JavaFX TextArea.append() is causing java.lang.NullPointerException

Question

I am reading a serial port and writing that to a text area as a scrolling value. I have created the SerialPortReader class which works well. My main code simply calls "updateTextArea()" in order to tell the class where the textarea is. This worked will for a while, but when I start throwing more data into the serial port - say ~100 bytes - Java sens a NullPointerException. If I comment out the "updateTextArea()" call (~line 13), then there is no exception. When I uncomment "updateTextArea", the program works perfectly for about a minute. I suspect that the textarea buffer is filling up, but I don't know how to increase the buffer size OR how to make the new text push the old text out of the buffer.

Printing to console works just fine for - presumably - forever. I have only run it for ~10min, but it hasn't so much as hiccuped.

class SerialPortReader implements SerialPortEventListener {
    byte[] serialData;
    TextArea textArea;

    @Override
    public void serialEvent(SerialPortEvent event) {
        try{
            serialData = serialPort.readBytes();
            for(byte dataReceived: serialData){
                System.out.println("Byte Received: " + dataReceived);
            }

            updateTextArea();
        } catch (SerialPortException ex){
            System.out.println(ex);
        }
    }

    public void initiateTextAreaUpdate(TextArea ta){
        textArea = ta;
    }

    public void updateTextArea(){
        if(textArea != null){
            for(int i = 0; i < serialData.length; i++){
                textArea.appendText(serialData[i] + "\n");
            }
        }
    }
}

The exception:

 Exception in thread "EventThread COM5" 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$TextAreaContent.insert(TextArea.java:196)
    at javafx.scene.control.TextInputControl.replaceText(TextInputControl.java:373)
    at javafx.scene.control.TextInputControl.insertText(TextInputControl.java:308)
    at javafx.scene.control.TextInputControl.appendText(TextInputControl.java:298)
    at mcsi.McsiGui$SerialPortReader.updateTextArea(McsiGui.java:489)
    at mcsi.McsiGui$SerialPortReader.serialEvent(McsiGui.java:476)
    at jssc.SerialPort$EventThread.run(SerialPort.java:1096)

Thank you for your insight!

j

Answer 1

I'm not sure if this is causing the null pointer exception, but you need to update the text area on the FX Application Thread. You should replace

updateTextArea();

with

Platform.runLater(() -> updateTextArea());

or, if you are still using Java 7,

Platform.runLater(new Runnable() {
    @Override
    public void run() {
        updateTextArea();
    }
});