Fluent JSON library for java
Switch branches/tags
Nothing to show
Clone or download
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Failed to load latest commit information.




You can download source, javadoc, and binaries from https://github.com/rzwitserloot/com.zwitserloot.json/releases

How to compile / develop



and that's that. All dependencies will be downloaded automatically. Once you've run ant, you can also open the project directory as an eclipse project. The runtime jar will be in the dist directory.

General principle

JSON comes from javascript, which is not an explicitly typed language. Therefore, when trying to work with JSON from java, it is a good idea to be explicit about what type you think some element in the JSON data should be and coerce whatever's there to this type; this matches how javascript and other similar languages deal with JSON.

This library does just that: You treat JSON as a directory-like structure and then query keys by stating what type you think it should be. This library will then find this element in the JSON and goes to some lengths to coerce it to the requested type, including parsing strings into numbers and upgrading single elements into lists with 1 element in them, if that's what you expected. Furthermore, all query methods have a second form with a default value to return if the element isn't in the JSON data.

This is not a library for 'marshalling' (the notion of converting JSON into java POJOs and back). That is much more complicated and heavyweight.

How to use

The simplest example of reading something:

String value = json.asString();
int value = json.asInt();

Similarly, to set a new value:

json.setString("Hello, world!");

A JSON instance isn't just a representation of an actual JSON structure, but it also includes your position inside it; a JSON blob has structure, after all. For example, if we have:

{ films: [ { name: "Serenity", director: { name: "Joss Whedon", age: 45 } }, { name: "A few good men", director: { name: "Rob Reiner" } } ] }

then you can query values from it like so:

JSON json = JSON.parse(theAboveJSONString);
int jossWhedonsAge = json.get("films").get(0).get("director").get("age").asInt(-1);

The above snippet also shows how all the asX methods take an optional default.

It is perfectly acceptable to get() your way into non-existent nodes; this does not cause an error, and if you try to get a value from such a non-existent node, you'll always get the default, or an exception if you didn't specify a default, or an empty list / keyset if you try to coerce the value to a list or map. This is not only convenient, as many JSON services simply omit information that isn't available, but is also the mechanism with which you can create new JSON. For example, to recreate the above JSON programatically:

JSON json = JSON.newMap();
JSON serenity = json.get("films").add();
serenity.get("director").get("name").setString("Joss Whedon");
JSON fewGoodMen = json.get("films").add();
fewGoodMen.get("name").setString("A few good men");
fewGoodMen.get("director").get("name").setString("Rob Reiner");
String jsonString = json.prettyPrint();

Reading from lists and maps

Once you've navigated your way to a map node, you can use keySet() to loop through each key. In JSON, all keys are always strings. For example:

JSON json = JSON.parse(movieData);
for (String key : json.get("films").get(0).keySet()) {

would print name followed by director.

To navigate through a list node, use the asList() method:

for (JSON movie : JSON.parse(movieData).get("films").asList()) {

The asList() method is smart enough to have zero size if the node doesn't exist, and to form a list containing just one element if you're on a simple (non-list, non-map) element.

As a convenience, there's also asStringList() which will coerce all elements inside the list to a string.

Writing lists and maps

You've already seen how to write maps; just get the key name then start using the setX methods. To write into a list, use the magic add() method. add() itself doesn't actually create anything, but once you start writing to a JSON instance returned by the add() method, writing occurs. See the 'write' example from earlier.


A JSON object can be rendered as minified JSON with the toJSON() method, or as pretty-printed JSON with the prettyPrint() method.

Advanced topics

  • You can use mixin(JSON) to merge 2 separate JSON lists or 2 separate JSON maps.
  • You can use setWithJSON(JSON) to put some JSON inside another JSON object.
  • You can use setIsMap() and setIsList() to enforce empty maps/lists.
  • You can use deepCopy() to create a 'deep' clone such that changes to one will not affect the other.
  • You can use getPath(), up() and top() to treat the JSON object as a directory pointer of sorts.



  • Pretty printing