Read JSON scalar as single-element double[] using Jackson

Question

Say I have the following two JSON files

{
  "a": [1, 2]
}

and

{
  "a": 1
}

I want to use Jackson to deserialize them both into an object of the following form -

public class Foo {
    public double[] a;
}

so I would end up with two objects, Foo{a=[1,2]} and Foo{a=[1]}. Is it possible to persuade Jackson to deserialize a scalar 1 to a double array [1], preferably using the jackson-databind api?

Answer 1

Yes you can.

By using the ObjectMapper#.configure(DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY, true); idiom.

Self-contained example here:

package test;

import java.util.Arrays;

import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.ObjectMapper;

public class Main {

    public static void main( String[] args ) throws Exception {
        ObjectMapper om = new ObjectMapper();
        // configuring as specified         
        om.configure(DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY, true);
        // scalar example
        String json = "{\"foo\":2}";
        // array example
        String otherJson = "{\"foo\":[3,4,5]}";
        // de-serializing scalar and printing value
        Main m = om.readValue(json, Main.class);
        System.out.println(Arrays.toString(m.foo));
        // de-serializing array and printing value
        Main otherM = om.readValue(otherJson, Main.class);
        System.out.println(Arrays.toString(otherM.foo));
    }
    @JsonProperty(value="foo")
    protected double[] foo;
}

Output

[2.0]
[3.0, 4.0, 5.0]

Quick note

About Jackson's version required. The docs for ACCEPT_SINGLE_VALUE_AS_ARRAY say:

Note that features that do not indicate version of inclusion were available in Jackson 2.0 (or earlier); only later additions indicate version of inclusion.

The feature has no @since javadoc annotation, so it should work in most recent-ish versions of Jackson.