We Keep Coding

Remove some keys when converting from custom type into Map using Jackson's convertValue

I have a rather simple Java object that I would like to convert it into a Map<String, Object> using Jackson's convertValue(fromValue, toValueTypeRef) — this is rather trivial.
At the same time, there are some fields/members (e.g.: createdOn and modifiedOn) I would like to remove from the resultant Map, but I can't remove them in all (de)serialization routines. I was wondering if there is a way to tell Jackson to do that, rather than removing them "by hand". I've usually seen #JsonView used for such purposes, but I think it doesn't work with convertValue — or I'm still unable to configure it correctly.
This is how I'm doing the conversion:
#NoArgsConstructor
public final class CustomTypeMapper {
private static final TypeReference<Map<String, Object>> MAP_TYPE_REFERENCE = new TypeReference<>() { };
private static final ObjectMapper OBJECT_MAPPER = new ObjectMapper(); // ...with some customizations
public Map<String, Object> convertFrom(final CustomType input) {
// OBJECT_MAPPER.writerWithView(Views.Public.class);
return OBJECT_MAPPER.convertValue(input, MAP_TYPE_REFERENCE);
}
}
CustomType looks roughly like this:
#Getter
#Builder
#Jacksonized
public final class CustomType {
#JsonProperty("id")
private String id;
#JsonProperty("user")
private User user;
...
#Builder.Default
#JsonProperty("created_on") // TODO: Exclude this field/member some times
#JsonView(Views.Internal.class)
private final Instant createdOn = Instant.now();
#Builder.Default
#JsonView(Views.Internal.class)
#JsonProperty("modified_on") // TODO: Exclude this field/member some times
private Instant modifiedOn = Instant.now();
}

Converting the unstructured object in java

I'm using MongoDb for unstructured documents. When I do the aggregations, I'm getting final output as unstructured objects. I post some sample data for the easiness. Actual objects have many fields.
Eg :
[
{ _id : "1", type: "VIDEO", videoUrl : "youtube.com/java"},
{ _id : "2", type: "DOCUMENT", documentUrl : "someurl.com/spring-boot-pdf"},
{ _id : "3", type: "ASSESSMENT", marks : 78}
]
The respective class for the types of above objects are
#Data
public class Video{
private String _id;
private String type;
private String videoUrl;
}
#Data
public class Document{
private String _id;
private String type;
private String documentUrl;
}
#Data
public class Assessment{
private String _id;
private String type;
private Integer marks;
}
Since I can't specify the converter class, I get all objects as list of Object.class which is a general type for all.
List<Object> list = mongoTemplate.aggregate(aggregation, mongoTemplate.getCollectionName(YOUR_COLLECTION.class), Object.class).getMappedResults();
It's working, but this is not readable and not maintainable for backend and front-end developers (eg : swagger ui). So I came up with a solution, that put all fields as a class.
#Data
#JsonInclude(JsonInclude.Include.NON_NULL)
class MyConvetor{
private String _id;
private String type;
private String videoUrl;
private String documentUrl;
private Integer marks;
}
Here Jackson helps to ignore all null fields
Now I can use MyConverter as Type
List<MyConverter> list = mongoTemplate.aggregate(aggregation, mongoTemplate.getCollectionName(YOUR_COLLECTION.class), MyConverter.class).getMappedResults();
But I feel this is not a good practice when we implementing a standard application. I'd like to know, is there any way to avoid the general type class (e.g. extending any abstract class)? Or is this the only way I can do?

