Many AJAX application developers have adopted JSON as the data format of choice for server communication. It is a relatively simple format based on the object-literal notation of JavaScript. If you choose to use JSON-encoded data within your application, you can use GWT classes to parse and manipulate JSON objects, as well as the very useful and elegant concept of JavaScript Overlay Types.

The JSON format is based on the syntax and data types of the JavaScript language. It supports strings, numbers, booleans, and null values. You can also combine multiple values into arrays and objects. JSON objects are simply unordered sets of name/value pairs, where the name is always a string and the value is any other valid JSON type (even another object). Here's an example of encoding product data in JSON:

{
  "product": {
    "name": "Widget",
    "company": "ACME, Inc",
    "partNumber": "7402-129",
    "prices": [
      { "minQty": 1, "price": 12.49 },
      { "minQty": 10, "price": 9.99 },
      { "minQty": 50, "price": 7.99 }
    ]
  }
}

See json.org/example.html for more JSON examples.

1. Parsing JSON
2. Mashups with JSON and JSNI

Parsing JSON

You can parse JSON Strings and convert them to a JavaScriptObject in GWT with a simple one liner JSNI method. However, you need to be careful since eval() in JavaScript can actually run code, so you need to absolutely trust the JSON String that you evaluate.

/*
 * Takes in a trusted JSON String and evals it.
 * @param JSON String that you trust
 * @return JavaScriptObject that you can cast to an Overlay Type
 */
public static native JavaScriptObject parseJson(String jsonStr) /*-{
  return eval(jsonStr);
}-*/;

Typically, you will receive JSON data as the response text of an HTTP request. Thus, you'll first have to convert that String into a Object that you can work with using a method like the one shown above. The recommended way for interacting with JavaScriptObjects is to use JavaScript Overlay Types.

Mashups with JSON and JSNI

If you're loading JSON-encoded data from your own server, you'll typically use the RequestBuilder and related classes to make HTTP requests. However, you can also retrieve JSON from remote servers in true mashup fashion using GWT's JavaScript Native Interface (JSNI) functionality. The techniques for cross-site JSON is explained more fully in the getting started tutorial. To see a working example, check out the Cross-site Client-Server Communication section of the Getting Started guide.