JavaFX开启新场景

Question

我希望我的代码能够在我单击按钮时打开一个新场景，但它不起作用，我也不知道为什么。

public void start(Stage primaryStage) throws Exception {
    window = primaryStage;

    Parent root = FXMLLoader.load(getClass().getResource("FXML/LoginScene.fxml"));
    scene = new Scene(root,400,400);
    openScene = new OpenScene(writer);
    window.setScene(scene);
    window.show();


}
public static void main(String[] args){
    launch(args);
}
@FXML protected void btnConnect(ActionEvent event) {
    System.out.println("hallo");
    try {
        openScene.start(window);
    } catch (Exception e) {

        e.printStackTrace();
    }
}

GUI 成功显示，但是当我按下按钮时，它抛出异常。

public class OpenScene extends Application{
    PrintWriter writer;

    @Override
    public void start(Stage window) throws Exception {
        Parent root = FXMLLoader.load(getClass().getResource("FXML/OpenScene.fxml"));
        Scene scene =  new Scene(root, 200 ,200);
        window.setScene(scene);
        window.show();

    }
    public OpenScene(PrintWriter writer){
        this.writer = writer;
    }
}

更新

我尝试将应用程序 class 与控制器 class 分开，如下面 James_D 的回答所示，但出现以下异常：

Exception in thread "JavaFX Application Thread" java.lang.RuntimeException: java.lang.reflect.InvocationTargetException
at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1762)
at javafx.fxml.FXMLLoader$ControllerMethodEventHandler.handle(FXMLLoader.java:1645)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
at javafx.event.Event.fireEvent(Event.java:198)
at javafx.scene.Node.fireEvent(Node.java:8216)
at javafx.scene.control.Button.fire(Button.java:185)
at com.sun.javafx.scene.control.behavior.ButtonBehavior.mouseReleased(ButtonBehavior.java:182)
at com.sun.javafx.scene.control.skin.BehaviorSkinBase.handle(BehaviorSkinBase.java:96)
at com.sun.javafx.scene.control.skin.BehaviorSkinBase.handle(BehaviorSkinBase.java:89)
at com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:218)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
at javafx.event.Event.fireEvent(Event.java:198)
at javafx.scene.Scene$MouseHandler.process(Scene.java:3724)
at javafx.scene.Scene$MouseHandler.access00(Scene.java:3452)
at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1728)
at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2461)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:348)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:273)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:382)
at com.sun.glass.ui.View.handleMouseEvent(View.java:553)
at com.sun.glass.ui.View.notifyMouse(View.java:925)
Caused by: java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:483)
at sun.reflect.misc.Trampoline.invoke(MethodUtil.java:71)
at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:483)
at sun.reflect.misc.MethodUtil.invoke(MethodUtil.java:275)
at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1759)
... 43 more
Caused by: java.lang.NullPointerException
at MainController.btnConnect(MainController.java:22)
... 53 more

Answer 1

您似乎正在尝试将主应用程序 class 用作控制器 class。这会让人感到困惑，你应该避免它。这是发生的事情：

当您启动应用程序时，它会调用 launch(...)。 launch 方法继承自 Application，将执行一系列重要的 "housekeeping"，例如启动 JavaFX 工具包和 JavaFX 应用程序线程。然后它会创建您的应用程序 subclass 的实例，创建初始 Stage 并在该实例中调用 start(...)。

在您的 start(...) 方法中，您初始化了几个实例变量（window 和 openScene），并加载了一个 FXML 文件，显示了它的内容。 FXMLLoader.load(...) 方法执行以下操作：

根据文件中的 XML 个元素创建 UI 个元素
创建由 FXML 文件中的 fx:controller 属性指定的 class 实例
将任何 @FXML 注释的字段注入控制器实例
注册 FXML 文件中指定的任何事件处理程序

请注意，如果您为应用程序和控制器指定相同的 class，将创建该 class 的两个实例。一种是launch方法创建的，一种是FXMLLoader方法创建的。请注意，只有 launch 方法创建的实例才会调用 start(...) 方法。由于您在 start(...) 方法中初始化实例变量，因此这些变量不会在 FXMLLoader 创建的实例中初始化。所以在FXMLLoader（"controller instance"，如果你愿意的话）创建的实例中，window和openScene没有被初始化。因此，行

    openScene.start(window);

会抛出一个NullPointerException.

由于应用程序和控制器确实具有完全不同的角色，因此您应该将它们分成不同的 classes。这将使事情变得不那么混乱。请注意，您始终可以通过调用

找到显示节点的 window

anyNode.getScene().getWindow();

所以不需要缓存 Stage 实例。

所以：

public class MainApp extends Application {

    @Override
    public void start(Stage primaryStage) throws Exception {

        // change LoginScene.fxml so it now has fx:controller="LoginController"
        Parent root = FXMLLoader.load(getClass().getResource("FXML/LoginScene.fxml"));

        Scene scene = new Scene(root, 400, 400);
        primaryStage.setScene(scene);
        primaryStage.show();
    }

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

并为控制器使用不同的 class：

public class LoginController {

    private OpenScene openScene ;

    @FXML
    private Button connectButton ; // needs fx:id in fxml file...

    public void initialize() throws Exception {
        PrintWriter writer = ... ;
        openScene = new OpenScene(writer);
    }

    @FXML // handler for connect button:
    private void btnConnect() throws Exception {
        Stage stage = (Stage) connectButton.getScene().getWindow();
        openScene.start(stage);
    }
}

另请注意，您的 OpenScene class 无需成为 Application 子class：您只需要一个这样的 class申请：

public class OpenScene {

    private final PrintWriter writer ;

    public OpenScene(PrintWriter writer) {
        this.writer = writer ;
    }

    // doesn't need to be called "start" any more...    
    public void start(Stage window) throws Exception {
        Parent root = FXMLLoader.load(getClass().getResource("FXML/OpenScene.fxml"));
        Scene scene =  new Scene(root, 200 ,200);
        window.setScene(scene);
        window.show();
    }
}

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