
}
}
class Dto {
private String name;
@JsonDeserialize(contentConverter = CustomConverter.class)
private List<Custom> customs;
}

You need to create a custom Deserializer.

public class CustomDeserializer extends StdDeserializer<Custom> { 
 
    public CustomDeserializer() { 
        this(null); 
    } 
 
    public CustomDeserializer(Class<?> vc) { 
        super(vc); 
    }
 
    @Override
    public Custom deserialize(JsonParser jp, DeserializationContext ctxt) 
      throws IOException, JsonProcessingException {
        JsonNode node = jp.getCodec().readTree(jp);
        int id = (Integer) ((IntNode) node.get("id")).numberValue();
        String name = node.get("name").asText();
        ...
        return new Custom(id, name, ...);
    }
}

and register the deserializer on the Custom class:

@JsonDeserialize(using = CustomDeserializer.class)
public class Custom {
  ...
}