hashing a Java JSONObject

Question

I'm currently writing a jsonrpc-client in java. A request is represented by an JSONObject like this:

{'method': 'do.stuff', 'params': ['asdf', 3, {'foo': 'bar'}]}

Now I want to implement a low-level request cache and for this I need a way to create a hash from a JSONObject that will always return the same value. Using the toString() is not an option since JSONObjects are unordered: the following example would do exactly the same as my first example even though the string representation is different:

{'params': ['asdf', 3, {'foo': 'bar'}], 'method': 'do.stuff'}

Whats the best solution to this problem? The jsonobject may be nested to any arbitrary depth of course. I'm fairly new to Java and thankful for any suggestion.

Answer 1

You want to have a look at Jackson: read your input as a JsonNode, which implements .equals() and .hashCode() correctly (that is, sticking to the contract established by java.lang.Object), for all JSON values (numbers, booleans, strings, objects recursively, arrays recursively, nulls).

Answer 2

I agree with @fge's solution. This is working for me across multiple variations of a JSON structure -

    int hash = 0;
    ObjectMapper mapper = new ObjectMapper();
    JsonFactory factory = mapper.getFactory();
    try {
        JsonNode jsonNode = mapper.readTree(factory.createParser(jsonString));
        hash = jsonNode.hashCode();
    } catch (IOException e) {
        LOG.error("Error generating hash for jsonString: {}", jsonString);
        e.printStackTrace();
    }


Example permutations - 
 "{ k1: v1, k2:v2, k3: v3}"
 "{ k3: v3, k2:v2, k1: v1}"
 "{ k3: v3, k1:v1, k2: v2}"

All 3 produced same hashcode everytime. In practice however my json also had ArrayList and Map within itself and that worked perfectly well too.