In Java, how do I use multithreading in TextArea? Do I need to synchronize my threads?

Question

I am rather inexperienced with Java and multi-threading so maybe you guys can help. I am having trouble printing out a series of numbers and letters in a TextArea box when I multi-thread. This is my code:

public class MultiThread extends Application {
    static TextArea outputArea = new TextArea();
@Override
    public void start(Stage primaryStage) throws Exception {

        outputArea.setWrapText(true);
        Runnable printA = new PrintChar('a', 100);
        Runnable printB = new PrintChar('b', 100);
        Runnable print100 = new PrintNum(100);

        // Create threads
        Thread thread1 = new Thread(printA);
        Thread thread2 = new Thread(printB);
        Thread thread3 = new Thread(print100);

        thread1.start();
        thread2.start();
        thread3.start();

        Scene scene = new Scene(outputArea, 250, 130);
        primaryStage.setTitle("Concurrent Output");
        primaryStage.setScene(scene);
        primaryStage.show();}


    public static void main(String[] args) {
        launch(args);
    }
}
class PrintChar extends MultiThread implements Runnable {
    private char charToPrint; // The character to print
    private int times; // The times to repeat

    public PrintChar(char c, int t) {
        charToPrint = c;
        times = t;
    }

    @Override
    public void run() {
        for (int i = 0; i < times; i++) {
            outputArea.appendText(charToPrint + "");
        }
    }
}

class PrintNum extends MultiThread implements Runnable {
    private int lastNum;
    private char charToPrint;

    public PrintNum(int n) {
        lastNum = n;
    }

    @Override
    public void run() {
        for (int i = 1; i <= lastNum; i++) {
            outputArea.appendText(i + "");
        }
    }
}

When I run the code, the threads do not usually print out all of what they are supposed to. For example, 'print100' does not always print to number 100 like it's supposed to. I also get random results and I never get the same output nor errors each time.

Exception in thread "Thread-6" Exception in thread "Thread-4" java.lang.IndexOutOfBoundsException
    at javafx.scene.control.TextInputControl.getText(TextInputControl.java:451)
    at javafx.scene.control.TextInputControl.updateContent(TextInputControl.java:564)
    at javafx.scene.control.TextInputControl.replaceText(TextInputControl.java:548)
    at javafx.scene.control.TextInputControl.insertText(TextInputControl.java:473)
    at javafx.scene.control.TextInputControl.appendText(TextInputControl.java:463)
    at threader.PrintChar.run(Threader.java:53)
    at java.lang.Thread.run(Thread.java:748)
java.lang.IndexOutOfBoundsException
    at javafx.scene.control.TextInputControl.getText(TextInputControl.java:451)
    at javafx.scene.control.TextInputControl.updateContent(TextInputControl.java:555)
    at javafx.scene.control.TextInputControl.replaceText(TextInputControl.java:548)
    at javafx.scene.control.TextInputControl.insertText(TextInputControl.java:473)
    at javafx.scene.control.TextInputControl.appendText(TextInputControl.java:463)
    at threader.PrintNum.run(Threader.java:71)
    at java.lang.Thread.run(Thread.java:748)

I am almost positive it is a problem with synchronizing the multiple threads in the code, but I can't seem to figure out how to do so for this specific code.

score 2 · Answer 1 · answered Nov 20 '19 at 08:52

Create a static function in MultiThread class as follows:

public static synchronized setText(String str) {
     outputArea.appendText(str);
}

and replace the line outputArea.appendText(...); with MultiThread.setText(...).

Problem: You are currently setting the text on outputArea from three threads and at one time they all are trying to append a text on the outputArea. So, the index within the outputArea to append the text is overlapping with another threads.

Solution: Make sure only one thread attempts to append text on outputArea.

score 1 · Accepted Answer · answered Nov 20 '19 at 07:53

The TextArea implementation is obviously not thread safe, so you should make sure that the current thread is not interupted while calling methods on your TextArea object. You can do this for example by synchronizing on the TextArea object itself:

synchronized (outputArea) {
    outputArea.appendText(charToPrint + "");
}

In Java, how do I use multithreading in TextArea? Do I need to synchronize my threads?

2 Answers2

Linked