小贝子编程

在VBox上调用方法会抛出NullPointerException



我正在开发一个客户机-服务器信使应用程序进行实践。在我的客户端模块中有client类、ClientController类、ClientGUI类和fxml文件、Message类、CSS文件和Main类。我在我的ClientGUI类和ClientController类中有一个方法，display(String message)，它将message对象添加到id为#messages的VBox中。

这是我的问题:在我的ClientController类内部是一个display()方法，它调用ClientGUI的display()方法(我知道这似乎是多余的，但这不是问题)。当display()方法在我的ClientGUI类中被调用时，例如在setOnMouseClick()方法中，它可以正常工作。然而，当从我的ClientController类调用此方法时，我得到一个NullPointerException，指向我的ClientGUI类的display()方法中VBox类型的消息变量。我已经尝试用VBox test = messages;替换messages.getChildren().add(new Message(message));

后者工作正常，而前者抛出异常。我还尝试在我的消息变量上调用其他方法，所有这些方法都会抛出相同的错误。为什么会发生这种情况?

(它仍在开发过程中，因此可能有其他不相关的问题)

客户:

package client;
import java.io.*;
import java.net.Socket;
/**
 * Gabe
 * 9/26/2016
 * Description: Represents client and its socket
 */
public class Client implements Serializable {
    private static Client instance;
    private Socket clientSocket;
    private BufferedReader in;
    private PrintWriter out;
    private ObjectOutputStream objectOutputStream;
    private boolean connected = false;
    private static final long serialVersionUID = 0L;
    private Client() {}
    public void connect(String targetAddress, String username_, int port_) {
        try {
            clientSocket = new Socket(targetAddress, port_);
            clientSocket.setReuseAddress(true); // Allows same port to be used successively without cool-down
            in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
            out = new PrintWriter(clientSocket.getOutputStream(), true);
            objectOutputStream = new ObjectOutputStream(clientSocket.getOutputStream());
            objectOutputStream.writeObject(username_);
            ClientController.getInstance().display("Attention: Successfully connected to server");
            connected = true;
        } catch (IOException e) {
            ClientController.getInstance().display("Attention: Unable to connect to server");
        }
    }
    public void disconnect() {
        try {
            if (in != null) {
                in.close();
            }
            if (out != null) {
                out.close();
            }
            if (objectOutputStream != null) {
                objectOutputStream.close();
            }
            if (clientSocket != null) {
                clientSocket.close();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
        connected = false;
    }
    public boolean connected() {
        return connected;
    }
    public static Client getInstance() {
        if (instance == null) {
            instance = new Client();
        }
        return instance;
    }
    public void sendMessage(String message) {
        out.println(message);
    }
}

ClientController: 

package client;
import javafx.scene.layout.VBox;
/**
 * Gabe Castelli
 * 9/28/2016
 * Description:
 */
public class ClientController implements Controller {
    private static ClientController instance;
    private ClientController() {}
    public static ClientController getInstance() {
        if (instance == null) {
            instance = new ClientController();
        }
        return instance;
    }
    // Controls
    public void display(String message) {
        ClientGUI.getInstance().display(message);
    }
    public void displayAndSend(String message) {
        display(message);
        Client.getInstance().sendMessage(message);
    }
    public void connect(String address_, String username_, int port_) {
        Client.getInstance().connect(address_, username_, port_);
    }
    public void disconnect() {
        Client.getInstance().disconnect();
    }
}

ClientGUI: 

