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POJava client / server thread null pointer exception when quickly communicating messages
 

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  1. POJava client / server thread null pointer exception when quickly communicating messages
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    <p>I'm using JavaFX to create a simple messenger client. The classes in question are:</p> <p>MainMenuController.class: this class handles JavaFX ui logic such as printing chat messages to text area</p> <p>Server.class: controls server socket and in/out logic</p> <p>Client.class: controls client socket and in/out logic</p> <p>I'm able to run the application perfectly on local host, except my program throws a nullpointerexception when I try to send messages too quickly. I'll past the relevant code, and then I'll explain what I've tried to solve this bug.</p> <p>MainMenuController.java (exception traced to this function)</p> <pre><code>public static void printMessage(String sender, String msg) { //msgHistory is a textArea msgHistory.appendText(sender + ": " +msg + "\n"); } </code></pre> <p>Server.java</p> <pre><code>try { servSocket = new ServerSocket(serverPort); MainMenuController.printMessage("Server running on port: " + serverPort + ". . ."); // block till we accept connection request clientSocket = servSocket.accept(); out = new PrintWriter(clientSocket.getOutputStream(), true); in = new BufferedReader(new InputStreamReader( clientSocket.getInputStream())); // receive and set friend's alias friendName = in.readLine(); aliasSet = true; String input; // while connection is present, poll server socket input for new // messages while (true) { Thread.sleep(100); input = in.readLine(); if (input == null) { break; } //update status MainMenuController.printMessage(friendName, input); //Line traced to exception } } </code></pre> <p>Client.java</p> <pre><code>try { socket = new Socket (serverAddr, portNum); MainMenuController.printMessage("Connected!"); out = new PrintWriter(socket.getOutputStream(), true); in = new BufferedReader(new InputStreamReader(socket.getInputStream())); //send server our alias out.println(myName); } catch (IOException e) { System.err.println("Accept failed."); } </code></pre> <p>I believe the exception is caused by the client and server trying to write to the textArea in MainMenuController at the same time. I've experimented with adding a thread.sleep(x) before the server prints to textArea. I've also implemented a public lock in the MainMenuController class. In each call to printMessage in both the server and client, I wait until the lock is free before calling the printMessage method (I set the lock before calling printMessage, then free it once the method has returned). I took this code out to make it more readable. Neither of these solutions have worked and I'm not sure where to look.</p> <p>the stack trace:</p> <pre><code>Exception in thread "Thread-4" java.lang.NullPointerException at com.sun.javafx.sg.prism.NGTextHelper$TextAttributes.computeLinePadding(NGTextHelper.java:405) at com.sun.javafx.sg.prism.NGTextHelper$TextAttributes.access$200(NGTextHelper.java:292) at com.sun.javafx.sg.prism.NGTextHelper.buildTextLines(NGTextHelper.java:2357) at com.sun.javafx.sg.prism.NGTextHelper.validateText(NGTextHelper.java:1847) at com.sun.javafx.sg.prism.NGTextHelper.getCaretShape(NGTextHelper.java:1435) at javafx.scene.text.Text.getDecorationShapes(Text.java:1150) at javafx.scene.text.Text.impl_geomChanged(Text.java:757) at javafx.scene.text.Text$1.invalidated(Text.java:214) at javafx.beans.property.StringPropertyBase.markInvalid(StringPropertyBase.java:127) at javafx.beans.property.StringPropertyBase.set(StringPropertyBase.java:161) at javafx.beans.property.StringPropertyBase.set(StringPropertyBase.java:67) at javafx.scene.text.Text.setText(Text.java:188) at com.sun.javafx.scene.control.skin.TextAreaSkin$17.invalidated(TextAreaSkin.java:610) at com.sun.javafx.binding.ExpressionHelper$Generic.fireValueChangedEvent(ExpressionHelper.java:359) at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:100) at javafx.scene.control.TextInputControl$TextProperty.fireValueChangedEvent(TextInputControl.java:1034) at javafx.scene.control.TextInputControl$TextProperty.markInvalid(TextInputControl.java:1038) at javafx.scene.control.TextInputControl$TextProperty.invalidate(TextInputControl.java:978) at javafx.scene.control.TextInputControl$TextProperty.access$200(TextInputControl.java:950) at javafx.scene.control.TextInputControl$1.invalidated(TextInputControl.java:119) at com.sun.javafx.binding.ExpressionHelper$SingleInvalidation.fireValueChangedEvent(ExpressionHelper.java:155) at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:100) at javafx.scene.control.TextArea$TextAreaContent.insert(TextArea.java:196) at javafx.scene.control.TextInputControl.replaceText(TextInputControl.java:373) at javafx.scene.control.TextInputControl.insertText(TextInputControl.java:308) at javafx.scene.control.TextInputControl.appendText(TextInputControl.java:298) at application.MainMenuController.printMessage(MainMenuController.java:96) at application.Server.run(Server.java:79) at java.lang.Thread.run(Unknown Source) </code></pre> <p>Edit: This error only occurs with textArea.appendText(string). No exception is thrown with textArea.setText(string).</p>
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