报错信息

Type definition error: [simple type, class cn.hutool.json.JSONNull]; nested exception is com.fasterxml.jackson.databind.exc.InvalidDefinitionException: No serializer found for class cn.hutool.json.JSONNull and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS)

1.用string替换null

str.replaceAll("null","\"\"");

2.或者

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.SerializerProvider;
import org.springframework.boot.jackson.JsonComponent;
import java.io.IOException;

@JsonComponent
public class JsonNullSerializer extends JsonSerializer<JSONNull> {
    @Override
    public void serialize(JSONNull jsonNull, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException {
        jsonGenerator.writeNull();
    }
}