Spring Boot String Deserializer for every form field - spring-boot

Question

In our web app create and update forms have a size validation. For instance:
#Size(min = 4, max = 20)
private String mobile;
As seen the field is not required. But at the front-end user wants to clear field. Then form validation fails because of length restriction. Incoming data is an empty string instead of null. So minimum length validation restricts the input.
Therefore I start to search a solution to convert empty strings to null values. I found a #InitBinder and StringTrimmerEditor solution but our system uses #ResponseBody approach. So It doesn't fit.
Adding #JsonDeserialize(using = CustomTrimDeserializer.class) annotation or writing a custom setter for every string field is not DRY solution.
I just want to add app wide custom deserializer for String fields.

Answer 1

I finally examine the JsonComponentModule class and noticed spring is looking for the JsonComponent annotation for deserializer registration.
This is a one file spring boot project for solution
#RestController
#SpringBootApplication
public class CheckNullApplication {
public static void main(String[] args) {
SpringApplication.run(CheckNullApplication.class, args);
}
#PostMapping("/check-null")
public boolean checkNull(#RequestBody final HelloForm form) {
return form.getName() == null;
}
public static class HelloForm {
private String name;
public String getName() { return name; }
public void setName(final String name) { this.name = name;}
}
#JsonComponent
public static class StringTrimmerDeserializer extends JsonDeserializer<String> {
#Override
public String deserialize(final JsonParser p, final DeserializationContext ctxt)
throws IOException, JsonProcessingException {
String result = StringDeserializer.instance.deserialize(p, ctxt);
if (result != null) {
result = result.trim();
if (StringUtils.isEmpty(result)) {
return null;
}
}
return result;
}
}
}

Answer 2

Instead of adding #JsonDeserialize annotation you may want to just register your custom deserializer via Module (for example, SimpleModule), and it will apply to all String valued properties. Something like:
SimpleModule module = new SimpleModule(...);
module.addDeserializer(String.class, new CustomTrimDeserializer());
mapper.registerModule(module);

Answer 3

Create a class as following and annotate with #JsonComponent. Spring boot will pick that up as a component.
import com.fasterxml.jackson.databind.deser.std.StringDeserializer;
#JsonComponent
public class WhitSpaceTrimmerDeserializer extends StringDeserializer {
#Override
public String deserialize(JsonParser p, DeserializationContext ctxt) throws IOException {
final String value = super.deserialize(p, ctxt);
return value!=null?value.trim():null;
}