不允许我多次执行每个动作

Question

我的代码存在问题，每当我尝试不止一次做某事时，程序就会崩溃。例如，如果我登录，然后注销然后再次重新登录，程序将崩溃。这种情况发生在其他我不想做的事情上，例如切换到另一个场景，回到家，然后再次尝试去另一个场景。

只需发布登录部分的代码。

主要

package libraryapp;

import java.io.File;
import java.io.IOException;
import java.util.prefs.Preferences;

import javafx.application.Application;
import javafx.collections.FXCollections;
import javafx.collections.ObservableList;
import javafx.fxml.FXMLLoader;
import javafx.scene.Scene;
import javafx.scene.control.Alert;
import javafx.scene.control.Alert.AlertType;
import javafx.scene.layout.AnchorPane;
import javafx.scene.layout.BorderPane;
import javafx.stage.Stage;
import javax.xml.bind.JAXBContext;
import javax.xml.bind.Marshaller;
import javax.xml.bind.Unmarshaller;
import libraryapp.model.Book;
import libraryapp.model.BookListWrapper;
import libraryapp.view.BrowseController;
import libraryapp.view.HomeOverviewController;
import libraryapp.view.LoginController;
import libraryapp.view.RootLayoutController;

public class LibraryApp extends Application {

private Stage primaryStage;
private BorderPane rootLayout;
/**
 * The data as an observable list of Books.
 */
private ObservableList<Book> bookData = FXCollections.observableArrayList();

/**
 * Constructor
 */
public LibraryApp() {
    // Add some sample data
    bookData.add(new Book("Hans", "Muster"));
    bookData.add(new Book("Ruth", "Mueller"));
    bookData.add(new Book("Heinz", "Kurz"));
    bookData.add(new Book("Cornelia", "Meier"));
    bookData.add(new Book("Werner", "Meyer"));
    bookData.add(new Book("Lydia", "Kunz"));
    bookData.add(new Book("Anna", "Best"));
    bookData.add(new Book("Stefan", "Meier"));
    bookData.add(new Book("Martin", "Mueller"));
}

/**
 * Returns the data as an observable list of Books. 
 * @return
 */
public ObservableList<Book> getBookData() {
    return bookData;
}




@Override
public void start(Stage primaryStage) {
    this.primaryStage = primaryStage;
    this.primaryStage.setTitle("LibraryApp"); 
    initRootLayout();
     showLogin();
}


/**
* Initializes the root layout and tries to load the last opened
* person file.
 */
public void initRootLayout() {
try {
    // Load root layout from fxml file.
    FXMLLoader loader = new FXMLLoader();
        loader.setLocation(LibraryApp.class.getResource("view/RootLayout.fxml"));
    rootLayout = (BorderPane) loader.load();

    // Show the scene containing the root layout.
    Scene scene = new Scene(rootLayout);
    primaryStage.setScene(scene);

    // Give the controller access to the main app.
    RootLayoutController controller = loader.getController();
    controller.setLibraryApp(this);

    primaryStage.show();
} catch (IOException e) {
    e.printStackTrace();
}

// Try to load last opened person file.
File file = getBookFilePath();
if (file != null) {
    loadBookDataFromFile(file);
}
}





/**
 * Shows the home overview inside the root layout.
 */
public void showHomeOverview() {
    try {

        FXMLLoader loader = new FXMLLoader();
        loader.setLocation(LibraryApp.class.getResource("view/HomeOverview.fxml"));
        AnchorPane homeOverview = (AnchorPane) loader.load();



        // Set home overview into the center of root layout.
        rootLayout.setCenter(homeOverview);

    // Give the controller access to the main app.
    HomeOverviewController controller = loader.getController();
    controller.setLibraryApp(this);


    } catch (IOException e) {
        e.printStackTrace();
    }
}

public void showLogin() {
    try {

        FXMLLoader loader = new FXMLLoader();
        loader.setLocation(LibraryApp.class.getResource("view/Login.fxml"));
        AnchorPane login = (AnchorPane) loader.load();



        // Set home overview into the center of root layout.
        rootLayout.setCenter(login);

    // Give the controller access to the main app.
    LoginController controller = loader.getController();
    controller.setLibraryApp(this);


    } catch (IOException e) {
        e.printStackTrace();
    }
}

public void showBrowse() {
try {

    FXMLLoader loader = new FXMLLoader();
    loader.setLocation(LibraryApp.class.getResource("view/Browse.fxml"));
    AnchorPane browse = (AnchorPane) loader.load();

    // Set browse into the center of root layout.
    rootLayout.setCenter(browse);