I don't think so (or I don't know) if MongoDB in Java provides this kind of dynamic conversion by some field (it would require specify what field and what classes). But you can do it by hand.
First, you need to define your types (enum values or some map) for matching string to class. You can create abstract parent class (eg. TypedObject) for easier usage and binding all target classes (Video, Document, Assessment) .
Next you have to read and map values from Mongo to anything because you want to read all data in code. Object is good but I recommend Map<String, Object> (your Object actually is that Map - you can check it by invoking list.get(0).toString()). You can also map to String or DBObject or some JSON object - you have to read "type" field by hand and get all data from object.
At the end you can convert "bag of data" (Map<String, Object> in my example) to target class.
Now you can use converted objects by target classes. For proving these are actually target classes I print objects with toString all fields.
Example implementation
Classes:
#Data
public abstract class TypedObject {
private String _id;
private String type;
}
#Data
#ToString(callSuper = true)
public class Video extends TypedObject {
private String videoUrl;
}
#Data
#ToString(callSuper = true)
public class Document extends TypedObject {
private String documentUrl;
}
#Data
#ToString(callSuper = true)
public class Assessment extends TypedObject {
private Integer marks;
}
Enum for mapping string types to classes:
#RequiredArgsConstructor
public enum Type {
VIDEO("VIDEO", Video.class),
DOCUMENT("DOCUMENT", Document.class),
ASSESSMENT("ASSESSMENT", Assessment.class);
private final String typeName;
private final Class<? extends TypedObject> clazz;
public static Class<? extends TypedObject> getClazz(String typeName) {
return Arrays.stream(values())
.filter(type -> type.typeName.equals(typeName))
.findFirst()
.map(type -> type.clazz)
.orElseThrow(IllegalArgumentException::new);
}
}
Method for converting "bag of data" from JSON to your target class:
private static TypedObject toClazz(Map<String, Object> objectMap, ObjectMapper objectMapper) {
Class<? extends TypedObject> type = Type.getClazz(objectMap.get("type").toString());
return objectMapper.convertValue(objectMap, type);
}
Read JSON to "bags of data" and use of the above:
String json = "[\n" +
" { _id : \"1\", type: \"VIDEO\", videoUrl : \"youtube.com/java\"},\n" +
" { _id : \"2\", type: \"DOCUMENT\", documentUrl : \"someurl.com/spring-boot-pdf\"},\n" +
" { _id : \"3\", type: \"ASSESSMENT\", marks : 78}\n" +
"]";
ObjectMapper objectMapper = new ObjectMapper();
objectMapper.configure(JsonParser.Feature.ALLOW_UNQUOTED_FIELD_NAMES, true);
List<Map<String, Object>> readObjects = objectMapper.readValue(json, new TypeReference<>() {});
for (Map<String, Object> readObject : readObjects) {
TypedObject convertedObject = toClazz(readObject, objectMapper);
System.out.println(convertedObject);
}
Remarks:
In example I use Jackson ObjectMapper for reading JSON. This makes the example and testing simpler. I think you can replace it with mongoTemplate.aggregate(). But anyway I need ObjectMapper in toClazz method for converting "bags of data".
I use Map<String, Object> instead of just Object. It is more complicated: List<Map<String, Object>> readObjects = objectMapper.readValue(json, new TypeReference<>() {});. If you want, you can do something like this: List<Object> readObjects2 = (List<Object>) objectMapper.readValue(json, new TypeReference<List<Object>>() {});
Result:
Video(super=TypedObject(_id=1, type=VIDEO), videoUrl=youtube.com/java)
Document(super=TypedObject(_id=2, type=DOCUMENT), documentUrl=someurl.com/spring-boot-pdf)
Assessment(super=TypedObject(_id=3, type=ASSESSMENT), marks=78)
Of course you can cast TypedObject to target class you need (I recommend checking instance of before casting) and use:
Video video = (Video) toClazz(readObjects.get(0), objectMapper);
System.out.println(video.getVideoUrl());
I assumed you read whole collection once and you get all types mixed up in one list (as in example in your question). But you can try find documents in MongoDB by field "type" and get data separately for each of type. With this you can easily convert to each type separately.

json serialization with a variable field as a string

I have a pojo that I am unmarshalling a REST response to. One of the fields ("variable value") is just a Json variable element (can be any form).
Is there a way to tell it to treat the field as a plain string for all cases instead of trying to deserialize to an object?
Here's a json obiect ("variable value" can be any form):
{"id":1, "variable value": {"name":"one", "age": 22, "data":{"key":"value"}}}
I would like to save this json as a class object using gson
public class SomeCommand {
private Long id;
private String data;
}

