How do I use a custom Serializer with Jackson

Question

I have two Java classes that I want to serialize to JSON using Jackson   public class User       public final int id      public final String name       public User int id  String name            this id   id          this name   name           public class Item       public final int id      public final String itemNr      public final User createdBy       public Item int id  String itemNr  User createdBy            this id   id          this itemNr   itemNr          this createdBy   createdBy            I want to serialize an Item to this JSON     id  7   itemNr   TEST    createdBy  3    with User serialized to only include the id  I will also be able to serilize all user objects to JSON like     id  3   name    Jonas    email    jonas example com     So I guess that I need to write a custom serializer for Item and tried with this   public class ItemSerializer extends JsonSerializer lt Item gt      Override public void serialize Item value  JsonGenerator jgen          SerializerProvider provider  throws IOException          JsonProcessingException       jgen writeStartObject        jgen writeNumberField  id   value id       jgen writeNumberField  itemNr   value itemNr       jgen writeNumberField  createdBy   value user id       jgen writeEndObject           I serialize the JSON with this code from Jackson How-to  Custom Serializers   ObjectMapper mapper   new ObjectMapper    SimpleModule simpleModule   new SimpleModule  SimpleModule                                                  new Version 1 0 0 null    simpleModule addSerializer new ItemSerializer     mapper registerModule simpleModule   StringWriter writer   new StringWriter    try       mapper writeValue writer  myItem     catch  JsonGenerationException e        e printStackTrace      catch  JsonMappingException e        e printStackTrace      catch  IOException e        e printStackTrace        But I get this error   Exception in thread  main  java lang IllegalArgumentException  JsonSerializer of type com example ItemSerializer does not define valid handledType    use alternative registration method       at org codehaus jackson map module SimpleSerializers addSerializer SimpleSerializers java 62      at org codehaus jackson map module SimpleModule addSerializer SimpleModule java 54      at com example JsonTest main JsonTest java 54    How can I use a custom Serializer with Jackson     This is how I would do it with Gson   public class UserAdapter implements JsonSerializer lt User gt          Override      public JsonElement serialize User src  java lang reflect Type typeOfSrc              JsonSerializationContext context            return new JsonPrimitive src id                GsonBuilder builder   new GsonBuilder        builder registerTypeAdapter User class  new UserAdapter         Gson gson   builder create        String json   gson toJson myItem       System out println  JSON    json     But I need to do it with Jackson now  since Gson doesn t have support for interfaces

User · Answer

Use  JsonValue   public class User       int id      String name        JsonValue     public int getId             return id             JsonValue only works on methods so you must add the getId method  You should be able to skip your custom serializer altogether

User · Answer

I tried doing this too  and there is a mistake in the example code on the Jackson web page that fails to include the type   class  in the call to addSerializer   method  which should read like this   simpleModule addSerializer Item class  new ItemSerializer       In other words  these are the lines that instantiate the simpleModule and add the serializer  with the prior incorrect line commented out    ObjectMapper mapper   new ObjectMapper    SimpleModule simpleModule   new SimpleModule  SimpleModule                                              new Version 1 0 0 null       simpleModule addSerializer new ItemSerializer     simpleModule addSerializer Item class  new ItemSerializer     mapper registerModule simpleModule     FYI  Here is the reference for the correct example code  http   wiki fasterxml com JacksonFeatureModules

User · Answer

In my case  Spring 3 2 4 and Jackson 2 3 1   XML configuration for custom serializer      lt mvc annotation-driven gt       lt mvc message-converters register-defaults  false  gt           lt bean class  org springframework http converter json MappingJackson2HttpMessageConverter  gt               lt property name  objectMapper  gt                   lt bean class  org springframework http converter json Jackson2ObjectMapperFactoryBean  gt                       lt property name  serializers  gt                           lt array gt                               lt bean class  com example business serializer json CustomObjectSerializer   gt                           lt  array gt                       lt  property gt                   lt  bean gt               lt  property gt           lt  bean gt       lt  mvc message-converters gt   lt  mvc annotation-driven gt    was in unexplained way overwritten back to default by something   This worked for me   CustomObject java     JsonSerialize using   CustomObjectSerializer class  public class CustomObject        private Long value       public Long getValue             return value             public void setValue Long value            this value   value            CustomObjectSerializer java    public class CustomObjectSerializer extends JsonSerializer lt CustomObject gt          Override     public void serialize CustomObject value  JsonGenerator jgen          SerializerProvider provider  throws IOException JsonProcessingException           jgen writeStartObject            jgen writeNumberField  y   value getValue             jgen writeEndObject                Override     public Class lt CustomObject gt  handledType             return CustomObject class            No XML configuration   lt mvc message-converters gt       lt  mvc message-converters gt   is needed in my solution

