Jackson - serializing a list containing null elements

Question

I'm using Jackson 2.4 to serialize objects to JSON.
When I serialize a list of objects, having some elements are null, the result JSON string contains some "null" strings.

How do I prevent "null" elements from being serialized? Is there any configuration for ObjectMapper? I have already set "setSerializationInclusion(Include.NON_NULL)"!

Here is my code :

List<String> strings = new ArrayList<>();
strings.add("string 1");
strings.add("string 2");
strings.add(null);
strings.add(null);

After serializing I got this :

[string 1, string 2, null, null]

How do I get the JSON string without "null"?

[string 1, string 2]

It's just an example, in some cases, a collection may contain null elements, I just want to know if Jackson is able to ignore null members in a collection, or I have to make the collection null-free. — toandv, Dec 03 '14 at 03:49

Ken de Guzman · Accepted Answer · 2014-12-03T05:11:09.247

Using @JsonInclude annotation.

@JsonInclude(Include.NON_NULL)
class Foo {
  String bar;

}

Edit

Also you can create your own serializer.
For example :

public static void main(String[] args) throws JsonProcessingException {

        List<String> strings = new ArrayList<>();
        strings.add("string 1");
        strings.add("string 2");
        strings.add(null);
        strings.add(null);

        ObjectMapper mapper=new ObjectMapper();
        mapper.getSerializerProvider().setNullValueSerializer(new NullSerializer());
        System.out.println(mapper.writeValueAsString(strings));
    }

NullSerializer.java

class NullSerializer extends JsonSerializer<Object>
{
  @Override
  public void serialize(Object obj, JsonGenerator jsonGen, SerializerProvider unused) 
      throws IOException, JsonProcessingException
  {
    jsonGen.writeFieldName("");
  }
}

Will print

["string 1","string 2","",""]

then you can remove jsonGen.writeFieldName(""); to print

["string 1","string 2"]

NullSerializer solves problem with null values in arrays, Include.NON_EMPTY solves skipping of keys with empty collections and keys with null values. — Michal Bernhard, Aug 06 '15 at 11:19

score 2 · Answer 2 · answered Dec 29 '17 at 19:08

Suggestion of implementing a custom NullSerializer is a great idea, in my case I don't want all null values to be ignored (eg, outside collections) so with just a small tweak you can have both things working.

class NullSerializer extends JsonSerializer<Object>{
    @Override
    public void serialize(Object obj, JsonGenerator jsonGen, SerializerProvider unused) 
  throws IOException, JsonProcessingException {
        //any null outside of a collection will be serialized (as long as SerilizationInclusion allows nulls)
        if (!jsonGen.getOutputContext().inArray()) {
            jsonGen.writeNull();
        }
    }
}

Using that custom NullSerializer, serializing following class:

class MyTest {
    public List<String> list = Arrays.asList("hello", null, "world", null);
    public Object goodNull = null;
}

Produces the JSON:

{ "list":["hello","world"], "goodNull":null}

score 1 · Answer 3 · edited May 23 '17 at 10:31

1

try using below

@JsonInclude(Include.NON_NULL)

also refer this for more detailed explanation.

edited May 23 '17 at 10:31

Community

1
1

answered Dec 02 '14 at 10:26

Ankur Singhal

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user3145373 ツ · Answer 4 · 2014-12-02T10:49:13.847

You can prevent it by :

@JsonSerialize(include=JsonSerialize.Inclusion.NON_NULL) private String checkNullThenSerialize;

put this on Member declaration.

or you can put class level also like :

@JsonInclude(Include.NON_NULL)
class CheckNullThenSerialize{
  private String fieldData;
  private String fieldNull;
}

You can also try Include.NON_EMPTY with @JsonInclude alter to Include.NON_NULL.

For ObjectMapper (Any null field in any class serialized through this mapper is going to be ignored):

ObjectMapper mapper = new ObjectMapper();
mapper.setSerializationInclusion(Include.NON_NULL);

Look Here's full details on this topic.

Jackson - serializing a list containing null elements

4 Answers4