Java Image Capture, HTML Snapshot, HTML to image, no external dependencies. Using JavaFX for supporting Operating Systems

JavaFXHtmlImageCapture.java

import javafx.animation.AnimationTimer;
import javafx.animation.PauseTransition;
import javafx.application.Application;
import javafx.application.Platform;
import javafx.embed.swing.SwingFXUtils;
import javafx.geometry.Rectangle2D;
import javafx.scene.Scene;
import javafx.scene.SnapshotParameters;
import javafx.scene.image.WritableImage;
import javafx.scene.layout.VBox;
import javafx.scene.web.WebView;
import javafx.stage.Stage;
import javafx.util.Duration;

import java.awt.image.BufferedImage;
import java.lang.reflect.Field;
import java.util.Objects;
import java.util.concurrent.atomic.AtomicReference;

@Service
public class JavaFXHtmlImageCapture extends Application {
    private static WebView webView;
    private static Stage stage;

    @PostConstruct
    public void initialize() {
        System.setProperty("javafx.macosx.embedded", "true");
        System.setProperty("glass.platform", "Monocle");
        System.setProperty("monocle.platform", "Headless");
        System.setProperty("prism.order", "sw");
        initMonocleHeadless(true);
        new Thread(() -> {
            Application.launch(JavaFXHtmlImageCapture.class);
        }).start();
    }

    @Override
    public void start(Stage stage) throws Exception {
        JavaFXHtmlImageCapture.stage = stage;
        webView = new WebView();
        webView.setPrefSize(800, 100);

        webView.getEngine().load("about:blank");

        VBox layout = new VBox(10);
        layout.getChildren().setAll(webView);

        stage.setScene(new Scene(layout));
        stage.show();
    }

    public synchronized static BufferedImage captureHtml(String html) {
        AtomicReference<BufferedImage> captured = new AtomicReference<>();
        AtomicReference<Throwable> throwable = new AtomicReference<>();
        runLater(() -> {
            try {
                webView.setPrefSize(800, 100);
                webView.setMaxWidth(800);
                if (html.startsWith("http")) {
                    webView.getEngine().load(html);
                } else {
                    webView.getEngine().loadContent(html, "text/html");
                }
                webView.autosize();
                stage.show();

                runLater(() -> {
                    try {
                        String heightText = webView.getEngine().executeScript("document.body.offsetHeight").toString();
                        double height = Double.valueOf(heightText) + 5;
                        stage.setHeight(height);
                        webView.setPrefHeight(height);
                        runLater(() -> {
                            try {
                                SnapshotParameters parameters = new SnapshotParameters();
                                parameters.setViewport(new Rectangle2D(0, 0, 760, Math.min(height, 1200) - 3));

                                WritableImage image = webView.snapshot(parameters, null);
                                BufferedImage bufferedImage = SwingFXUtils.fromFXImage(image, null);
                                captured.set(bufferedImage);
//                                ImageIO.write(bufferedImage, "png", new File("capture.png"));
                            } catch (Throwable t) {
                                throwable.set(t);
                            }
                        });
                    } catch (Throwable t) {
                        throwable.set(t);
                    }
                });
            } catch (Throwable t) {
                throwable.set(t);
            }
        });
        try {
            Thread.sleep(1500);
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
        for (int i = 0; i < 200; i++) {
            if (captured.get() == null && throwable.get() == null) {
                try {
                    Thread.sleep(100);
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                }
            }
        }
        if (throwable.get() != null) {
            throw new RuntimeException(throwable.get());
        }
        return captured.get();
    }

    public static void runInPlatform(Runnable runnable) {
        Platform.runLater(() -> {
            final AnimationTimer timer = new AnimationTimer() {
                private int pulseCounter;