It sounds that you would like to parse the given JSON string to transform variable value into String object. You can achieve this by creating 2 classes - SomeCommandOriginal and SomeCommand as follows:
First, convert the JSON string to SomeCommandOriginal to map the value of variable value to JsonNode.
class SomeCommandOriginal {
private Long id;
#JsonProperty("variable value")
private JsonNode variableValue;
//general getters and setters
}
class SomeCommand {
private Long id;
private String data;
public SomeCommand(SomeCommandOriginal someCommandOriginal) {
super();
this.id = someCommandOriginal.id;
this.data = someCommandOriginal.variableValue.toString();
}
//general getters and setters
}
Second, initialize an instance of SomeCommand and pass someCommandOriginal as the argument of customized constructor:
ObjectMapper mapper = new ObjectMapper();
SomeCommandOriginal someCommandOriginal = mapper.readValue(jsonStr, SomeCommandOriginal.class);
SomeCommand someCommand = new SomeCommand(someCommandOriginal);
System.out.println(someCommand.getData());
Console output:
{"name":"one", "age": 22, "data":{"key":"value"}}
UPDATED
If you are using Gson, just modify the datatype of variableValue to be JsonObject and switch to #SerializedName annotation as follows:
class SomeCommandOriginal {
private Long id;
#SerializedName("variable value")
private JsonObject variableValue;
//general getters and setters
}
And then you can get the same result as well:
Gson gson = new Gson();
SomeCommandOriginal someCommandOriginal = gson.fromJson(jsonStr, SomeCommandOriginal.class);
SomeCommand someCommand = new SomeCommand(someCommandOriginal);
System.out.println(someCommand.getData());

change field name to lowercase while deserializing POJO to JSON using GSON?

I have a POJO class like this. I am deserializing my JSON to below POJO first..
public class Segment implements Serializable {
#SerializedName("Segment_ID")
#Expose
private String segmentID;
#SerializedName("Status")
#Expose
private String status;
#SerializedName("DateTime")
#Expose
private String dateTime;
private final static long serialVersionUID = -1607283459113364249L;
...
...
...
// constructors
// setters
// getters
// toString method
}
Now I am serializing my POJO to a JSON like this using Gson and it works fine:
Gson gson = new GsonBuilder().create();
String json = gson.toJson(user.getSegments());
System.out.println(json);
I get my json printed like this which is good:
[{"Segment_ID":"543211","Status":"1","DateTime":"TueDec2618:47:09UTC2017"},{"Segment_ID":"9998877","Status":"1","DateTime":"TueDec2618:47:09UTC2017"},{"Segment_ID":"121332121","Status":"1","DateTime":"TueDec2618:47:09UTC2017"}]
Now is there any way I can convert "Segment_ID" to all lowercase while deserializing? I mean "Segment_ID" should be "segment_id" and "Status" should be "status". Is this possible to do using gson? So it should print like this instead.
[{"segment_id":"543211","status":"1","datetime":"TueDec2618:47:09UTC2017"},{"segment_id":"9998877","status":"1","datetime":"TueDec2618:47:09UTC2017"},{"segment_id":"121332121","status":"1","datetime":"TueDec2618:47:09UTC2017"}]
if I change the "SerializedName" then while deserializing my JSON to POJO, it doesn't work so not sure if there is any other way.

You need to provide alternative names for deserialisation process and primary (value property) for serialisation.
class Segment {
#SerializedName(value = "segment_id", alternate = {"Segment_ID"})
#Expose
private String segmentID;
#SerializedName(value = "status", alternate = {"Status"})
#Expose
private String status;
#SerializedName(value = "datetime", alternate = {"DateTime"})
#Expose
private String dateTime;
}
Now, you can deserialise fields: Segment_ID, DateTime, Status and still be able to serialise as desired.

Parsing JSON with Retrofit and GSON, error when trying to parse and obtain callback.

