Gson How to exclude specific fields from Serialization without annotations

Question

I m trying to learn Gson and I m struggling with field exclusion  Here are my classes  public class Student         private Long                id    private String              firstName           Philip     private String              middleName          J      private String              initials            P F     private String              lastName            Fry     private Country             country    private Country             countryOfBirth     public class Country         private Long                id    private String              name    private Object              other      I can use the GsonBuilder and add an ExclusionStrategy for a field name like firstName or country but I can t seem to manage to exclude properties of certain fields like country name   Using the method public boolean shouldSkipField FieldAttributes fa   FieldAttributes doesn t contain enough information to match the field with a filter like country name   P S  I want to avoid annotations since I want to improve on this and use RegEx to filter fields out   Edit  I m trying to see if it s possible to emulate the behavior of Struts2 JSON plugin  using Gson   lt interceptor-ref name  json  gt     lt param name  enableSMD  gt true lt  param gt     lt param name  excludeProperties  gt      login password      studentList    sin    lt  param gt   lt  interceptor-ref gt    Edit      I reopened the question with the following addition   I added a second field with the same type to futher clarify this problem  Basically I want to exclude country name but not countrOfBirth name  I also don t want to exclude Country as a type  So the types are the same it s the actual place in the object graph that I want to pinpoint and exclude

User · Answer

Any fields you don t want serialized in general you should use the  transient  modifier  and this also applies to json serializers  at least it does to a few that I have used  including gson    If you don t want name to show up in the serialized json give it a transient keyword  eg   private transient String name    More details in the Gson documentation

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Kotlin s  Transientannotation also does the trick apparently   data class Json       field SerializedName  serialized field 1   val field1  String       field SerializedName  serialized field 2   val field2  String       Transient val field3  String     Output     serialized field 1   VALUE1   serialized field 2   VALUE2

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This what I always use   The default behaviour implemented in Gson is that null object fields are ignored   Means Gson object does not serialize fields with null values to JSON  If a field in a Java object is null  Gson excludes it   You can use this function to convert some object to null or well set by your own                convert object to json         public String toJson Object obj           Convert emtpy string and objects to null so we don t serialze them     setEmtpyStringsAndObjectsToNull obj       return gson toJson obj                   Sets all empty strings and objects  all fields null  including sets to null             param obj any object         public void setEmtpyStringsAndObjectsToNull Object obj        for  Field field   obj getClass   getDeclaredFields            field setAccessible true         try           Object fieldObj   field get obj           if  fieldObj    null              Class fieldType   field getType              if  fieldType isAssignableFrom String class                 if fieldObj equals                      field set obj  null                             else if  fieldType isAssignableFrom Set class                 for  Object item    Set  fieldObj                  setEmtpyStringsAndObjectsToNull item                             boolean setFielToNull   true              for  Object item    Set  field get obj                   if item    null                    setFielToNull   false                  break                                            if setFielToNull                  setFieldToNull obj  field                             else if   isPrimitiveOrWrapper fieldType                 setEmtpyStringsAndObjectsToNull fieldObj               boolean setFielToNull   true              for  Field f   fieldObj getClass   getDeclaredFields                    f setAccessible true                 if f get fieldObj     null                    setFielToNull   false                  break                                            if setFielToNull                  setFieldToNull obj  field                                               catch  IllegalAccessException e            System err println  Error while setting empty string or object to null      e getMessage                          private void setFieldToNull Object obj  Field field  throws IllegalAccessException       if  Modifier isFinal field getModifiers             field set obj  null                private boolean isPrimitiveOrWrapper Class fieldType         return fieldType isPrimitive              fieldType isAssignableFrom Integer class             fieldType isAssignableFrom Boolean class             fieldType isAssignableFrom Byte class             fieldType isAssignableFrom Character class             fieldType isAssignableFrom Float class             fieldType isAssignableFrom Long class             fieldType isAssignableFrom Double class             fieldType isAssignableFrom Short class

