JavaFX WebEngine JavaScript 调用

Question

在 java Windows 10 pro x64 jre 1.8.0_60 中，以下代码生成预期的输出（单击 html 按钮后）：

Hello World

但是在 java Windows 10 pro x64 jre 1.8.0_152 中似乎存在某种断开连接，因为它不会在单击按钮时向控制台输出任何内容

为什么在 java 的最新版本（当时为 152）上，我的代码给出了不可预测且通常不需要的结果。我试图给出创建以下场景的最少代码。

import javafx.application.Application;
import javafx.beans.value.ChangeListener;
import javafx.beans.value.ObservableValue;
import javafx.concurrent.Worker;
import javafx.scene.Scene;
import javafx.scene.layout.BorderPane;
import javafx.scene.web.WebEngine;
import javafx.scene.web.WebView;
import javafx.stage.Stage;
import netscape.javascript.JSObject;

public class Main extends Application {

public static void main(String[] args) {
    launch(args);
}

@Override
public void start(Stage primaryStage) {
    WebView browser = new WebView();
    final WebEngine webEngine = browser.getEngine();
    webEngine.setJavaScriptEnabled(true);
    webEngine.load("https://api.ipify.org/?format=json");
    webEngine.getLoadWorker().stateProperty().addListener(
            new ChangeListener<Worker.State>() {
                @Override
                public void changed(ObservableValue ov, Worker.State oldState, Worker.State newState) {
                    if (newState == Worker.State.SUCCEEDED) {
                        JSObject jso = (JSObject) webEngine.executeScript("window");
                        webEngine.executeScript(
                                "var button = document.createElement(\"button\");\n" +
                                        "button.innerHTML = \"Do Something\";\n" +
                                        "var body = document.getElementsByTagName(\"body\")[0];\n" +
                                        "body.appendChild(button);\n" +
                                        "button.addEventListener (\"click\", function() {java.exit();});");
                        jso.setMember("java", new Bridge());

                    }
                }
            });
    BorderPane panel = new BorderPane(browser);
    Scene scene = new Scene(panel, 700, 700);
    primaryStage.setScene(scene);
    primaryStage.show();
}

public class Bridge {
    public void exit() {
        System.out.println("Hello World");
    }
}}

Answer 1

您正在将 new Bridge() 传递给 setMember 方法。由于没有变量保存 Bridge 实例，因此在您按下按钮之前它会被垃圾回收。

来自WebEngine documentation：

Note that in the above example, the application holds a reference to the JavaApplication instance. This is required for the callback from JavaScript to execute the desired method.

In the following example, the application does not hold a reference to the Java object:

JSObject window = (JSObject) webEngine.executeScript("window");
window.setMember("app", new JavaApplication());

In this case, since the property value is a local object, "new JavaApplication()", the value may be garbage collected in next GC cycle.

When a user clicks the link, it does not guarantee to execute the callback method exit.

将您的 Bridge 对象保存在一个字段中以防止对其进行垃圾回收：

new ChangeListener<Worker.State>() {
    private final Bridge bridge = new Bridge();

    @Override
    public void changed(ObservableValue ov, Worker.State oldState, Worker.State newState) {
        if (newState == Worker.State.SUCCEEDED) {

            // ...

            jso.setMember("java", bridge);
        }
    }

为什么在 Java 的早期版本中没有出现？因为不同的 Java 版本可以自由改变垃圾收集的时间和行为。你很幸运，但在以后的版本中，你的运气运行消失了。

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