Implementing callbacks and event handling in Nashorn

06 Oct 2023

Nashorn is a JavaScript engine for Java that allows you to embed JavaScript code within your Java applications. In addition to executing JavaScript code, Nashorn also provides support for callbacks and event handling. This feature allows you to define and handle callbacks in your JavaScript code, and invoke them from your Java application. In this blog post, we will explore how to implement callbacks and event handling in Nashorn.

Callbacks in JavaScript
Creating Callback Functions in Nashorn
Passing Callbacks to JavaScript Functions
Invoking Callbacks from Java
Event Handling in Nashorn
- Registering Event Listeners
- Triggering Events from Java
Conclusion

Callbacks in JavaScript

In JavaScript, a callback is a function that is passed as an argument to another function. The function that receives the callback can then invoke it at a later time. This pattern is commonly used for asynchronous operations, such as network requests or file I/O, where the result is not immediately available.

Creating Callback Functions in Nashorn

In Nashorn, you can create callback functions using the Java.extend function. This function allows you to extend a Java interface and implement its methods using JavaScript code. Here’s an example of creating a callback function that implements the Runnable interface:

var callback = Java.extend(java.lang.Runnable, {
    run: function() {
        print("Callback executed!");
    }
});

In this example, the run method of the Runnable interface is implemented using JavaScript code. The print function is used to output a message indicating that the callback has been executed.

Passing Callbacks to JavaScript Functions

Once you have created a callback function, you can pass it as an argument to other JavaScript functions. Here’s an example of passing a callback function to a JavaScript function:

function performAsyncOperation(callback) {
    // Simulating an asynchronous operation
    setTimeout(callback, 1000);
}

performAsyncOperation(callback);

In this example, the performAsyncOperation function accepts a callback function as an argument. The setTimeout function is used to simulate an asynchronous operation that invokes the callback after a delay of 1000 milliseconds.

Invoking Callbacks from Java

To invoke a callback function from your Java application, you can use the invokeFunction method provided by the ScriptEngine class. Here’s an example of invoking a callback function from Java:

import javax.script.*;

public class CallbackExample {
    public static void main(String[] args) throws ScriptException, NoSuchMethodException {
        ScriptEngineManager manager = new ScriptEngineManager();
        ScriptEngine engine = manager.getEngineByName("nashorn");

        engine.eval("var callback = Java.extend(java.lang.Runnable, { run: function() { print('Callback executed from Java!'); } });");

        Invocable invocable = (Invocable) engine;
        invocable.invokeFunction("callback.run");
    }
}

In this example, we first create a ScriptEngine instance and evaluate the JavaScript code that defines our callback function. We then cast the engine to an Invocable and use the invokeFunction method to invoke the run method of the callback.

Event Handling in Nashorn

In addition to callbacks, Nashorn also provides support for event handling. You can register event listeners in JavaScript code and trigger events from your Java application.

Registering Event Listeners

To register an event listener in JavaScript, you can use the addEventListener function provided by the EventTarget interface. Here’s an example of registering an event listener for the click event:

var button = document.getElementById("myButton");
button.addEventListener("click", function() {
    print("Button clicked!");
});

In this example, the click event listener is registered for a button with the id “myButton”. When the button is clicked, the callback function is invoked and the message “Button clicked!” is printed.

Triggering Events from Java

To trigger an event from your Java application, you can use the dispatchEvent method provided by the EventTarget interface. Here’s an example of triggering a click event:

import javax.script.*;

public class EventHandlingExample {
    public static void main(String[] args) throws ScriptException, NoSuchMethodException {
        ScriptEngineManager manager = new ScriptEngineManager();
        ScriptEngine engine = manager.getEngineByName("nashorn");

        engine.eval("var button = document.getElementById('myButton');");

        Invocable invocable = (Invocable) engine;
        invocable.invokeMethod(engine.eval("button"), "dispatchEvent", engine.eval("new Event('click')"));
    }
}

In this example, we first obtain a reference to the button element in our JavaScript code. We then cast the engine to an Invocable and use the invokeMethod method to invoke the dispatchEvent method of the button object, passing a new Event object for the click event.

Conclusion

In this blog post, we have explored how to implement callbacks and event handling in Nashorn. We have seen how to create callback functions, pass them to JavaScript functions, and invoke them from Java. We have also seen how to register event listeners and trigger events from your Java application. These features allow you to seamlessly integrate JavaScript code with your Java applications and handle asynchronous operations and user interactions.

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