Jackson JSON custom serialization for certain fields

Question

Is there a way using Jackson JSON Processor to do custom field level serialization  For example  I d like to have the class  public class Person       public String name      public int age      public int favoriteNumber      serialized to the follow JSON      name    Joe    age   25   favoriteNumber    123      Note that age 25 is encoded as a number while favoriteNumber 123 is encoded as a string  Out of the box Jackson marshalls int to a number  In this case I want favoriteNumber to be encoded as a string

User · Answer

Jackson-databind  at least 2 1 3  provides special ToStringSerializer  com fasterxml jackson databind ser std ToStringSerializer   Example   public class Person       public String name      public int age       JsonSerialize using   ToStringSerializer class      public int favoriteNumber

User · Answer

You can create a custom serializer inline in the mixin  Then annotate a field with it  See example below that appends  quot  - something else  quot  to lang field  This is kind of hackish - if your serializer requires something like a repository or anything injected by spring  this is going to be a problem  Probably best to use a custom deserializer serializer instead of a mixin  package com test   import com fasterxml jackson annotation JsonAutoDetect  import com fasterxml jackson annotation JsonAutoDetect Visibility  import com fasterxml jackson annotation JsonProperty  import com fasterxml jackson annotation JsonPropertyOrder  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 com fasterxml jackson databind annotation JsonSerialize  import com test Argument  import java io IOException  import java util HashMap  import java util List  import java util Map    Serialize only fields explicitly mentioned by this mixin   JsonAutoDetect      fieldVisibility   Visibility NONE      setterVisibility   Visibility NONE      getterVisibility   Visibility NONE      isGetterVisibility   Visibility NONE      creatorVisibility   Visibility NONE    JsonPropertyOrder   quot lang quot    quot name quot    quot value quot    public abstract class V2ArgumentMixin       JsonProperty  quot name quot     private String name      JsonSerialize using   LangCustomSerializer class  as String class     JsonProperty  quot lang quot     private String lang      JsonProperty  quot value quot     private Object value         public static class LangCustomSerializer extends JsonSerializer lt String gt          Override     public void serialize String value                            JsonGenerator jsonGenerator                            SerializerProvider serializerProvider          throws IOException  JsonProcessingException         jsonGenerator writeObject value toString      quot   - something else quot

User · Answer

with the help of  JsonView we can decide fields of model classes to serialize which satisfy the minimal criteria   we have to define the criteria  like we can have one core class with 10 properties but only 5 properties can be serialize which are needful for client only  Define our Views by simply creating following class   public class Views       static class Android        static class IOS        static class Web        Annotated model class with views   public class Demo        public Demo                  JsonView Views IOS class  private String iosField    JsonView Views Android class  private String androidField    JsonView Views Web class  private String webField       getters setters            Now we have to write custom json converter by simply extending HttpMessageConverter class from spring as       public class CustomJacksonConverter implements HttpMessageConverter lt Object gt             public CustomJacksonConverter                          super              this delegate getObjectMapper   setConfig this delegate getObjectMapper   getSerializationConfig   withView Views ClientView class            this delegate getObjectMapper   configure MapperFeature DEFAULT VIEW INCLUSION  true           this delegate getObjectMapper   setSerializationInclusion Include NON NULL                  a real message converter that will respond to methods and do the actual work     private MappingJackson2HttpMessageConverter delegate   new MappingJackson2HttpMessageConverter          Override     public boolean canRead Class lt   gt  clazz  MediaType mediaType            return delegate canRead clazz  mediaType               Override     public boolean canWrite Class lt   gt  clazz  MediaType mediaType            return delegate canWrite clazz  mediaType               Override     public List lt MediaType gt  getSupportedMediaTypes             return delegate getSupportedMediaTypes                Override     public Object read Class lt   extends Object gt  clazz              HttpInputMessage inputMessage  throws IOException              HttpMessageNotReadableException           return delegate read clazz  inputMessage               Override     public void write Object obj  MediaType contentType  HttpOutputMessage outputMessage  throws IOException  HttpMessageNotWritableException                synchronized this                         String userAgent     ServletRequestAttributes  RequestContextHolder getRequestAttributes    getRequest   getHeader  userAgent                if   userAgent    null                                  switch  userAgent                                     case  IOS                        this delegate getObjectMapper   setConfig this delegate getObjectMapper   getSerializationConfig   withView Views IOS class                        break                  case  Android                        this delegate getObjectMapper   setConfig this delegate getObjectMapper   getSerializationConfig   withView Views Android class                        break                  case  Web                        this delegate getObjectMapper   setConfig this delegate getObjectMapper   getSerializationConfig   withView  Views Web class                        break                  default                      this delegate getObjectMapper   setConfig this delegate getObjectMapper   getSerializationConfig   withView  null                         break                                              else                                  reset to default view                 this delegate getObjectMapper   setConfig this delegate getObjectMapper   getSerializationConfig   withView  null                               delegate write obj  contentType  outputMessage                        Now there is need to tell spring to use this custom json convert by simply putting this in dispatcher-servlet xml   lt mvc annotation-driven gt           lt mvc message-converters register-defaults  true  gt               lt bean id  jsonConverter  class  com mactores org CustomJacksonConverter   gt               lt  bean gt           lt  mvc message-converters gt       lt  mvc annotation-driven gt    That s how you will able to decide which fields to get serialize

User · Answer

You can implement a custom serializer as follows    public class Person       public String name      public int age       JsonSerialize using   IntToStringSerializer class  as String class      public int favoriteNumber      public class IntToStringSerializer extends JsonSerializer lt Integer gt          Override     public void serialize Integer tmpInt                             JsonGenerator jsonGenerator                             SerializerProvider serializerProvider                             throws IOException  JsonProcessingException           jsonGenerator writeObject tmpInt toString               Java should handle the autoboxing from int to Integer for you

User · Answer

Add a  JsonProperty annotated getter  which returns a String  for the favoriteNumber field   public class Person       public String name      public int age      private int favoriteNumber       public Person String name  int age  int favoriteNumber            this name   name          this age   age          this favoriteNumber   favoriteNumber              JsonProperty     public String getFavoriteNumber             return String valueOf favoriteNumber              public static void main String    args  throws Exception           Person p   new Person  Joe   25  123           ObjectMapper mapper   new ObjectMapper            System out println mapper writeValueAsString p                  name   Joe   age  25  favoriteNumber   123

User · Answer

In case you don t want to pollute your model with annotations and want to perform some custom operations  you could use mixins   ObjectMapper mapper   new ObjectMapper    SimpleModule simpleModule   new SimpleModule    simpleModule setMixInAnnotation Person class  PersonMixin class   mapper registerModule simpleModule     Override age   public abstract class PersonMixin        JsonSerialize using   PersonAgeSerializer class      public String age      Do whatever you need with the age   public class PersonAgeSerializer extends JsonSerializer lt Integer gt         Override     public void serialize Integer integer  JsonGenerator jsonGenerator  SerializerProvider serializerProvider  throws IOException           jsonGenerator writeString String valueOf integer   52      months

User · Answer

jackson-annotations provides  JsonFormat which can handle a lot of customizations without the need to write the custom serializer    For example  requesting a STRING shape for a field with numeric type will output the numeric value as string  public class Person       public String name      public int age       JsonFormat shape   JsonFormat Shape STRING      public int favoriteNumber      will result in the desired output    name   Joe   age  25  favoriteNumber   123

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