                @Override
                public void handle(long now) {
                    pulseCounter += 1;
                    if (pulseCounter > 2) {
                        stop();
                        runnable.run();
                    }
                }
            };
            timer.start();
        });
    }

    public static void runLater(Runnable runnable) {
        final PauseTransition pt = new PauseTransition();
        pt.setDuration(Duration.millis(500));
        pt.setOnFinished(actionEvent -> runInPlatform(runnable::run));
        pt.play();
    }

    private static void initMonocleHeadless(boolean headless) {
        if (checkSystemPropertyEquals("testfx.headless", "true") || headless) {
            try {
                assignMonoclePlatform();
                assignHeadlessPlatform();
            } catch (ClassNotFoundException var3) {
                throw new IllegalStateException("Monocle headless platform not found", var3);
            } catch (Exception var4) {
                throw new RuntimeException(var4);
            }
        }

    }

    private static boolean checkSystemPropertyEquals(String propertyName, String valueOrNull) {
        return Objects.equals(System.getProperty(propertyName, null), valueOrNull);
    }

    private static void assignMonoclePlatform() throws Exception {
        Class platformFactoryClass = Class.forName("com.sun.glass.ui.PlatformFactory");
        Object platformFactoryImpl = Class.forName("com.sun.glass.ui.monocle.MonoclePlatformFactory").newInstance();
        assignPrivateStaticField(platformFactoryClass, "instance", platformFactoryImpl);
    }

    private static void assignHeadlessPlatform() throws Exception {
        Class nativePlatformFactoryClass = Class.forName("com.sun.glass.ui.monocle.NativePlatformFactory");
        Object nativePlatformImpl = Class.forName("com.sun.glass.ui.monocle.headless.HeadlessPlatform").newInstance();
        assignPrivateStaticField(nativePlatformFactoryClass, "platform", nativePlatformImpl);
    }

    private static void assignPrivateStaticField(Class<?> cls, String name, Object value) throws Exception {
        Field field = cls.getDeclaredField(name);
        field.setAccessible(true);
        field.set(cls, value);
        field.setAccessible(false);
    }
}

Usage:
new JavaFXHtmlImageCapture().initialize()
BufferedImage image = JavaFXHtmlImageCapture.captureHtml(url or html content);

Does this require usage of OpenJDK?

Seems so. Tried it on a Mac with Oracle Java 8 JDK, and it complained about some monocle class not existing.

If you are using Maven, then adding this dependencies, should solve compilation problems:

<dependency>
  <!-- for Oracle JDK 1.8 -->
  <groupId>com.oracle</groupId>
  <artifactId>javafx</artifactId>
  <version>2.2</version>
  <systemPath>${java.home}/lib/ext/jfxrt.jar</systemPath>
  <scope>system</scope>
</dependency>
<dependency>
  <groupId>org.springframework</groupId>
  <artifactId>spring-context</artifactId>
  <version>4.2.5.RELEASE</version>
</dependency>
<dependency>
  <groupId>org.jfxtras</groupId>
  <artifactId>openjfx-monocle</artifactId>
  <version>1.8.0_20</version>
</dependency>

Won't work with JDK11. Need to use:

    private static void assignHeadlessPlatform() throws Exception {
        Class<?> nativePlatformFactoryClass = Class.forName("com.sun.glass.ui.monocle.NativePlatformFactory");
        try {
            Constructor<?> nativePlatformCtor = Class.forName(
                    "com.sun.glass.ui.monocle.HeadlessPlatform").getDeclaredConstructor();
            nativePlatformCtor.setAccessible(true);
            assignPrivateStaticField(nativePlatformFactoryClass, "platform", nativePlatformCtor.newInstance());
        }
        catch (ClassNotFoundException exception) {
            // Before Java 8u40 HeadlessPlatform was located inside of a "headless" package.
            Constructor<?> nativePlatformCtor = Class.forName(
                    "com.sun.glass.ui.monocle.headless.HeadlessPlatform").getDeclaredConstructor();
            nativePlatformCtor.setAccessible(true);
            assignPrivateStaticField(nativePlatformFactoryClass, "platform", nativePlatformCtor.newInstance());
        }
    }