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EO principles respected here DevOps By

CircleCI Codecov Hits-of-Code Maven Central License

Nereid* for Jackson is an object oriented JSON library wrapper for jackson-databind It allows developers to work with JSON documents in a purely object oriented way: everything is instantiated via constructors, there are no static methods, no nulls and no "mappers" or "builders". Most importantly, the core Json interface lends itself to easy custom implementations, making Nereides very extensible.

*(Nereides are the sea nymphs who guided Jason's ship safely through the Wandering Rocks in his quest for the Golden Fleece.)

Available Nereides:
-- Nereid for Jackson (recommended)
-- Nereid for javax.json

Maven dependency


Simple usage

Create new Json object

// From String:
String jsonAsString = "{\"nymph\": \"nereid\"}";
Json json = new Json.Of(jsonAsString);

// From InputStream:
InputStream stream = new ByteArrayInputStream(jsonAsString.getBytes());
json = new Json.Of(stream);

// From Jackson's JsonNode:
JsonNode node = new ObjectMapper().readTree(jsonAsString);
json = new Json.Of(node);


Once we have the Json object, to use it in various ways, the Smart Object pattern is employed.

// Convert it to String:
String textual = new SmartJson(json).textual();

// Convert it to pretty formatted String:
String pretty = new SmartJson(json).pretty();

// Convert it to byte array:
byte[] bytes = new SmartJson(json).byteArray();

// Get a String field value:
Optional<String> leaf = new SmartJson(json).leaf("nymph");

// Get a deeply nested Json:
SmartJson nested = new SmartJson(json).at("/path/to/nested/json");

// Get a deeply nested int:
int nestedInt = new SmartJson(json).at("/path/to/nested/int");


While the main purpose of this library is to enable making custom implementations of the Json interface (see more on that below), if you need to quickly assemble a Json by hand, MutableJson can be used. This API has a very declarative notation.

Json json = new MutableJson().with(
    new MutableJson().with(
        new MutableJson()
            .with("name", "Thetis")
            .with("hair", "black")
        new MutableJson()
            .with("name", "Actaea")
            .with("hair", "blonde")
    .with("stormy", true)
System.out.println(new SmartJson(json).pretty());

The code above would print this:

  "ocean" : {
    "nereid1" : {
      "name" : "Thetis",
      "hair" : "black"
    "nereid2" : {
      "name" : "Actaea",
      "hair" : "blonde"
    "stormy" : true

Custom implementations

If you have an object which needs to be able to display itself as JSON, sometimes it might be useful to just treat it as a JSON to begin with. In that case that object will have to implement a JSON interface. In most (all?) other libraries, JSON interfaces are huge, making it very difficult to implement them. With Nereides, all you need to do is provide the JSON representation in a stream of bytes. The easiest way to do this is to encapsulate another Json and delegate to it, or construct one on the spot.

Let's say we have a bank account which we need to display as JSON. We need its IBAN, nickname and balance, which (to make this a less trivial example) we get from another service. One way to implement it is this:

public final class BankAccount implements Json {
    private final String iban;
    private final String nickname;
    private final TransactionHistory transactions;

    // Constructor...

    public void makePayment(double amount) { /* Implementation... */ }
    // Other public methods...

    public InputStream bytes() {
        return new MutableJson()
            .with("iban", iban)
            .with("nickname", nickname)
            .with("balance", transactions.balance(iban))

Even simpler way is to extend the JsonEnvelope, then you don't even need to implement bytes():

public final class BankAccount extends JsonEnvelope {
    public BankAccount(String iban, String nickname, TransactionHistory transactions) {
        super(new MutableJson()
            .with("iban", iban)
            .with("nickname", nickname)
            .with("balance", transactions.balance(iban))

    public void makePayment(double amount) { /* Implementation... */ }
    // Other public methods...

We can then make an HTTP response directly, e.g. with Spring:

return new ResponseEntity<>(
    new SmartJson(
        new BankAccount(iban, nickname, transactions)

...or with Takes:

return new RsWithType(
    new RsWithStatus(
        new RsWithBody(
            new BankAccount(iban, nickname, transactions).bytes()

...or insert it in some JSON datastore:

accounts.insert(new BankAccount(iban, nickname));

...or compose it within a larger JSON:

Json accounts = new MutableJson()
    .with("name", "John")
    .with("surname", "Smith")
    .with("account", new BankAccount(iban, nickname));

Additional functionality

If available functionality in the current version of Nereid is not enough, the developer can always fall back to jackson-databind. Convert Json to ObjectNode, do what you need with it, and construct a new Json.

ObjectNode node = new SmartJson(json).objectNode();
// Do stuff with node using Jackson's API.
Json updated = new Json.Of(node);


To contribute:

  1. Fork this repository.
  2. Clone your fork.
  3. Create a branch from master.
  4. Make changes in your branch.
  5. Make sure the build passes with mvn clean install -Pwall. ("wall" is Maven profile with quality wall)
  6. Commit and push your changes to your fork.
  7. Make a pull request from your fork to this repository.

You can read more about contributing in GitHub in this article.