I am using to Retrofit to handle Calls to my API for an Android Application. I am trying to get Retrofit to handle the parsing of the JSON, and creating a list of Objects in accordance with the POJO i have created.
The error i receive is "com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected a string but was BEGIN_OBJECT at line 1 column 176".
I used JsonSchema2Pojo to generate my java classes. The classes and associated JSON are as follows.
{"status":"success","data":[{"sort_key":1,"event_id":1947357,"title":"2014 US Open Tennis Session 15 (Mens\/Womens Round of 16)","datetime_utc":"2014-09-01T15:00:00","venue":{"city":"Flushing","name":"Louis Armstrong Stadium","extended_address":"Flushing, NY 11368","url":"https:\/\/seatgeek.com\/venues\/louis-armstrong-stadium\/tickets\/?aid=10918","country":"US","display_location":"Flushing, NY","links":[],"slug":"louis-armstrong-stadium","state":"NY","score":0.73523,"postal_code":"11368","location":{"lat":40.7636,"lon":-73.83},"address":"1 Flushing Meadows Corona Park Road","timezone":"America\/New_York","id":2979},"images":["https:\/\/chairnerd.global.ssl.fastly.net\/images\/performers-landscape\/us-open-tennis-45e2d9\/5702\/huge.jpg","https:\/\/chairnerd.global.ssl.fastly.net\/images\/performers\/5702\/us-open-tennis-c1ccf7\/medium.jpg","https:\/\/chairnerd.global.ssl.fastly.net\/images\/performers\/5702\/us-open-tennis-01f513\/large.jpg","https:\/\/chairnerd.global.ssl.fastly.net\/images\/performers\/5702\/us-open-tennis-4e07f2\/small.jpg"]}
From this i believe i need to generate 3 POJO's, my higher level "EventObject" Class, A Location Class, and a Venue Class. These classes and their variables follow:
EventObject Class:
public class EventObject {
private Integer sortKey;
private Integer eventId;
private String title;
private String datetimeUtc;
private Venue venue;
private List<String> images = new ArrayList<String>();
private Map<String, Object> additionalProperties = new HashMap<String, Object>();
Location Class:
public class Location {
private Float lat;
private Float lon;
private Map<String, Object> additionalProperties = new HashMap<String, Object>();
Venue Class:
public class Venue {
private String city;
private String name;
private String extendedAddress;
private String url;
private String country;
private String displayLocation;
private List<Object> links = new ArrayList<Object>();
private String slug;
private String state;
private Float score;
private String postalCode;
private Location location;
private String address;
private String timezone;
private Integer id;
private Map<String, Object> additionalProperties = new HashMap<String, Object>();
My interface for the Api Call is as follows:
public interface UserEvents {
#GET("/user/get_events")
void getEvents(#Header("Authorization")String token_id,
#Query("event_type")String event_type,
#Query("postal_code")int postalCode,
#Query("per_page") int perPage ,
#Query("lat") int lat,
#Query("lon") int lon,
#Query("month")int month,
#Query("page")int page,
Callback<List<EventObject>>callback) ;
}
Here is its implementation in my code :
UserEvents mUserEvents = mRestAdapter.create(UserEvents.class);
mUserEvents.getEvents(token_Id, "sports",11209,25,0, 0, 9, 2, new Callback <List<EventObject>>() {
#Override
public void success(List<EventObject> eventObjects, retrofit.client.Response response) {
Log.d(TAG,"Success");
}
There is alot going on here, but i believe that i am probably going wrong with how i am handling the JSON. When i copied and pasted in my JSON to the Pojo generator, i did not include "status:success, " data:{
I essentially just used the entire entry of an element in the Array ( everything from {sort_key onward until the next sort key ) and pushed that through the converter.
This is my first try at Retrofit and API work, and parsing anything this complicated.
I am hoping its something that someone else will be able to point out. I have googled as well i could to sort this out with no luck.
Thanks for looking.

The main problem is that you are not getting the root element of the response. You need to create an entity "response" that gets the items status and data. It would look something like this:
public class RootObject {
#Expose
private String status;
#Expose
private EventObject data;
//getters and setters here
}
Then when you make the callback you should point to your RootObject, mUserEvents.getEvents(token_Id, "sports",11209,25,0, 0, 9, 2, new Callback <RootObject>()
One more thing, Retrofit uses GSON to parse your json reponse. It means that when you create the entities, the variables need to match the name of the objects coming in the response. If it doesn't you need to tell GSON how to map the variables, like this:
#SerializedName("extended_address")
#Expose
private String extendedAddress;
In that case the value coming in the json is "extended_address" and will be mapped to the String extendedAddress. If you don't put that #SerializedName line the parsing will fail. If you want to skip that line then you can call your variable "extended_address" so it matches the response.
The #Expose is needed by GSON to parse the variable below it. If a variable doesn't have it then GSON will ignore that parsing. So you need to fix both the #Expose and #SerializedName on your entities so GSON works correctly.
Hope it helps.