使用（JavaFX）调用方法时出现NullPointerException

Question

当我在LoginViewController类中调用login方法时，我收到一个NullPointerException。我知道用户对象不是null，因为它使用toString打印出对象。我是否必须以某种方式在LoginView.fxml中注册Client和User类？我想要实现的是，当用户点击登录时，该方法应该实例化一个将User对象发送到服务器的新客户端。

异常

User: Username Password
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.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:8411)
    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.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$MouseEventNotification.run(GlassViewEventHandler.java:380)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:294)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$354(GlassViewEventHandler.java:416)
    at com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:389)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:415)
    at com.sun.glass.ui.View.handleMouseEvent(View.java:555)
    at com.sun.glass.ui.View.notifyMouse(View.java:937)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.lambda$null$148(WinApplication.java:191)
    at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at sun.reflect.misc.Trampoline.invoke(Unknown Source)
    at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at sun.reflect.misc.MethodUtil.invoke(Unknown Source)
    at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1771)
    ... 48 more
Caused by: java.lang.NullPointerException
    at dinnerTime.Client.sendToServer(Client.java:44)
    at dinnerTime.LoginViewController.login(LoginViewController.java:34)
    ... 58 more

Main.java

package dinnerTime;

import java.io.IOException;
import javafx.application.Application;
import javafx.fxml.FXML;
import javafx.fxml.FXMLLoader;
import javafx.scene.Scene;
import javafx.scene.layout.BorderPane;
import javafx.stage.Stage;

public class Main extends Application {
    private Stage primaryStage;
    private BorderPane mainLayout;

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

    @Override
    public void start(Stage primaryStage) throws Exception {
        this.primaryStage = primaryStage;
        showLoginView();
    }

    private void showLoginView() throws IOException {
        FXMLLoader loader = new FXMLLoader();
        loader.setLocation(Main.class.getResource("LoginView.fxml"));
        mainLayout = loader.load();
        Scene scene = new Scene(mainLayout, 540, 400);
        primaryStage.setScene(scene);
        primaryStage.setResizable(false);
        primaryStage.show();
    }
}

LoginViewController.java

package dinnerTime;

import java.io.IOException;
import java.net.URL;
import java.util.ResourceBundle;
import javafx.fxml.FXML;
import javafx.fxml.Initializable;
import javafx.scene.control.Button;
import javafx.scene.control.PasswordField;
import javafx.scene.control.TextField;

public class LoginViewController {
    @FXML
    private Main main;
    @FXML
    private TextField username;
    @FXML
    private PasswordField password;
    @FXML
    private Button login;
    @FXML
    private Button register;

    private Client client;

    private User user;

    @FXML
    public void login() throws IOException {
        client = new Client("127.0.0.1", 3250);
        client.start();
        user = new User(username.getText(), password.getText());
        System.out.println("User: " + user.toString());
        client.sendToServer(user);
    }
}

LoginView.fxml

<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.scene.control.Button?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.control.PasswordField?>
<?import javafx.scene.control.TextField?>
<?import javafx.scene.image.Image?>
<?import javafx.scene.image.ImageView?>
<?import javafx.scene.layout.AnchorPane?>
<?import javafx.scene.layout.BorderPane?>
<?import javafx.scene.text.Font?>

