How to use specific custom objectMapper/JsonSerializer for specific requestMapping - json

Question

I know thar Jackson allow us to use custom serializer for specific domain/entity/modle, like this:
#JsonSerialize(using = CustomSerializer.class)
public class SimpleDomain {
}
And, is there any idea to specify custom serializer for some requestMapping,
(only for the specific requestMapping(that method), not to set global objectMapper.) like this:
#RequestMapping(method = RequestMethod.GET, value = "hello")
#JsonSerialize(nullsUsing = NullToEmptyStrSerializer.class)
public #ResponseBody
Object get() {
return new HashMap<String, Object>() {{
put("aa", null);
put("bb", "");
}};
}
I execute the code above, it returns:
{
"aa": null,
"bb": ""
}
instead of:
{
"aa": "",
"bb": ""
}
which exactly i wanted.
NullToEmptyStrSerializer :
public class NullToEmptyStrSerializer extends JsonSerializer {
#Override
public void serialize(Object value, JsonGenerator gen, SerializerProvider serializers) throws IOException, JsonProcessingException {
gen.writeString("");
}
}
Or, can i get HttpServletRequest in NullToEmptyStrSerializer, if yes, i can filter in NullToEmptyStrSerializer.

Answer 1

It may be a known limitation (#JsonSerialize with nullUsing option not working for String properties) and reported in this github issue.
It is scheduled to be released in jackson 2.9.3 and 2.8.11.
Fix will be in 2.9.3 but I also back-ported it in 2.8 branch in case
2.8.11 might be released at some point.

Answer 2

Finally, I found it out by myself.
I realized this point:
Or, can i get HttpServletRequest in NullToEmptyStrSerializer, if yes, i can filter in NullToEmptyStrSerializer.
My purpose :
Custom convert null to "" for specific Url(RequestMapping)
My solution :
Define a bean to keep ApplicationContext storing in a static field, also, a static getter:
#Component
public class ContextHolder {
private static ApplicationContext applicationContext;
#Resource
public void setApplicationContext(ApplicationContext applicationContext) {
ContextHolder.applicationContext = applicationContext;
}
public static ApplicationContext get(){
return applicationContext;
}
}
Define a bean below, this bean will create for every request, thus, i store ServletRequest in this bean.
#Component("servletRequestHolder")
#Scope(value = WebApplicationContext.SCOPE_REQUEST, proxyMode = ScopedProxyMode.TARGET_CLASS)
public class ServletRequestHolder {
#Resource
private HttpServletRequest request;
public HttpServletRequest getRequest(){
return request;
}
}
Then, a serializer is needed.
public class NullToEmptyStringSerializer extends JsonSerializer.None {
public static List<String> convertUrls = new ArrayList<String>(){{
add("/hello");
}};
#Override
public void serialize(Object value, JsonGenerator gen, SerializerProvider serializers) throws IOException {
HttpServletRequest request = ContextHolder.get().getBean(ServletRequestHolder.class).getRequest();
if (request != null) {
String currentUrl = request.getRequestURI();
boolean match = convertUrls.contains(currentUrl);
if (match) {
gen.writeString("");
} else {
gen.writeObject(null);
}
} else {
gen.writeObject(null);
}
}
}
Add the Serializer to objectMapper:
DefaultSerializerProvider.Impl sp = new DefaultSerializerProvider.Impl();
sp.setNullValueSerializer(new NullToEmptyStringSerializer());
objectMapper.setSerializerProvider(sp);
Finally, test it :
#GetMapping({"hello", "hello1"})
public Object get() {
return new HashMap<String, Object>() {{
put("a", null);
put("b", "");
}};
}
In case of requesting localhost:20000/hello, the client receive:
{
"aa": "",
"bb": ""
}
And for requesting localhost:20000/hello1, the client receive:
{
"aa": null,
"bb": ""
}