Java - Strange behaviour on PrintStream with custom OutputStream

Question

I am trying to write a program that redirects System.out to a JTextArea (it doesn't have to be a JTextArea), but when I call System.out.println("Test!") the output to the text area is like so:

\n
st!
\n

The code for my OutputStream:

package gui;

import java.awt.*;
import java.io.*;
import javax.swing.text.*;

public class LogOutputStream extends OutputStream
{
    public void write(final int b) throws IOException
    {
        EventQueue.invokeLater(new Runnable()
        {
            public void run()
            {
                write0(b);
            }
        });
    }

    public void write(final byte[] b, final int off, final int len)
    {
        EventQueue.invokeLater(new Runnable()
        {
            public void run()
            {
                write0(b, off, len);
            }
        });
    }

    public void write(final byte[] b)
    {
        EventQueue.invokeLater(new Runnable()
        {
            public void run()
            {
                write0(b);
            }
        });
    }

    private void write0(int b)
    {
        Document doc = FernflowerGUI.frame.textArea.getDocument();
        try
        {
            doc.insertString(doc.getLength(), String.valueOf((char)b), null);
        }
        catch(BadLocationException impossible)
        {

        }
    }

    private void write0(byte[] b, int off, int len)
    {
        Document doc = FernflowerGUI.frame.textArea.getDocument();
        try
        {
            doc.insertString(doc.getLength(), new String(b, off, len), null);
        }
        catch(BadLocationException impossible)
        {

        }
    }

    private void write0(byte[] b)
    {
        write0(b, 0, b.length);
    }
}

The code that creates the PrintStream:

PrintStream ps = new PrintStream(new LogOutputStream(), true);

Can anyone please tell me what on Earth is going on?

That code shows compilation errors here. For better help sooner, post an [SSCCE](http://sscce.org/). — Andrew Thompson, Apr 20 '13 at 08:15
Is the `\n` shown verbatim? What is the type returned by `getDocument()`, and how is it meant to handle linefeed characters? — Jon Skeet, Apr 20 '13 at 08:18
@Telthien write is called by the PrintStream, I am not responsible for that — condorcraft110, Apr 20 '13 at 08:26
@Jon The \n is shown as a linebreak, but I'm not familiar with StackOverflow formatting so I just put a \n. Looking through JTextArea's source, the authors themselves don't seem to know what getDocument() will return... — condorcraft110, Apr 20 '13 at 08:28
Lines 42 & 55 `Document doc = FernflowerGUI.frame.textArea.getDocument();` ... `package FernflowerGUI.frame does not exist` — Andrew Thompson, Apr 20 '13 at 08:28
@Telthien You mean write(b, off, len) calls write0 with 0 for off? I tried that and got the same result. — condorcraft110, Apr 20 '13 at 08:30
So it's actually blank line, then "st!" then another blank line? — Jon Skeet, Apr 20 '13 at 08:31
@Jon Yep! I cannot see any possible reason for this, though... — condorcraft110, Apr 20 '13 at 08:35

score 1 · Accepted Answer · answered Apr 20 '13 at 08:39

Your code isn't thread-safe, basically.

You're accepting a synchronous call accepting a byte array - and then you're using that byte array later, and assuming it will still have the same content. What if the caller to write() overwrites the data in the byte array immediately after the method returns? By the time you get to use it, you won't have the right data.

I would extract the String from the byte array in your write call, and then use that String in the call to write0.

(I'd also personally use a Writer rather than an OutputStream - fundamentally you want to deal with text data, not binary data.)

Java - Strange behaviour on PrintStream with custom OutputStream

1 Answers1