User · Answer

I wrote an example for a custom Timestamp class serialization deserialization  but you could use it for what ever you want   When creating the object mapper do something like this   public class JsonUtils        public static ObjectMapper objectMapper   null       static           objectMapper   new ObjectMapper            SimpleModule s   new SimpleModule            s addSerializer Timestamp class  new TimestampSerializerTypeHandler             s addDeserializer Timestamp class  new TimestampDeserializerTypeHandler             objectMapper registerModule s              for example in java ee you could initialize it with this   import java time LocalDateTime   import javax ws rs ext ContextResolver  import javax ws rs ext Provider   import com fasterxml jackson databind ObjectMapper  import com fasterxml jackson databind module SimpleModule    Provider public class JacksonConfig implements ContextResolver lt ObjectMapper gt         private final ObjectMapper objectMapper       public JacksonConfig             objectMapper   new ObjectMapper            SimpleModule s   new SimpleModule            s addSerializer Timestamp class  new TimestampSerializerTypeHandler             s addDeserializer Timestamp class  new TimestampDeserializerTypeHandler             objectMapper registerModule s                Override     public ObjectMapper getContext Class lt   gt  type            return objectMapper            where the serializer should be something like this   import java io IOException  import java sql Timestamp   import com fasterxml jackson core JsonGenerator  import com fasterxml jackson core JsonProcessingException  import com fasterxml jackson databind JsonSerializer  import com fasterxml jackson databind SerializerProvider   public class TimestampSerializerTypeHandler extends JsonSerializer lt Timestamp gt          Override     public void serialize Timestamp value  JsonGenerator jgen  SerializerProvider provider  throws IOException  JsonProcessingException           String stringValue   value toString            if stringValue    null  amp  amp   stringValue isEmpty    amp  amp   stringValue equals  null                  jgen writeString stringValue             else               jgen writeNull                          Override     public Class lt Timestamp gt  handledType             return Timestamp class            and deserializer something like this   import java io IOException  import java sql Timestamp   import com fasterxml jackson core JsonGenerator  import com fasterxml jackson core JsonParser  import com fasterxml jackson core JsonProcessingException  import com fasterxml jackson databind DeserializationContext  import com fasterxml jackson databind JsonDeserializer  import com fasterxml jackson databind SerializerProvider   public class TimestampDeserializerTypeHandler extends JsonDeserializer lt Timestamp gt          Override     public Timestamp deserialize JsonParser jp  DeserializationContext ds  throws IOException  JsonProcessingException           SqlTimestampConverter s   new SqlTimestampConverter            String value   jp getValueAsString            if value    null  amp  amp   value isEmpty    amp  amp   value equals  null                return  Timestamp  s convert Timestamp class  value           return null              Override     public Class lt Timestamp gt  handledType             return Timestamp class

User · Answer

Jackson s JSON Views might be a simpler way of achieving your requirements  especially if you have some flexibility in your JSON format   If   id  7   itemNr   TEST    createdBy   id 3   is an acceptable representation then this will be very easy to achieve with very little code   You would just annotate the name field of User as being part of a view  and specify a different view in your serialisation request  the un-annotated fields would be included by default   For example  Define the views   public class Views       public static class BasicView       public static class CompleteUserView       Annotate the User   public class User       public final int id        JsonView Views CompleteUserView class      public final String name       public User int id  String name            this id   id          this name   name            And serialise requesting a view which doesn t contain the field you want to hide  non-annotated fields are serialised by default    objectMapper getSerializationConfig   withView Views BasicView class

User · Answer

You have to override method handledType and everything will work   Override public Class lt Item gt  handledType       return Item class

User · Answer

The problem in your case is the ItemSerializer is missing the method handledType   which needs to be overridden from JsonSerializer      public class ItemSerializer extends JsonSerializer lt Item gt          Override     public void serialize Item value  JsonGenerator jgen              SerializerProvider provider  throws IOException              JsonProcessingException           jgen writeStartObject            jgen writeNumberField  id   value id           jgen writeNumberField  itemNr   value itemNr           jgen writeNumberField  createdBy   value user id           jgen writeEndObject               Override    public Class lt Item gt  handledType            return Item class           Hence you are getting the explicit error that handledType   is not defined  Exception in thread  main  java lang IllegalArgumentException  JsonSerializer of type com example ItemSerializer does not define valid handledType      Hope it helps someone  Thanks for reading my answer

