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

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

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

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

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 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

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

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

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

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

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