Why doesn't #JsonUnwrapped work for Lists? - jackson

Question

I am using Jackson 2.1.0. Given:
public static final class GetCompanies
{
private final List<URI> companies;
/**
* Creates a new GetCompanies.
* <p/>
* #param companies the list of available companies
* #throws NullPointerException if companies is null
*/
#JsonCreator
public GetCompanies(#JsonUnwrapped #NotNull List<URI> companies)
{
Preconditions.checkNotNull(companies, "companies");
this.companies = ImmutableList.copyOf(companies);
}
/**
* #return the list of available companies
*/
#JsonUnwrapped
#SuppressWarnings("ReturnOfCollectionOrArrayField")
public List<URI> getCompanies()
{
return companies;
}
}
When the input list contains http://test.com/, Jackson generates:
{"companies":["http://test.com/"]}
instead of:
["http://test.com/"]
Any ideas?
UPDATE: See https://github.com/FasterXML/jackson-core/issues/41 for a related discussion.

Answer 1

In this case, if this was to work, you'd end up trying to produce following:
{ "http://test.com" }
which is not legal JSON. #JsonUnwrapped really just removes one layer of wrapping. And although it theoretically could be made to work for "arrays in arrays" case, it does not.
And in fact I wonder if adding this feature was a mistake: mostly because it encourages use that is often against data-binding best practices (simplicity, one-to-one mapping).
But what would work instead is #JsonValue:
#JsonValue
private final List<URI> companies;
which means "use value of this property instead of serializing the object that contains it".
And the creator method would actually work as-is, no need for either #JsonUnwrapped or #JsonProperty.
Here is the corrected code:
public static final class GetCompanies
{
private final List<URI> companies;
/**
* Creates a new GetCompanies.
* <p/>
* #param companies the list of available companies
* #throws NullPointerException if companies is null
*/
#JsonCreator
public GetCompanies(#NotNull List<URI> companies)
{
Preconditions.checkNotNull(companies, "companies");
this.companies = ImmutableList.copyOf(companies);
}
/**
* #return the list of available companies
*/
#JsonValue
#SuppressWarnings("ReturnOfCollectionOrArrayField")
public List<URI> getCompanies()
{
return companies;
}
}