User · Answer

You can put  JsonSerialize using   CustomDateSerializer class  over any date field of object to be serialized   public class CustomDateSerializer extends SerializerBase lt Date gt         public CustomDateSerializer             super Date class  true               Override     public void serialize Date value  JsonGenerator jgen  SerializerProvider provider          throws IOException  JsonProcessingException           SimpleDateFormat formatter   new SimpleDateFormat  EEE MMM dd yyyy HH mm ss  GMT ZZZ  z             String format   formatter format value           jgen writeString format

User · Answer

As mentioned   JsonValue is a good way  But if you don t mind a custom serializer  there s no need to write one for Item but rather one for User -- if so  it d be as simple as   public void serialize Item value  JsonGenerator jgen      SerializerProvider provider  throws IOException      JsonProcessingException     jgen writeNumber id       Yet another possibility is to implement JsonSerializable  in which case no registration is needed   As to error  that is weird -- you probably want to upgrade to a later version  But it is also safer to extend org codehaus jackson map ser SerializerBase as it will have standard implementations of non-essential methods  i e  everything but actual serialization call

User · Answer

These are behavior patterns I have noticed while trying to understand Jackson serialization   1  Assume there is an object Classroom and a class Student  I ve made everything public and final for ease    public class Classroom       public final double double1   1234 5678      public final Double Double1   91011 1213      public final Student student1   new Student       public class Student       public final double double2   1920 2122      public final Double Double2   2324 2526      2  Assume that these are the serializers we use for serializing the objects into JSON  The writeObjectField uses the object s own serializer if it is registered with the object mapper  if not  then it serializes it as a POJO  The writeNumberField exclusively only accepts primitives as arguments   public class ClassroomSerializer extends StdSerializer lt Classroom gt        public ClassroomSerializer Class lt Classroom gt  t            super t               Override     public void serialize Classroom value  JsonGenerator jgen  SerializerProvider provider  throws IOException  JsonGenerationException           jgen writeStartObject            jgen writeObjectField  double1-Object   value double1           jgen writeNumberField  double1-Number   value double1           jgen writeObjectField  Double1-Object   value Double1           jgen writeNumberField  Double1-Number   value Double1           jgen writeObjectField  student1   value student1           jgen writeEndObject             public class StudentSerializer extends StdSerializer lt Student gt        public StudentSerializer Class lt Student gt  t            super t               Override     public void serialize Student value  JsonGenerator jgen  SerializerProvider provider  throws IOException  JsonGenerationException           jgen writeStartObject            jgen writeObjectField  double2-Object   value double2           jgen writeNumberField  double2-Number   value double2           jgen writeObjectField  Double2-Object   value Double2           jgen writeNumberField  Double2-Number   value Double2           jgen writeEndObject              3  Register only a DoubleSerializer with DecimalFormat output pattern       0 000  in SimpleModule and the output is        double1    1234 5678     Double1           value     91 011 121          student1           double2    1920 2122       Double2             value     2 324 253                You can see that the POJO serialization differentiates between double and Double  using the DoubleSerialzer for Doubles and using a regular String format for doubles   4  Register DoubleSerializer and ClassroomSerializer  without the StudentSerializer  We expect that the output is such that if we write a double as an object  it behaves like a Double  and if we write a Double as a number  it behaves like a double  The Student instance variable should be written as a POJO and follow the pattern above since it does not register        double1-Object           value     1 234 568          double1-Number    1234 5678     Double1-Object           value     91 011 121          Double1-Number    91011 1213     student1           double2    1920 2122       Double2             value     2 324 253                5  Register all serializers  The output is        double1-Object           value     1 234 568          double1-Number    1234 5678     Double1-Object           value     91 011 121          Double1-Number    91011 1213     student1           double2-Object             value     1 920 212              double2-Number    1920 2122       Double2-Object             value     2 324 253              Double2-Number    2324 2526         exactly as expected   Another important note  If you have multiple serializers for the same class registered with the same Module  then the Module will select the serializer for that class that is most recently added to the list  This should not be used - it s confusing and I am not sure how consistent this is  Moral  if you want to customize serialization of primitives in your object  you must write your own serializer for the object  You cannot rely on the POJO Jackson serialization

User · Answer

If your only requirement in your custom serializer is to skip serializing the name field of User  mark it as transient  Jackson will not serialize or deserialize transient fields     see also  Why does Java have transient fields

[java] How do I use a custom Serializer with Jackson?

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