Redirect console output to string in java

Question

I have one function whose return type is VOID and it prints directly on console.

However I need that output in string so that I can work on it.

As I cant make any changes with function with return type is VOID so I have to redirect that output to string.

How can I redirect it in JAVA?

There are many questions regarding redirecting stdout to string but they redirect only input taken from user and not output of some function...

Answer 1

Here is a utility Class named ConsoleOutputCapturer. It allows the output to go to the existing console however behind the scene keeps capturing the output text. You can control what to capture with the start/stop methods. In other words call start to start capturing the console output and once you are done capturing you can call the stop method which returns a String value holding the console output for the time window between start-stop calls. This class is not thread-safe though.

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.io.PrintStream;
import java.util.Arrays;
import java.util.List;

public class ConsoleOutputCapturer {
    private ByteArrayOutputStream baos;
    private PrintStream previous;
    private boolean capturing;

    public void start() {
        if (capturing) {
            return;
        }

        capturing = true;
        previous = System.out;      
        baos = new ByteArrayOutputStream();

        OutputStream outputStreamCombiner = 
                new OutputStreamCombiner(Arrays.asList(previous, baos)); 
        PrintStream custom = new PrintStream(outputStreamCombiner);

        System.setOut(custom);
    }

    public String stop() {
        if (!capturing) {
            return "";
        }

        System.setOut(previous);

        String capturedValue = baos.toString();             

        baos = null;
        previous = null;
        capturing = false;

        return capturedValue;
    }

    private static class OutputStreamCombiner extends OutputStream {
        private List<OutputStream> outputStreams;

        public OutputStreamCombiner(List<OutputStream> outputStreams) {
            this.outputStreams = outputStreams;
        }

        public void write(int b) throws IOException {
            for (OutputStream os : outputStreams) {
                os.write(b);
            }
        }

        public void flush() throws IOException {
            for (OutputStream os : outputStreams) {
                os.flush();
            }
        }

        public void close() throws IOException {
            for (OutputStream os : outputStreams) {
                os.close();
            }
        }
    }
}

Answer 2

If the function is printing to System.out, you can capture that output by using the System.setOut method to change System.out to go to a PrintStream provided by you. If you create a PrintStream connected to a ByteArrayOutputStream, then you can capture the output as a String.

Example:

    // Create a stream to hold the output
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    PrintStream ps = new PrintStream(baos);
    // IMPORTANT: Save the old System.out!
    PrintStream old = System.out;
    // Tell Java to use your special stream
    System.setOut(ps);
    // Print some output: goes to your special stream
    System.out.println("Foofoofoo!");
    // Put things back
    System.out.flush();
    System.setOut(old);
    // Show what happened
    System.out.println("Here: " + baos.toString());

This program prints just one line:

    Here: Foofoofoo!