Jackson: deserialize JSON with XML-like lists - json

Question

A service I don't control is sending me JSON that looks like this when there is one item:
{ items: {
item: { ... }
}
... like this when there is no item:
{ items: null }
... and like this when there are two or more items:
{ items: {
item: [
{ ... },
{ ... }
]
}
}
I believe this is because the service was designed to produce XML like this:
<items>
<item>....</item>
<item>....</item>
</items>
... and has thrown the results at an XML->JSON convertor without paying much attention to the result.
Using Jackson, I can handle the "sometimes null, sometimes object, sometimes array" issue in Jackson by configuring my ObjectMapper with DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY.
I can handle the two-layer structure by defining an Items class with a field List<Item> item.
However, the Items class is only there as a sop to the weird JSON format. Other than by writing a whole custom deserializer, can I persuade Jackson to deserialize to an object as if the structure was:
{ items: [ { ... }, { ... } ] }
?

Answer 1

It looks like you have a root object which contains Items property. In this case you can use #JsonUnwrapped annotation. See below class:
class Root {
#JsonUnwrapped
private Items items = new Items();
public Items getItems() {
return items;
}
public void setItems(Items items) {
this.items = items;
}
}
Simple usage:
ObjectMapper mapper = new ObjectMapper();
Items items = new Items();
items.setItem(Arrays.asList(new Item(), new Item()));
Root wrapper = new Root();
wrapper.setItems(items);
String json = mapper.writeValueAsString(wrapper);
System.out.println(json);
Above program prints:
{"item":[{"name":"X"},{"name":"X"}]}

Answer 2

Usually the easiest way to handle mapping is to reflect structure of data format in your POJO structure. So in this case, you would add one functionally unnecessary, but mapping-wise necessary, property:
public class POJO {
public ItemHolder items;
// might want convenience method for Java access, but not with getter name
public List<Item> items() {
return (items == null) ? null : items.item;
}
}
public class ItemHolder {
public List<Item> item;
}
and you do still also need to enable DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY to handle mapping from single Item into List<Item>.
But perhaps the other answer of using #JsonUnwrapped also helps; it is another option.