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The Google Web Toolkit (GWT) is a Java-based framework that allows developers to write client-side web applications using Java, which is then compiled into optimized JavaScript that can run on any browser. The JavaScript Native Interface (JSNI) is a feature of GWT that provides a way to integrate Java code with JavaScript code and browser APIs.

JSNI bindings allow Java code to interact with JavaScript code in two main ways: calling JavaScript functions from Java, and exposing Java methods to be called from JavaScript. To call a JavaScript function from Java, the JSNI syntax is used to create a JavaScript method reference, which can then be invoked from Java code. For example:

native void alert(String message) /*-{
  $wnd.alert(message);
}-*/;

In this example, the alert() function is a JavaScript function that displays a message in an alert dialog box. The native keyword indicates that the method is implemented in JavaScript, and the JSNI syntax is used to wrap the JavaScript code in a Java method.

We can convert a JavaScriptObject (GWT) to a JSON string using the very same approach:

private static native String convertToJsonString(JavaScriptObject jso) /*-{
    return JSON.stringify(jso);
}-*/;

This way we can use the JS logic in our GWT code.

Built-in method

Another method would be to use GWT’s built in JsonUtils (com.google.gwt.core.client) which takes a JavaScriptObject parameter:

JavaScriptObject someobject = ...;
JsonUtils.stringify(someobject);