package client;
import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.geometry.Pos;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.control.TextField;
import javafx.scene.input.KeyCode;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;
/**
 * Gabe Castelli
 * 9/26/2016
 * Description: GUI for client-side using JavaFX
 */
public class ClientGUI extends Application {
    private Scene scene;
    private TextField targetAddress;
    private TextField fieldUsername;
    private TextField fieldPort;
    private VBox messages;
    private TextField input;
    private Button btnConnect;
    private String username = "Anonymous";
    private int port = 2000;
    private static ClientGUI instance;
    public static ClientGUI getInstance() {
        if (instance == null) {
            instance = new ClientGUI();
        }
        return instance;
    }
    @Override
    public void start(Stage stage) throws Exception {
        Parent root = FXMLLoader.load(getClass().getResource("ClientGUI.fxml"));
        scene = new Scene(root, 480, 360);
        stage.setTitle("MyMessenger");
        stage.setScene(scene);
        stage.setMinWidth(660);
        stage.setMinHeight(495);
        stage.setMaxWidth(990);
        stage.show();
        targetAddress = (TextField) scene.lookup("#targetAddress");
        fieldUsername = (TextField) scene.lookup("#fieldUsername");
        fieldPort = (TextField) scene.lookup("#fieldPort");
        messages = (VBox) scene.lookup("#messages");
        input = (TextField) scene.lookup("#input");
        btnConnect = (Button) scene.lookup("#btnConnect");
        input.setOnKeyPressed(event -> {
            if (event.getCode() == KeyCode.ENTER) {
                if (Client.getInstance().connected()) {
                    ClientController.getInstance().displayAndSend(input.getText());
                    input.setText("");
                } else {
                    display("Attention: Not connected to server");
                    input.setText("");
                }
            }
        });
        btnConnect.setOnMouseClicked(event -> {
            if (!Client.getInstance().connected()) {
                if (!targetAddress.getText().equals("")) {
                    if (!fieldUsername.getText().equals("")) {
                        username = fieldUsername.getText();
                    }
                    if (!fieldPort.getText().equals("")) {
                        port = Integer.valueOf(fieldPort.getText());
                    }
                    ClientController.getInstance().connect(targetAddress.getText(), username, port);
                    btnConnect.setText("Disconnect");
                } else {
                    display("Attention: Address required");
                }
            } else {
                ClientController.getInstance().disconnect();
                display("Attention: Disconnecting...");
                btnConnect.setText("Connect");
            }
        });
    }
    public void display(String message) {
        messages.getChildren().add(new Message(message));
    }
}

堆栈跟踪:

Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
    at client.ClientGUI.display(ClientGUI.java:96)
    at client.ClientController.display(ClientController.java:26)
    at client.Client.connect(Client.java:34)
    at client.ClientController.connect(ClientController.java:35)
    at client.ClientGUI.lambda$start$1(ClientGUI.java:81)
    at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
    at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
    at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
    at javafx.event.Event.fireEvent(Event.java:198)
    at javafx.scene.Scene$ClickGenerator.postProcess(Scene.java:3470)
    at javafx.scene.Scene$ClickGenerator.access$8100(Scene.java:3398)
    at javafx.scene.Scene$MouseHandler.process(Scene.java:3766)
    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$MouseEventNotification.run(GlassViewEventHandler.java:352)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:275)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$355(GlassViewEventHandler.java:388)
    at com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:389)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:387)
    at com.sun.glass.ui.View.handleMouseEvent(View.java:555)
    at com.sun.glass.ui.View.notifyMouse(View.java:937)

空指针异常正在发生，因为您从ClientGUI.getInstance()获得的ClientGUI实例不是作为JavaFX启动过程的一部分创建的实例，并且在其上调用start()方法。因为messages是在start()中初始化的，所以从ClientGUI.getInstance()获得的实例没有初始化messages。所以当你调用ClientGUI.getInstance().display(message)时，它调用messages.getChildren()...: messages在ClientGUI实例中是空的，你会得到一个空指针异常。

这里的潜在问题是整体结构。您确实不能创建这些单例—特别是Application子类—因为它们是由框架实例化的。ClientGUI是由JavaFX启动进程实例化的，而使用FXMLLoader的方式，ClientController是由FXMLLoader.load()实例化的(所以ClientController.getInstance()可能也没有给你所希望的实例)。

这不是很清楚，当然没有意义，为什么你试图使这些类的单例。我的猜测是，你正试图绕过你现有的结构，这是完全不标准的。你所定义的方法几乎都在类中，这些类不应该对这些方法实现的功能负责。

特别地:Application子类负责应用程序生命周期。它应该有一个start方法，它只是加载初始视图(FMXL文件)，把它放在一个窗口中，然后显示它。如果您有/需要其他服务，它也可能启动它们。如果需要，它可以选择覆盖init()和stop()。

控制器类负责更新视图。这是您应该引用FXML文件中定义的UI控件的唯一地方，也是您应该修改显示的唯一地方。

模型类(Client)应该完全不知道视图或控制器。它应该只表示正在呈现的状态或数据。这是控制器的责任(实际上是一个呈现者，在其他一些体系结构中这可以由视图完成)根据模型所表示的数据来更新视图。

