Writing to console and text file

Question

I found the code below from the internet, works but it doesn't write the printed console to omt.txt, it only writes the System.out.println statements after the second catch block.If you run the code once you will understand what I mean.All I want is to write what is on console to the "omt.txt" file that is all...

After some answers, I see that my question wasn't clear, sorry for that. I want to save console output to omt.txt text file. If on the console "Hello 123" is printed , it should be also in omt.txt file.In other words whatever on the console is printed should be simultaneously written on the om.txt file or can be after the console execution but should be 1-to-1 the same!

import java.io.File;
import java.io.FileOutputStream;
import java.io.PrintStream;

public class Wrt_file {

    public static void main(String[] args) {
        System.out.println("THIS is what I see on the console. but not on TEXT file"); 

          File f = new File("omt.txt");
          if(!f.exists())
          {
            try {
                       f.createNewFile();
                } catch (Exception e) {
                    e.printStackTrace();
                }
          }

        try {
                FileOutputStream fos = new FileOutputStream(f);
                PrintStream ps = new PrintStream(fos);
                System.setOut(ps);
        } catch (Exception e) {
            e.printStackTrace();
        }
        System.out.println("THIS is what I see on the text file, but not on CONSOLE");      

        for (int i=0; i<10; i++){

            System.out.println("Testing");  
        }

    }

}

Answer 1

Updated answer after learning that OP wants to duplicate streams

Since you want to write data in both streams try using TeeOutputStream from Apache Commons. Change your code in second try to

try {
    FileOutputStream fos = new FileOutputStream(f);
    Runtime.getRuntime().addShutdownHook(new Thread(() -> {
        try {
            fos.flush();
        }
        catch (Throwable t) {
            // Ignore
        }
    }, "Shutdown hook Thread flushing " + f));
    //we will want to print in standard "System.out" and in "file"
    TeeOutputStream myOut=new TeeOutputStream(System.out, fos);
    PrintStream ps = new PrintStream(myOut, true); //true - auto-flush after println
    System.setOut(ps);
} catch (Exception e) {
    e.printStackTrace();
}

Now results from System.out will also be placed in your file.

Answer 2

The reason is :

The java.lang.System.setOut() method reassigns the "standard" output stream.

so when you use System.out.println it will print only in the text file

So , if you want to print on the text file and on the console , Try this :

   

    FileOutputStream fos = new FileOutputStream(f);
    PrintStream ps = new PrintStream(fos);
    ps.println("THIS is what I see on the text file, but not on CONSOLE");
    System.out.println("THIS is what I see on the text file, but not on CONSOLE");

        for (int i = 0; i < 4; i++) {

            ps.println("Testing");
            System.out.println("Testing");
        } 

Answer 3

My solution to this problem is to simply define your own PrintStream which overrides the methods you are using:

import java.io.FileNotFoundException;
import java.io.PrintStream;

public class DualStream extends PrintStream {
    public PrintStream consoleOutput = null;
    public PrintStream fileOutput = null;

    public DualStream(final PrintStream consoleOutput, final PrintStream fileOutput) throws FileNotFoundException {
        super(fileOutput, true);
        this.consoleOutput = consoleOutput;
        this.fileOutput = fileOutput;
    }

    @Override
    public void println() {
        consoleOutput.println();
        super.println();
    }

    @Override
    public void println(final Object output) {
       consoleOutput.println(output);
        super.println(output);
    }

    @Override
    public void println(final String output) {
        consoleOutput.println(output);
        super.println(output);
    }

    @Override
    public PrintStream printf(final String output, final Object... variables) {
        consoleOutput.printf(output, variables);
        super.printf(output, variables);
        return this;
    }
}

Every method we don't override will default to writting the output to the file only. (To default to console you can switch 1 line in the Constructor or switch both printstreams position in the constructor call)

Now simply define 2 printstreams, one of which will be writting to your file, let's make it a bufferedoutputstream to ensure good performance aswell:

public static void outputFile(final String file) {
    PrintStream CombinedOutput = null;
    try {
        CombinedOutput = new DualStream(System.out, new PrintStream(new BufferedOutputStream(new FileOutputStream(file))));
    } catch (final FileNotFoundException e) {
        e.printStackTrace();
    }
    System.setOut(CombinedOutput);
}

Answer 4

In System.java, this is the declaration of the out property:

public final static PrintStream out

You'll see that it can only be one PrintSteam object at a time. So it's either the console or the file, but not both.

At this line, you have effectively re-channelled the destination:

System.setOut(ps);

So your output stops displaying on the console.