How can I use Jackson to bind Map parameters to a bean?

Question

I'd like to use Jackson to bind a Map<String, String> to a bean. The pitfall here is that the not all the fields are collections so it doesn't work.

I would need to tweak the ObjectMapper to only bind collections when the corresponding bean property is a collection.

public class MapperTest {

    public static class Person {
        public String fname;
        public Double age;
        public List<String> other;
    }

    public static void main(String[] args) {
        ObjectMapper mapper = new ObjectMapper();
        Map<String, String[]> props = new HashMap<>();
        props.put("fname", new String[] {"mridang"});
        props.put("age", new String[] {"1"});
        props.put("other", new String[] {"one", "two"});
        mapper.convertValue(props, Person.class);
    }
}

The above example doesn't work as Jackson expects all the fields to be collections.

I cannot change the Map structure as this is a legacy system I'm dealing with so I'm pretty much stuck with Map<String, String[]>

You are providing collections so why would Jackson expect otherwise? — Antoniossss, Jul 20 '18 at 07:12
A bit of a backstory here. I'm on a legacy web framework here that exposes all the URL query params as that hackish map. I'd like to migrate to a standardized ser-deser library and that seems to be my only way out. — Mridang Agarwalla, Jul 20 '18 at 07:14
http://fasterxml.github.io/jackson-databind/javadoc/2.0.0/com/fasterxml/jackson/databind/DeserializationFeature.html#ACCEPT_SINGLE_VALUE_AS_ARRAY Thats close, but you need the other way around ;) — Antoniossss, Jul 20 '18 at 07:16
I cannot check that, but you can try `@JsonFormat(shape = JsonFormat.Shape.Array)` on non array fields. Maybeit will work on deserialization as well. — Antoniossss, Jul 20 '18 at 07:21
@Antoniossss `DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY` won't word here. However `DeserializationFeature.UNWRAP_SINGLE_VALUE_ARRAYS` does. See my [answer](https://stackoverflow.com/a/51438473/1426227). — cassiomolin, Jul 20 '18 at 08:40
Great, I didnt know that exists and managed to find that only so I was on good track;) — Antoniossss, Jul 20 '18 at 08:41

cassiomolin · Accepted Answer · 2018-07-21T08:16:50.320

You can use DeserializationFeature.UNWRAP_SINGLE_VALUE_ARRAYS, as follows:

ObjectMapper mapper = new ObjectMapper();
mapper.enable(DeserializationFeature.UNWRAP_SINGLE_VALUE_ARRAYS);

Map<String, String[]> props = new HashMap<>();
props.put("fname", new String[] {"mridang"});
props.put("age", new String[] {"1"});
props.put("other", new String[] {"one", "two"});

Person person = mapper.convertValue(props, Person.class);

How can I use Jackson to bind Map parameters to a bean?

1 Answers1