Life is really simple, but men insist on making it complicated.

Summary

In some case, to return objects for a RESTful call declared in Controller, via Spring or SpringBoot. You may face an exception like below

HttpMessageNotWritableException: Could not write JSON: Value out of range. Value:”20220406” Radix:10;

Error details

Here is one full error log for such case

.w.s.m.s.DefaultHandlerExceptionResolver : Resolved [org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON: Value out of range. Value:"20220406" Radix:10; nested exception is com.fasterxml.jackson.databind.JsonMappingException: Value out of range. Value:"20220406" Radix:10 (through reference chain: org.springframework.statemachine.support.DefaultExtendedState["variables"]->org.springframework.statemachine.support.ObservableMap["my-workflow-step1-datetime"]→com.google.gson.JsonPrimitive["asByte"])]

Troubleshoot & explanation

As you know, when you define one output for a RESTful mapping method (e.g. GetMapping). Internally, Spring will invoke a mechanism called HttpMessageConverter to transform message.

In Spring-web, there is one class for this case is AbstractGenericHttpMessageConverter.java

Afterwards, corresponding concreate Serializer will determined based on data type. E.g. if method return type is a Map, one MapSerializer.java from jackson-databind will be employed. As per below screenshot

The root cause of this error is Jackson will guess and try to convert value to a String. For this test case, it will try to convert it to byte which will failed at last.

As below highlighted errors:

solution

Due to the message is in Gson.JsonPrimitive, we can let Spring to use Google Gson library as default Json converter library, to replace JackSon Json as Spring’s default Json converter.

To config it, update your application.yml or application.properties.

spring:
  mvc:
    converters:
      preferred-json-mapper: gson

–HTH–