User · Answer

So  you want to exclude firstName and country name  Here is what your ExclusionStrategy should look like      public class TestExclStrat implements ExclusionStrategy            public boolean shouldSkipClass Class lt   gt  arg0                return false                     public boolean shouldSkipField FieldAttributes f                 return  f getDeclaringClass      Student class  amp  amp  f getName   equals  firstName                   f getDeclaringClass      Country class  amp  amp  f getName   equals  name                        If you see closely it returns true for Student firstName and Country name  which is what you want to exclude   You need to apply this ExclusionStrategy like this       Gson gson   new GsonBuilder            setExclusionStrategies new TestExclStrat               serializeNulls    lt -- uncomment to serialize NULL fields as well          create        Student src   new Student        String json   gson toJson src       System out println json     This returns      middleName    J     initials    P F    lastName    Fry    country      id   91     I assume the country object is initialized with id   91L in student class     You may get fancy  For example  you do not want to serialize any field that contains  name  string in its name  Do this   public boolean shouldSkipField FieldAttributes f        return f getName   toLowerCase   contains  name         This will return      initials    P F    country      id   91        EDIT  Added more info as requested   This ExclusionStrategy will do the thing  but you need to pass  Fully Qualified Field Name   See below       public class TestExclStrat implements ExclusionStrategy            private Class lt   gt  c          private String fieldName          public TestExclStrat String fqfn  throws SecurityException  NoSuchFieldException  ClassNotFoundException                       this c   Class forName fqfn substring 0  fqfn lastIndexOf                     this fieldName   fqfn substring fqfn lastIndexOf      1                     public boolean shouldSkipClass Class lt   gt  arg0                return false                     public boolean shouldSkipField FieldAttributes f                 return  f getDeclaringClass      c  amp  amp  f getName   equals fieldName                       Here is how we can use it generically       Gson gson   new GsonBuilder            setExclusionStrategies new TestExclStrat  in naishe test Country name               serializeNulls            create        Student src   new Student        String json   gson toJson src       System out println json      It returns      firstName    Philip     middleName    J     initials    P F    lastName    Fry    country      id   91

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After reading all available answers I found out  that most flexible  in my case  was to use custom  Exclude annotation  So  I implemented simple strategy for this  I didn t want to mark all fields using  Expose nor I wanted to use transient which conflicted with in app Serializable serialization     Annotation    Retention RetentionPolicy RUNTIME   Target ElementType FIELD  public  interface Exclude       Strategy   public class AnnotationExclusionStrategy implements ExclusionStrategy         Override     public boolean shouldSkipField FieldAttributes f            return f getAnnotation Exclude class     null              Override     public boolean shouldSkipClass Class lt   gt  clazz            return false            Usage   new GsonBuilder   setExclusionStrategies new AnnotationExclusionStrategy    create

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I used this strategy   i excluded every field which is not marked with  SerializedName annotation  i e    public class Dummy         SerializedName  VisibleValue       final String visibleValue      final String hiddenValue       public Dummy String visibleValue  String hiddenValue            this visibleValue   visibleValue          this hiddenValue   hiddenValue            public class SerializedNameOnlyStrategy implements ExclusionStrategy         Override     public boolean shouldSkipField FieldAttributes f            return f getAnnotation SerializedName class     null              Override     public boolean shouldSkipClass Class lt   gt  clazz            return false            Gson gson   new GsonBuilder                    setExclusionStrategies new SerializedNameOnlyStrategy                     create     Dummy dummy   new Dummy  I will see this   I will not see this    String json   gson toJson dummy     It returns       VisibleValue   I will see this

User · Answer

I ran into this issue  in which I had a small number of fields I wanted to exclude only from serialization  so I developed a fairly simple solution that uses Gson s  Expose annotation with custom exclusion strategies   The only built-in way to use  Expose is by setting GsonBuilder excludeFieldsWithoutExposeAnnotation    but as the name indicates  fields without an explicit  Expose are ignored  As I only had a few fields I wanted to exclude  I found the prospect of adding the annotation to every field very cumbersome   I effectively wanted the inverse  in which everything was included unless I explicitly used  Expose to exclude it  I used the following exclusion strategies to accomplish this   new GsonBuilder            addSerializationExclusionStrategy new ExclusionStrategy                  Override             public boolean shouldSkipField FieldAttributes fieldAttributes                    final Expose expose   fieldAttributes getAnnotation Expose class                   return expose    null  amp  amp   expose serialize                                Override             public boolean shouldSkipClass Class lt   gt  aClass                    return false                                    addDeserializationExclusionStrategy new ExclusionStrategy                  Override             public boolean shouldSkipField FieldAttributes fieldAttributes                    final Expose expose   fieldAttributes getAnnotation Expose class                   return expose    null  amp  amp   expose deserialize                                Override             public boolean shouldSkipClass Class lt   gt  aClass                    return false                                    create      Now I can easily exclude a few fields with  Expose serialize   false  or  Expose deserialize   false  annotations  note that the default value for both  Expose attributes is true   You can of course use  Expose serialize   false  deserialize   false   but that is more concisely accomplished by declaring the field transient instead  which does still take effect with these custom exclusion strategies