<BorderPane prefHeight="400.0" prefWidth="540.0" xmlns="http://javafx.com/javafx/8.0.102" xmlns:fx="http://javafx.com/fxml/1" fx:controller="dinnerTime.LoginViewController">
   <center>
      <AnchorPane prefHeight="400.0" prefWidth="540.0" BorderPane.alignment="CENTER">
         <children>
            <ImageView fitHeight="122.0" fitWidth="124.0" layoutX="238.0" layoutY="14.0" pickOnBounds="true" preserveRatio="true">
               <image>
                  <Image url="@../../images/dt-logo-original.png" />
               </image>
            </ImageView>
            <Label layoutX="14.0" layoutY="14.0" text="Dinner" textFill="#464646">
               <font>
                  <Font size="70.0" />
               </font>
            </Label>
            <Label layoutX="372.0" layoutY="14.0" text="Time" textFill="#464646">
               <font>
                  <Font size="70.0" />
               </font></Label>
            <Label layoutX="91.0" layoutY="155.0" text="Username" />
            <TextField layoutX="164.0" layoutY="151.0" prefHeight="25.0" prefWidth="216.0" />
            <Label layoutX="91.0" layoutY="213.0" text="Password" />
            <PasswordField layoutX="164.0" layoutY="209.0" prefHeight="25.0" prefWidth="216.0" />
            <Button fx:id="login" layoutX="162.0" layoutY="271.0" mnemonicParsing="false" onAction="#login" prefHeight="59.0" prefWidth="216.0" text="Login" />
            <Button layoutX="242.0" layoutY="352.0" mnemonicParsing="false" text="Register" />
         </children>
      </AnchorPane>
   </center>
</BorderPane>

Client.java

package dinnerTime;

import java.io.*;
import java.net.*;

public class Client extends Thread {
    private String ip;
    private int port;
    private Socket socket;
    private ObjectOutputStream oos;
    private ObjectInputStream ois;

    public Client(String ip, int port) {
        this.ip = ip;
        this.port = port;
    }

    public void run() {
        try {
            socket = new Socket(ip, port);
            oos = new ObjectOutputStream(socket.getOutputStream());
            oos.flush();
            ois = new ObjectInputStream(socket.getInputStream());

            while (true) {
                try {
                    Object obj = ois.readObject();

                    if (obj instanceof Recipe) {

                    }


                } catch (IOException | ClassNotFoundException e) {
                }
            }
        } catch (IOException e) {
        }
    }

    public void sendToServer(Object obj) {
        try {
            oos.writeObject(obj);
            oos.flush();
        } catch (IOException e) {
        }
    }
}

User.java

package dinnerTime;

import java.io.Serializable;

public class User implements Serializable {
    private String name, password;

    public User(String name, String password){
        this.name = name;
        this.password = password;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String toString() {
        return name + " " + password;
    }
}

Answer 1

客户的oos字段中有race condition。您在run()方法中初始化它，该方法在后台线程上执行，但您尝试在sendToServer()中访问它，该login()在控制器中从sendToServer()调用，并在FX上执行应用程序线程。

因为它们位于不同的线程上，所以不能保证首先执行哪个：发生的事情是oos在后​​台线程初始化oos.writeObject(...)之前执行，因此{{ 1}}抛出空指针异常。

要解决此问题，您需要确保在客户端完全连接之前不要调用sendToServer()。一种方法是在建立连接时提供某种类型的回调来执行：

public class Client extends Thread {
    private String ip;
    private int port;
    private Socket socket;
    private ObjectOutputStream oos;
    private ObjectInputStream ois;

    private Runnable onConnected ;

    public void setOnConnected(Runnable onConnected) {
        this.onConnected = onConnected ;
    }

    public Client(String ip, int port) {
        this.ip = ip;
        this.port = port;
    }

    public void run() {
        try {
            socket = new Socket(ip, port);
            oos = new ObjectOutputStream(socket.getOutputStream());
            oos.flush();

            if (onConnected != null) {
                onConnected.run();
            }

            ois = new ObjectInputStream(socket.getInputStream());

            while (true) {
                try {
                    Object obj = ois.readObject();

                    if (obj instanceof Recipe) {

                    }


                } catch (IOException | ClassNotFoundException e) {
                }
            }
        } catch (IOException e) {
        }
    }

    public void sendToServer(Object obj) {
        try {
            oos.writeObject(obj);
            oos.flush();
        } catch (IOException e) {
        }
    }
}

然后

@FXML
public void login() throws IOException {
    client = new Client("127.0.0.1", 3250);
    user = new User(username.getText(), password.getText());
    client.setOnConnected(() -> client.sendToServer(user));
    client.start();
}