NullPointerException when calling method with (JavaFX) - javafx

Question

When I call the login method in the LoginViewController class I receive a NullPointerException. I know that the user object is not null since it prints out the object with a toString. Do I have to register the Client and User class in the LoginView.fxml in some way? What Im trying to achieve is that when the user clicks on login the method should instantiate a new Client which sends the User object to the server.
Exception
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

You have a race condition on the client's oos field. You initialize it in the run() method, which is executed on the background thread, but you try to access it in sendToServer() which is called from login() in the controller, and executed on the FX Application Thread.
Because these are on different threads, there is no guarantee as to which will be executed first: what is happening is that sendToServer() is getting executed before the background thread initializes oos, and hence oos.writeObject(...) is throwing a null pointer exception.
To fix this, you need to ensure that you do not call sendToServer() until the client is fully connected. One approach is to provide a callback of some kind to execute when the connection is established:
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) {
}
}
}
and then
#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();
}