Setting default values to null fields when mapping with Jackson

Question

I am trying to map some JSON objects to Java objects with Jackson  Some of the fields in the JSON object are mandatory which I can mark with  NotNull  and some are optional   After the mapping with Jackson  all the fields that are not set in the JSON object will have a null value in Java  Is there a similar annotation to  NotNull that can tell Jackson to set a default value to a Java class member  in case it is null   Edit  To make the question more clear here is some code example   The Java object   class JavaObject        NotNull     public String notNullMember        DefaultValue  Value       public String optionalMember      The JSON object can be either          notNullMember     notNull      or          notNullMember     notNull        optionalMember     optional      The  DefaultValue annotations is just to show what I am asking  It s not a real annotation  If the JSON object is like in the first example I want the value of the optionalMember to be  Value  and not null  Is there an annotation that does such a thing

User · Answer

Looks like the solution is to set the value of the properties inside the default constructor. So in this case the java class is:

class JavaObject {

    public JavaObject() {

        optionalMember = "Value";
    }

    @NotNull
    public String notNullMember;

    public String optionalMember;
}

After the mapping with Jackson, if the optionalMember is missing from the JSON its value in the Java class is "Value".

However, I am still interested to know if there is a solution with annotations and without the default constructor.

User · Answer

There is no annotation to set default value  You can set default value only on java class level    public class JavaObject        public String notNullMember       public String optionalMember    Value

User · Answer

I had a similar problem  but in my case the default value was in database  Below is the solution for that     Configuration  public class AppConfiguration     Autowired  private AppConfigDao appConfigDao     Bean  public Jackson2ObjectMapperBuilder builder        Jackson2ObjectMapperBuilder builder   new Jackson2ObjectMapperBuilder           deserializerByType SomeDto class    new SomeDtoJsonDeserializer appConfigDao findDefaultValue         return builder       Then in SomeDtoJsonDeserializer use ObjectMapper to deserialize the json and set default value if your field object is null

User · Answer

Another option is to use InjectableValues and  JacksonInject  It is very useful if you need to use not always the same value but one get from DB or somewhere else for the specific case  Here is an example of using JacksonInject   protected static class Some       private final String field1      private final String field2       public Some  JsonProperty  field1   final String field1               JsonProperty  field2    JacksonInject value    defaultValueForField2                       useInput   OptBoolean TRUE  final String field2            this field1   requireNonNull field1           this field2   requireNonNull field2              public String getField1             return field1             public String getField2             return field2            Test public void testReadValueInjectables   throws JsonParseException  JsonMappingException  IOException       final ObjectMapper mapper   new ObjectMapper        final InjectableValues injectableValues               new InjectableValues Std   addValue  defaultValueForField2    somedefaultValue        mapper setInjectableValues injectableValues        final Some actualValueMissing   mapper readValue     field1      field1value      Some class       assertEquals actualValueMissing getField1     field1value        assertEquals actualValueMissing getField2     somedefaultValue         final Some actualValuePresent               mapper readValue     field1      field1value      field2      field2value      Some class       assertEquals actualValuePresent getField1     field1value        assertEquals actualValuePresent getField2     field2value        Keep in mind that if you are using constructor to create the entity  this usually happens when you use  Value or  AllArgsConstructor in lombok   and you put  JacksonInject not to the constructor but to the property it will not work as expected - value of the injected field will always override value in json  no matter whether you put useInput   OptBoolean TRUE in  JacksonInject  This is because jackson injects those properties after constructor is called  even if the property is final  - field is set to the correct value in constructor but then it is overrided  check  https   github com FasterXML jackson-databind issues 2678 and https   github com rzwitserloot lombok issues 1528 issuecomment-607725333 for more information   this test is unfortunately passing   protected static class Some       private final String field1        JacksonInject value    defaultValueForField2   useInput   OptBoolean TRUE      private final String field2       public Some  JsonProperty  field1   final String field1               JsonProperty  field2    JacksonInject value    defaultValueForField2                       useInput   OptBoolean TRUE  final String field2            this field1   requireNonNull field1           this field2   requireNonNull field2              public String getField1             return field1             public String getField2             return field2            Test public void testReadValueInjectablesIncorrectBehavior   throws JsonParseException  JsonMappingException  IOException       final ObjectMapper mapper   new ObjectMapper        final InjectableValues injectableValues               new InjectableValues Std   addValue  defaultValueForField2    somedefaultValue        mapper setInjectableValues injectableValues        final Some actualValueMissing   mapper readValue     field1      field1value      Some class       assertEquals actualValueMissing getField1     field1value        assertEquals actualValueMissing getField2     somedefaultValue         final Some actualValuePresent               mapper readValue     field1      field1value      field2      field2value      Some class       assertEquals actualValuePresent getField1     field1value           unfortunately  field2value  is overrided because of putting   JacksonInject  to the field     assertEquals actualValuePresent getField2     somedefaultValue         Hope this helps to someone with a similar problem   P S  I m using jackson v  2 9 6

User · Answer

Only one proposed solution keeps the default-value when some-value null was set explicitly  POJO readability is lost there and it s clumsy   Here s how one can keep the default-value and never set it to null   JsonProperty  some-value   public String someValue    default-value     JsonSetter  some-value   public void setSomeValue String s        if  s    null             someValue   s

User · Answer

There are already a lot of good suggestions  but here s one more  You can use  JsonDeserialize to perform an arbitrary  sanitizer  which Jackson will invoke post-deserialization    JsonDeserialize converter Message1  Sanitizer class    public class Message1 extends MessageBase       public String string1           public int integer1       public static class  Sanitizer extends StdConverter lt Message1 Message1 gt             Override         public Message1 convert Message1 message                if  message string1    null  message string1                   return message

User · Answer

Make the member private and add a setter getter pair  In your setter  if null  then set default value instead  Additionally  I have shown the snippet with the getter also returning a default when internal value is null   class JavaObject       private static final String DEFAULT  Default Value        public JavaObject                 NotNull     private String notNullMember      public void setNotNullMember String value               if  value  null    notNullMember DEFAULT  return                notNullMember value              return             public String getNotNullMember                if  notNullMember  null    return DEFAULT               return notNullMember             public String optionalMember

User · Answer

You can create your own JsonDeserializer and annotate that property with  JsonDeserialize as   DefaultZero class   For example  To configure BigDecimal to default to ZERO   public static class DefaultZero extends JsonDeserializer lt BigDecimal gt        private final JsonDeserializer lt BigDecimal gt  delegate       public DefaultZero JsonDeserializer lt BigDecimal gt  delegate            this delegate   delegate              Override     public BigDecimal deserialize JsonParser jsonParser  DeserializationContext deserializationContext  throws IOException  JsonProcessingException           return jsonParser getDecimalValue                Override     public BigDecimal getNullValue DeserializationContext ctxt  throws JsonMappingException           return BigDecimal ZERO            And usage   class Sth        JsonDeserialize as   DefaultZero class     BigDecimal property

[java] Setting default values to null fields when mapping with Jackson

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