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I solved this problem with custom annotations  This is my  SkipSerialisation  Annotation class     Target  ElementType FIELD  public  interface SkipSerialisation        and this is my GsonBuilder   gsonBuilder addSerializationExclusionStrategy new ExclusionStrategy         Override public boolean shouldSkipField  FieldAttributes f         return f getAnnotation SkipSerialisation class     null           Override public boolean shouldSkipClass  Class lt   gt  clazz         return false            Example    public class User implements Serializable      public String firstName     public String lastName      SkipSerialisation   public String email

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Or can say whats fields not will expose with   Gson gson   gsonBuilder excludeFieldsWithModifiers Modifier TRANSIENT  create      on your class on attribute   private   transient   boolean nameAttribute

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I have Kotlin version    Retention AnnotationRetention RUNTIME   Target AnnotationTarget FIELD  internal annotation class JsonSkip  class SkipFieldsStrategy   ExclusionStrategy        override fun shouldSkipClass clazz  Class lt   gt    Boolean           return false            override fun shouldSkipField f  FieldAttributes   Boolean           return f getAnnotation JsonSkip  class java     null           and how You can add this to Retrofit GSONConverterFactory    val gson   GsonBuilder                    setExclusionStrategies SkipFieldsStrategy                       serializeNulls                      setDateFormat DateFormat LONG                     setFieldNamingPolicy FieldNamingPolicy UPPER CAMEL CASE                     setPrettyPrinting                      registerTypeAdapter Id class  IdTypeAdapter                     create           return GsonConverterFactory create gson

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You can explore the json tree with gson   Try something like this     gson toJsonTree student  getAsJsonObject    get  country   getAsJsonObject   remove  name      You can add some properties also    gson toJsonTree student  getAsJsonObject   addProperty  isGoodStudent   false     Tested with gson 2 2 4

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I m working just by putting the  Expose annotation  here my version that I use  compile  com squareup retrofit2 retrofit 2 0 2  compile  com squareup retrofit2 converter-gson 2 0 2    In Model class    Expose int number   public class AdapterRestApi     In the Adapter class   public EndPointsApi connectRestApi         OkHttpClient client   new OkHttpClient Builder                connectTimeout 90000  TimeUnit SECONDS               readTimeout 90000 TimeUnit SECONDS  build         Retrofit retrofit   new Retrofit Builder                baseUrl ConstantRestApi ROOT URL               addConverterFactory GsonConverterFactory create                 client client               build         return retrofit create   EndPointsApi class

User · Answer

Nishant provided a good solution  but there s an easier way  Simply mark the desired fields with the  Expose annotation  such as    Expose private Long id    Leave out any fields that you do not want to serialize  Then just create your Gson object this way   Gson gson   new GsonBuilder   excludeFieldsWithoutExposeAnnotation   create

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I came up with a class factory to support this functionality  Pass in any combination of either fields or classes you want to exclude   public class GsonFactory        public static Gson build final List lt String gt  fieldExclusions  final List lt Class lt   gt  gt  classExclusions            GsonBuilder b   new GsonBuilder            b addSerializationExclusionStrategy new ExclusionStrategy                  Override             public boolean shouldSkipField FieldAttributes f                    return fieldExclusions    null   false   fieldExclusions contains f getName                                 Override             public boolean shouldSkipClass Class lt   gt  clazz                    return classExclusions    null   false   classExclusions contains clazz                                     return b create               To use  create two lists  each is optional   and create your GSON object   static    List lt String gt  fieldExclusions   new ArrayList lt String gt      fieldExclusions add  id     fieldExclusions add  provider     fieldExclusions add  products      List lt Class lt   gt  gt  classExclusions   new ArrayList lt Class lt   gt  gt      classExclusions add Product class    GSON   GsonFactory build null  classExclusions      private static final Gson GSON   public String getSomeJson        List lt Provider gt  list   getEntitiesFromDatabase        return GSON toJson list

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Another approach  especially useful if you need to make a decision to exclude a field at runtime  is to register a TypeAdapter with your gson instance  Example below   Gson gson   new GsonBuilder    registerTypeAdapter BloodPressurePost class  new BloodPressurePostSerializer      In the case below  the server would expect one of two values but since they were both ints then gson would serialize them both  My goal was to omit any value that is zero  or less  from the json that is posted to the server   public class BloodPressurePostSerializer implements JsonSerializer lt BloodPressurePost gt          Override     public JsonElement serialize BloodPressurePost src  Type typeOfSrc  JsonSerializationContext context            final JsonObject jsonObject   new JsonObject             if  src systolic  gt  0                jsonObject addProperty  systolic   src systolic                      if  src diastolic  gt  0                jsonObject addProperty  diastolic   src diastolic                      jsonObject addProperty  units   src units            return jsonObject

[java] Gson: How to exclude specific fields from Serialization without annotations

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