javafx WebView and WebEngine Communication between Java app and Javascript in the web page


Example

When using a WebView to display your own custom webpage and this webpage contains Javascript, it might be necessary to establish a two-way communication between the Java program and the Javascript in the web page.

This example shows how to setup such a communication.

The webpage shall display an input field and a button. On clicking the button, the value from the input field is sent to the Java application, which processes it. After processing a result is sent to the Javascript which in turn displays the result on the web page.

The basic principle is that for communication from Javascript to Java an object is created in Java which is set into the webpage. And for the other direction, an object is created in Javascript and extracted from the webpage.

The following code shows the Java part, I kept it all in one file:

package com.sothawo.test;

import javafx.application.Application;
import javafx.concurrent.Worker;
import javafx.scene.Scene;
import javafx.scene.web.WebEngine;
import javafx.scene.web.WebView;
import javafx.stage.Stage;
import netscape.javascript.JSObject;

import java.io.File;
import java.net.URL;

/**
 * @author P.J. Meisch (pj.meisch@sothawo.com).
 */
public class WebViewApplication extends Application {

    /** for communication to the Javascript engine. */
    private JSObject javascriptConnector;

    /** for communication from the Javascript engine. */
    private JavaConnector javaConnector = new JavaConnector();;

    @Override
    public void start(Stage primaryStage) throws Exception {
        URL url = new File("./js-sample.html").toURI().toURL();

        WebView webView = new WebView();
        final WebEngine webEngine = webView.getEngine();

        // set up the listener
        webEngine.getLoadWorker().stateProperty().addListener((observable, oldValue, newValue) -> {
            if (Worker.State.SUCCEEDED == newValue) {
                // set an interface object named 'javaConnector' in the web engine's page
                JSObject window = (JSObject) webEngine.executeScript("window");
                window.setMember("javaConnector", javaConnector);

                // get the Javascript connector object. 
                javascriptConnector = (JSObject) webEngine.executeScript("getJsConnector()");
            }
        });

        Scene scene = new Scene(webView, 300, 150);
        primaryStage.setScene(scene);
        primaryStage.show();

        // now load the page
        webEngine.load(url.toString());
    }

    public class JavaConnector {
        /**
         * called when the JS side wants a String to be converted.
         *
         * @param value
         *         the String to convert
         */
        public void toLowerCase(String value) {
            if (null != value) {
                javascriptConnector.call("showResult", value.toLowerCase());
            }
        }
    }
}

When the page has loaded, a JavaConnector object (which is defined by the inner class and created as a field) is set into the web page by these calls:

JSObject window = (JSObject) webEngine.executeScript("window");
window.setMember("javaConnector", javaConnector);

The javascriptConnector object is retrieved from the webpage with

javascriptConnector = (JSObject) webEngine.executeScript("getJsConnector()");

When the toLowerCase(String) method from the JavaConnector is called, the passed in value is converted and then sent back via the javascriptConnector object.

And this is the html and javascript code:

<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="UTF-8">
        <title>Sample</title>
    </head>
    <body>
        <main>
    
            <div><input id="input" type="text"></div>
            <button onclick="sendToJava();">to lower case</button>
            <div id="result"></div>
    
        </main>
    
        <script type="text/javascript">
            function sendToJava () {
                var s = document.getElementById('input').value;
                javaConnector.toLowerCase(s);
            };
    
            var jsConnector = {
                showResult: function (result) {
                    document.getElementById('result').innerHTML = result;
                }
            };
    
            function getJsConnector() {
                return jsConnector;
            };
        </script>
    </body>
</html>

The sendToJava function calls the method of the JavaConnector which was set by the Java code:

function sendToJava () {
    var s = document.getElementById('input').value;
    javaConnector.toLowerCase(s);
};

and the function called by the Java code to retrieve the javascriptConnector just returns the jsConnector object:

var jsConnector = {
    showResult: function (result) {
        document.getElementById('result').innerHTML = result;
    }
};
    
function getJsConnector() {
    return jsConnector;
};

The argument type of the calls between Java and Javascript are not limited to Strings. More info on the possible types and conversion are found in the JSObject API doc.