Error “Can not write a field name, expecting a value;” while Writing JSON on Spring MVC

The error started since i upgraded my jackson library, somehow it never happen before. This is my full stacktrace

com.fasterxml.jackson.core.JsonGenerationException: Can not write a field name, expecting a value
	com.fasterxml.jackson.core.JsonGenerator._reportError(JsonGenerator.java:1649)
	com.fasterxml.jackson.core.json.UTF8JsonGenerator.writeFieldName(UTF8JsonGenerator.java:186)
	com.edw.test.jackson.TestSerializer.serialize(TestSerializer.java:27)
	com.edw.test.jackson.TestSerializer.serialize(TestSerializer.java:19)
	com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:130)
	com.fasterxml.jackson.databind.ObjectWriter$Prefetch.serialize(ObjectWriter.java:1387)
	com.fasterxml.jackson.databind.ObjectWriter.writeValue(ObjectWriter.java:889)
	org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.writeInternal(AbstractJackson2HttpMessageConverter.java:292)
	org.springframework.http.converter.AbstractGenericHttpMessageConverter.write(AbstractGenericHttpMessageConverter.java:106)
	org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:231)
	org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.handleReturnValue(RequestResponseBodyMethodProcessor.java:174)
	org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite.handleReturnValue(HandlerMethodReturnValueHandlerComposite.java:81)
	org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:113)
	org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:827)
	org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:738)
	org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85)
	org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:967)
	org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:901)
	org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970)
	org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:861)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:622)
	org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
	org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
	org.netbeans.modules.web.monitor.server.MonitorFilter.doFilter(MonitorFilter.java:393)

Looks like this error happen on my custom json serializer,

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:p="http://www.springframework.org/schema/p"
       xmlns:context="http://www.springframework.org/schema/context"
       xmlns:mvc="http://www.springframework.org/schema/mvc"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-4.0.xsd
        http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc-4.0.xsd
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-4.0.xsd">
    
    <mvc:annotation-driven>
        <mvc:message-converters register-defaults="false">
            <bean class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter" p:objectMapper-ref="testObjectMapper"/>
        </mvc:message-converters>
    </mvc:annotation-driven>
    
    <context:component-scan base-package="com.edw.test.jackson"/>
</beans>

And this is the content of my objectMapper java class,

package com.edw.test.jackson;

import com.edw.test.jackson.bean.Test;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;
import com.fasterxml.jackson.databind.module.SimpleModule;
import org.springframework.stereotype.Component;

@Component
public class TestObjectMapper extends ObjectMapper {
    public TestObjectMapper() {
        super();
        
        SimpleModule module = new SimpleModule();
        module.addSerializer(Test.class, new TestSerializer());
        registerModule(module);
        configure(SerializationFeature.INDENT_OUTPUT, false);
    }
}

And as you can see, the main culprit is on class TestSerializer

package com.edw.test.jackson;

import com.edw.test.jackson.bean.Test;
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.SerializerProvider;
import java.io.IOException;

public class TestSerializer extends JsonSerializer<Test> {

    @Override
    public void serialize(Test t, JsonGenerator jgen, SerializerProvider sp) throws IOException, JsonProcessingException {
        jgen.writeStartObject();
        jgen.writeRaw("{");
        jgen.writeFieldName("test01");
        jgen.writeRaw(t.getTest01());
        jgen.writeFieldName("test02");
        jgen.writeRaw(t.getTest02());
        jgen.writeRaw("}");
        jgen.writeEndObject();
    }
}

replacing writeFieldName method with writeRaw method solve my problem.

package com.edw.test.jackson;

import com.edw.test.jackson.bean.Test;
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.SerializerProvider;
import java.io.IOException;

public class TestSerializer extends JsonSerializer<Test> {
    @Override
    public void serialize(Test t, JsonGenerator jgen, SerializerProvider sp) throws IOException, JsonProcessingException {
        jgen.writeStartObject();
        jgen.writeRaw("test01");
        jgen.writeRaw(t.getTest01());
        jgen.writeRaw("test02");
        jgen.writeRaw(t.getTest02());
        jgen.writeEndObject();
    }
}

Hope it helps :)

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