In Java, how can I redirect System.out to null then back to stdout again?

Question

I've tried to temporarily redirect System.out to /dev/null using the following code but it doesn't work.

System.out.println("this should go to stdout");

PrintStream original = System.out;
System.setOut(new PrintStream(new FileOutputStream("/dev/null")));
System.out.println("this should go to /dev/null");

System.setOut(original);
System.out.println("this should go to stdout"); // This is not getting printed!!!

Anyone have any ideas?

I see both lines fine on my system. I'm using Java 6 update 22. — Peter Lawrey, Jan 25 '11 at 21:29
BTW: Beware of manipulating System.out like this without broader synchronization to prevent concurrent interactions by multiple threads. — Lawrence Dol, Jan 25 '11 at 21:50
Since JDK11, a nullOutputStream is builtin. See [my answer below](https://stackoverflow.com/a/58183767/2448440) — Qw3ry, Oct 01 '19 at 11:22

score 53 · Answer 1 · answered Jan 25 '11 at 21:44

53

Man, this is not so good, because Java is cross-platform and '/dev/null' is Unix specific (apparently there is an alternative on Windows, read the comments). So your best option is to create a custom OutputStream to disable output.

try {
    System.out.println("this should go to stdout");

    PrintStream original = System.out;
    System.setOut(new PrintStream(new OutputStream() {
                public void write(int b) {
                    //DO NOTHING
                }
            }));
    System.out.println("this should go to /dev/null, but it doesn't because it's not supported on other platforms");

    System.setOut(original);
    System.out.println("this should go to stdout");
}
catch (Exception e) {
    e.printStackTrace();
}

answered Jan 25 '11 at 21:44

Ilya Saunkin

18,934
9
36
50

4

There is in alternative on windows, the NUL device; IIRC it's "NUL:" as a "filename". – Lawrence Dol Jan 25 '11 at 21:46
oh wow, this is really first time i hear about it! Gonna edit the post. Thank you! – Ilya Saunkin Jan 25 '11 at 21:46
4

But +1, because this is the right platform-independent answer. I would, however, also override write(byte[],int,int), for efficiency. – Lawrence Dol Jan 25 '11 at 21:47
5

I will point out that in most cases you should probably restore the original OutputStream in a finally block. – javacoder Mar 07 '13 at 20:50
2

There is new NullOutputStream in Apache Commons IO. – volkovs Oct 23 '14 at 06:18
Since Java 11 there is a static method `OutputStream.nullOutputStream()`. – Peter Verhas Oct 21 '20 at 04:15

score 20 · Answer 2 · answered Sep 14 '13 at 16:44

You can use the class NullPrintStream below as:

PrintStream original = System.out;
System.setOut(new NullPrintStream());
System.out.println("Message not shown.");
System.setOut(original);

And the class NullPrintStream is...

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.io.PrintStream;

public class NullPrintStream extends PrintStream {

  public NullPrintStream() {
    super(new NullByteArrayOutputStream());
  }

  private static class NullByteArrayOutputStream extends ByteArrayOutputStream {

    @Override
    public void write(int b) {
      // do nothing
    }

    @Override
    public void write(byte[] b, int off, int len) {
      // do nothing
    }

    @Override
    public void writeTo(OutputStream out) throws IOException {
      // do nothing
    }

  }

}

Where can I get the NullPrintStream? Which library? Or do I need to create the class myself? — Daniil Shevelev, Nov 07 '13 at 15:14
I really needed this for my code... It is definitely a bit of a life saver for silencing stdout. — Claudia, Jan 17 '14 at 01:59

score 10 · Answer 3 · edited Jul 14 '23 at 20:17

10

Since JDK 11 there is OutputStream.nullOutputStream(). It does exactly what you are looking for:

System.setOut(new PrintStream(OutputStream.nullOutputStream()));

edited Jul 14 '23 at 20:17

Ellen Spertus

6,576
9
50
101

answered Oct 01 '19 at 11:21

Qw3ry

1,319
15
31

score 3 · Answer 4 · answered Oct 09 '13 at 23:42

3

Old question, I know, but would this small line do the trick on Windows?

System.setOut(new PrintStream(new File("NUL")));

Much less code and looks pretty direct to me.

answered Oct 09 '13 at 23:42

E.S.

2,733
6
36
71

`System.setOut(new PrintStream("NUL"));` works too. – AndrWeisR Sep 08 '22 at 05:48

score -4 · Answer 5 · answered Feb 02 '15 at 21:52

-4

I think System.setOut(null); should work too. At least it worked for me.

answered Feb 02 '15 at 21:52

pavlini4

1

1

Any subsequent calls to System.out.anything will cause a NullPointerException to be thrown. If that worked for you, then you must be catching the exception somewhere. – John Oct 19 '15 at 17:43
While this might be what you want to do (make System.out fail fast and noisy), it is certainly not what OP asks, and it does not work with his example code either – Qw3ry Sep 30 '19 at 13:26

In Java, how can I redirect System.out to null then back to stdout again?

5 Answers5

Linked

Related