How to tell Jackson to ignore a field during serialization if its value is null?

With Jackson > 1.9.11 and < 2.x use @JsonSerialize annotation to do that:

@JsonSerialize(include=JsonSerialize.Inclusion.NON_NULL)

Just to expand on the other answers - if you need to control the omission of null values on a per-field basis, annotate the field in question (or alternatively annotate the field's 'getter').

example - here only fieldOne will be omitted from the JSON if it is null. fieldTwo will always be included in the JSON regardless of if it is null.

public class Foo {

    @JsonInclude(JsonInclude.Include.NON_NULL) 
    private String fieldOne;

    private String fieldTwo;
}

To omit all null values in the class as a default, annotate the class. Per-field/getter annotations can still be used to override this default if necessary.

example - here fieldOne and fieldTwo will be omitted from the JSON if they are null, respectively, because this is the default set by the class annotation. fieldThree however will override the default and will always be included, because of the annotation on the field.

@JsonInclude(JsonInclude.Include.NON_NULL)
public class Foo {

    private String fieldOne;

    private String fieldTwo;
    
    @JsonInclude(JsonInclude.Include.ALWAYS)
    private String fieldThree;
}

---

UPDATE

The above is for Jackson 2. For earlier versions of Jackson you need to use:

@JsonSerialize(include=JsonSerialize.Inclusion.NON_NULL) 

instead of

@JsonInclude(JsonInclude.Include.NON_NULL)

If this update is useful, please upvote ZiglioUK's answer below, it pointed out the newer Jackson 2 annotation long before I updated my answer to use it!

To suppress serializing properties with null values using Jackson >2.0, you can configure the ObjectMapper directly, or make use of the @JsonInclude annotation:

mapper.setSerializationInclusion(Include.NON_NULL);

or:

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

Alternatively, you could use @JsonInclude in a getter so that the attribute would be shown if the value is not null.

A more complete example is available in my answer to How to prevent null values inside a Map and null fields inside a bean from getting serialized through Jackson.