Jackson ObjectMapper：java.util.Set是按升序排序顺序为json数组吗？

Question

There's an object mapper feature to serialize maps sorted by map keys, but nothing is said about java sets. If I store items in a java.util.Set in my POJO, will the Jackson ObjectMapper serialize them to the json array in any kind of order?

If not, I'm thinking of writing a custom serializer for the set for this purpose - is there a better way that someone can suggest?

Update:

Ultimately, the goal is to deserialize an array of unique integers into some sort of container in a "request" pojo. Then process the request and pass that input field into the corresponding output field (unchanged) in a "response" object.

I'd like to store the data in the POJOs as java.util.Set just to keep all the users happy (if you try to store it as as subtype of Set then consumers of the POJO's interface may be upset that they can't just get a Set but have to expect a HashSet or a TreeSet).

Will this work?

1. json array [5,4,3,2,1] is deserialized into Set BUT, I use @JsonDeserialize(as=TreeSet.class) on the setter method. So the runtime type of the object is now a TreeSet (even though it's stored in the POJO as a Set).
2. The Set is passed around through the server and ultimately ends being stored in the response POJO (runtime type is still TreeSet, even though the POJO field type is Set).
3. ObjectMapper serializes a TreeSet to json array [1,2,3,4,5] (because TreeSet is sorted naturally).

Item 3 above is the one I'm wondering about - will ObjectMapper use the runtime type of the POJO's field, or the declared type?

Answer 1

A JSON Array is an ordered collection, exactly like a Java array is:

An array is an ordered collection of values. An array begins with [ (left bracket) and ends with ] (right bracket). Values are separated by , (comma).

However, ordered does not mean sorted . It just means that it retains the order of the given values:

• If the Set is sorted (eg TreeSet ), then the JSON array is sorted , according to the sort order of the Set :

  TreeSet : The elements are ordered using their natural ordering, or by a Comparator provided at set creation time, depending on which constructor is used.

• If the Set is unordered (eg HashSet ), then the JSON array is in the arbitrary order returned by the Set .

  HashSet : It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time.

This is unlike the fields of a JSON Object:

An object is an unordered set of name/value pairs. An object begins with { (left brace) and ends with } (right brace). Each name is followed by : (colon) and the name/value pairs are separated by , (comma).