因此，您可能希望按照以下方式重构应用程序。这取决于您可能需要的其他东西，但这至少应该让您进入一个可行的轨道:

ClientGUI: 

public class ClientGUI extends Application {
    @Override
    public void start(Stage stage) throws Exception {
        Parent root = FXMLLoader.load(getClass().getResource("ClientGUI.fxml"));
        scene = new Scene(root, 480, 360);
        stage.setTitle("MyMessenger");
        stage.setScene(scene);
        stage.setMinWidth(660);
        stage.setMinHeight(495);
        stage.setMaxWidth(990);
        stage.show();
    }
}

ClientController: 

public class ClientController implements Controller {
    @FXML
    private TextField targetAddress;
    @FXML
    private TextField fieldUsername;
    @FXML
    private TextField fieldPort;
    @FXML
    private VBox messages;
    @FXML
    private TextField input;
    @FXML
    private Button btnConnect;
    private String username = "Anonymous";
    private int port = 2000;
    private Client client ;
    public void initialize() {
        client = new Client();
    }
    @FXML
    private void connect() {
        if (!client.connected()) {
            if (!targetAddress.getText().equals("")) {
                if (!fieldUsername.getText().equals("")) {
                    username = fieldUsername.getText();
                }
                if (!fieldPort.getText().equals("")) {
                    port = Integer.valueOf(fieldPort.getText());
                }
                try {
                    client.connect(targetAddress.getText(), username, port);
                    btnConnect.setText("Disconnect");
                    display("Attention: Successfully connected to server");
                } catch (IOException exc) {
                    display("Attention: Unable to connect to server");
                }
            } else {
                display("Attention: Address required");
            }
        } else {
            client.disconnect();
            display("Attention: Disconnecting...");
            btnConnect.setText("Connect");
        }
    }
    @FXML
    private void message() {
        if (client.connected()) {
            displayAndSend(input.getText());
            input.setText("");
        } else {
            display("Attention: Not connected to server");
            input.setText("");
        }
    }
    public void display(String message) {
        messages.getChildren().add(new Message(message));
    }
    public void displayAndSend(String message) {
        display(message);
        client.sendMessage(message);
    }
}

然后

// Note: why on earth is this Serializable? What state are you intending to serialize???
public class Client implements Serializable {
    private Socket clientSocket;
    private BufferedReader in;
    private PrintWriter out;
    private ObjectOutputStream objectOutputStream;
    private boolean connected = false;
    private static final long serialVersionUID = 0L;

    public void connect(String targetAddress, String username_, int port_) throws IOException {
        clientSocket = new Socket(targetAddress, port_);
        clientSocket.setReuseAddress(true); // Allows same port to be used successively without cool-down
        in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
        out = new PrintWriter(clientSocket.getOutputStream(), true);
        objectOutputStream = new ObjectOutputStream(clientSocket.getOutputStream());
        objectOutputStream.writeObject(username_);
        connected = true;
    }
    public void disconnect() {
        try {
            if (in != null) {
                in.close();
            }
            if (out != null) {
                out.close();
            }
            if (objectOutputStream != null) {
                objectOutputStream.close();
            }
            if (clientSocket != null) {
                clientSocket.close();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
        connected = false;
    }
    public boolean connected() {
        return connected;
    }

    public void sendMessage(String message) {
        out.println(message);
    }
}

要使控制器工作，需要根元素定义一个fx:controller="client.ClientController"属性，ClientGUI.fxml中的控件定义与控制器类中的字段名称匹配的fx:id属性，并指定事件处理程序方法，例如:

<!-- ... -->
<TextField fx:id="targetAddress" />
<TextField fx:id="fieldUserName" />
<TextField fx:id="fieldPort" />
<VBox fx:id="messages" />
<TextField fx:id="input" onAction="#message" />
<Button text="Connect" fx:id="btnConnect" onAction="#connect" />
<!-- ... -->

(注意，当按enter键时，文本字段会触发一个ActionEvent，因此不需要打乱按下键的处理程序等)

你可能需要稍微修改一下，以使应用程序的其余功能正常工作，但只要你坚持基本原则:只有控制器应该修改UI;Client类应该不了解演示细节等，那么您将走上正确的道路。如果你让任何 static，当然，如果你试图做任何这些单例，你做错了。

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