    // Give the controller access to the main app.
    BrowseController controller = loader.getController();
    controller.setLibraryApp(this);

} catch (IOException e) {
    e.printStackTrace();
}
}


/**
 * Returns the main stage.
 * @return
 */
public Stage getPrimaryStage() {
    return primaryStage;
}

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

登录控制器

package libraryapp.view;

import java.io.File;
import java.io.IOException;
import java.net.URL;
import java.util.ResourceBundle;
import javafx.event.ActionEvent;
import javafx.fxml.FXML;
import javafx.fxml.FXMLLoader;
import javafx.fxml.Initializable;
import javafx.scene.control.Label;
import javafx.scene.control.PasswordField;
import javafx.scene.control.TextField;
import javafx.scene.layout.AnchorPane;
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;
import libraryapp.LibraryApp;
import org.w3c.dom.Document;
import org.w3c.dom.NodeList;
import org.xml.sax.SAXException;

/**
* FXML Controller class
*
* @author Michael
*/
public class LoginController implements Initializable {

/**
 * Initializes the controller class.
 */

    @FXML
private AnchorPane loginPane;



@FXML
private Label lblMessage;  
@FXML
private TextField txtUsername;
@FXML
private PasswordField txtPassword;

private LibraryApp libraryApp;


public String getString(String tagName) throws SAXException, IOException,     ParserConfigurationException{
    File file = new File("src/libraryapp/account.xml");
    DocumentBuilderFactory docBuildFactory =   DocumentBuilderFactory.newInstance();
    DocumentBuilder docBuilder = docBuildFactory.newDocumentBuilder();
    Document document = docBuilder.parse(file);

    NodeList list = document.getElementsByTagName(tagName);
    if (list != null && list.getLength() > 0){
        NodeList subList = list.item(0).getChildNodes();

        if (subList != null && subList.getLength() > 0) {
            return subList.item(0).getNodeValue();
        }
    }
        return null;
   }

@FXML
private void login(ActionEvent event) throws IOException, ParserConfigurationException, SAXException {


     String email = getString("email");

     String password = getString("password");


    if (txtUsername.getText().equals(email) && txtPassword.getText().equals(password)){

        libraryApp.showHomeOverview();
    } else{
        lblMessage.setText("Username or password invalid!");
    }


}

    @FXML
private void register(ActionEvent event) throws IOException {

   AnchorPane pane = FXMLLoader.load(getClass().getResource("Register.fxml"));
   loginPane.getChildren().setAll(pane);
}

@Override
public void initialize(URL url, ResourceBundle rb) {
    // TODO
}    

 public void setLibraryApp(LibraryApp libraryApp) {
    this.libraryApp = libraryApp;
}

}

这是我得到的错误

Exception in thread "JavaFX Application Thread" java.lang.RuntimeException: java.lang.reflect.InvocationTargetException
at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1774)
at javafx.fxml.FXMLLoader$ControllerMethodEventHandler.handle(FXMLLoader.java:1657)
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.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:8413)
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$1.handle(BehaviorSkinBase.java:96)
at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(BehaviorSkinBase.java:89)
at com.sun.javafx.event.CompositeEventHandler$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.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$MouseHandler.process(Scene.java:3757)
at javafx.scene.Scene$MouseHandler.access$1500(Scene.java:3485)
at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1762)
at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2494)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:381)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:295)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$354(GlassViewEventHandler.java:417)
at com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:389)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:416)
at com.sun.glass.ui.View.handleMouseEvent(View.java:555)
at com.sun.glass.ui.View.notifyMouse(View.java:937)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.lambda$null$148(WinApplication.java:191)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at sun.reflect.misc.Trampoline.invoke(MethodUtil.java:71)
at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at sun.reflect.misc.MethodUtil.invoke(MethodUtil.java:275)
at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1769)
... 52 more
Caused by: java.lang.NullPointerException
at libraryapp.view.LoginController.login(LoginController.java:82)
... 62 more

注销

@FXML
public void logout(ActionEvent event) throws IOException {

        File bookFile = libraryApp.getBookFilePath();
        libraryApp.saveBookDataToFile(bookFile);


    AnchorPane pane = FXMLLoader.load(getClass().getResource("Login.fxml"));
    browsePane.getChildren().setAll(pane);
}

Answer 1

logout()的实现会再次加载登录fxml文件，但不会在新控制器上设置libraryApp字段。因此，该字段在控制器的该实例中为空，并且您获得空指针异常。

由于您已经引用了libraryApp，因此只需调用

即可重用现有代码来加载登录屏幕

libraryApp.showLogin();