Why doesn't my Java program write String into file?

Question

I have two Threads and each Thread should write its name and an increasing number into a file - but it doesn't work.

If i use the System.out.println() method the threads are working perfectly only the writing into the file does fail. Any idea why?

This is how my Threads look like:

package ThreadTest;

import java.io.*;

public class Thread1 implements Runnable {

public void run() {
    int x = 0;
    while (true) {

        try {

            BufferedWriter p1 = new BufferedWriter(new FileWriter("C:\\Users\\User\\Desktop\\a1.txt"));
            x++;
            p1.write("Thread11: " + x);
            Thread.sleep(500);
            p1.close();
        } catch (Exception e) {
            System.out.println(e.getMessage());
        }

    }
  }

}

The main class looks like this:

package ThreadTest;

public class ThreadTestTest {

    public static void main(String args[]) {

        try {
            Thread1 t11 = new Thread1();
            Thread t1 = new Thread(t11);

            Thread2 t22 = new Thread2();
            Thread t2 = new Thread(t22);

            t2.start();
            t1.start();

            t1. join();
             t2. join();


        } catch (Exception e) {
            e.getMessage();
        }

    }

}

And you don't need to pass `t11` and `t22` as arguments to another threads objects. The instances itself are threads. — elias, Jan 28 '16 at 19:23

score 1 · Answer 1 · answered Jan 28 '16 at 19:17

1

After you close the file, you immedately open it again and thereby truncate it to zero length.

answered Jan 28 '16 at 19:17

Henry

42,982
7
68
84

Also a good idea to mention how they can [open the file for appending](http://stackoverflow.com/a/1625263/740553) – Mike 'Pomax' Kamermans Jan 28 '16 at 19:18

score 0 · Answer 2 · answered Jan 28 '16 at 20:07

As Henry already pointed out, you essentially need to make your BufferedWriter shared between the threads so you don't constantly overwrite the file again.

The following will do:

public class ThreadTest {
    public static void main(String[] args) {
        class ThreadSafeBufferedWriter extends BufferedWriter {
            public ThreadSafeBufferedWriter(Writer out) {
                super(out);     
            }

            @Override
            public synchronized void write(String str) throws IOException {
                super.write(str);
            }
        }

        try (BufferedWriter p1 = new ThreadSafeBufferedWriter(new FileWriter("/tmp/out.txt"))) {            
            Thread t1 = new Thread(new Payload("Thread1", p1));
            Thread t2 = new Thread(new Payload("Thread2", p1));         
            t2.start();
            t1.start();                
            t2.join();
            t1.join();                
        }
        catch (Exception e) {
            // do something smart
        }
    }
}

The actual payload comes in a Runnable which goes here:

class Payload implements Runnable {
    private String name;
    private Writer p1;

    public Payload(String _name, Writer _p1) {
        this.name = _name;
        this.p1 = _p1;
    }

    @Override
    public void run() {
        int x = 0;
        while (x < 10) { // you have to make sure these threads actually die at some point
            try {       
                x++;
                this.p1.write(this.name + ": " + x + "\n");
                Thread.sleep(500);      
            } catch (Exception e) {
                // do something smart
            }
        }
    }
}

score 0 · Answer 3 · answered Jan 28 '16 at 21:03

0

Thanks guys :)!

I just added a true to the file location:

PrintWriter p2 = new PrintWriter(new FileWriter("C:\\Users\\User\\Desktop\\a1.txt",true));

And now java appends the text instead of overwriting the old one.

answered Jan 28 '16 at 21:03

KlingKlong

1

Why doesn't my Java program write String into file?

3 Answers3