Is there a way to automate user input with JLine's LineReader?

Question

When handling user input through Scanner(System.in), you can use System.setIn() to automate user input.

Like this:

package edu.ntnu.idi.idat;

import java.io.ByteArrayInputStream;
import java.io.InputStream;
import java.util.Scanner;

public final class TestCode {
    private TestCode() { }
    /**
     * Test code.
     * @param args
     */
    public static void main(
        final String[] args
    ) {
        InputStream input = System.in;
        String testInputString = "test" + System.lineSeparator() + "test2";
        ByteArrayInputStream testInput
            = new ByteArrayInputStream(testInputString.getBytes());

        System.setIn(testInput);

        Scanner inputScanner = new Scanner(System.in);

        System.out.println(inputScanner.nextLine());
        System.out.println(inputScanner.nextLine());

        inputScanner.close()
    }
}

But when trying the same method for JLine's LineReader, it doesn't work:

package edu.ntnu.idi.idat;

import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStream;

import org.jline.reader.LineReader;
import org.jline.reader.LineReaderBuilder;
import org.jline.terminal.Terminal;
import org.jline.terminal.TerminalBuilder;

public final class TestCode {
    private TestCode() { }
    /**
     * Test code.
     * @param args
     * @throws IOException
     */
    public static void main(
        final String[] args
    ) throws IOException {
        InputStream input = System.in;
        String testInputString = "test" + System.lineSeparator() + "test2";
        ByteArrayInputStream testInput
            = new ByteArrayInputStream(testInputString.getBytes());

        System.setIn(testInput);

        Terminal terminal = TerminalBuilder.builder().build();
        LineReader lineReader = LineReaderBuilder
            .builder()
            .terminal(terminal)
            .build();

        System.out.println(lineReader.readLine());
        System.out.println(lineReader.readLine());

        terminal.close()
    }
}

I believe this is because inputScanner uses System.in (as can be seen when constructing it), while LineReader doesn't seem to.

My question is: is there a way to automate user input for LineReader as well?

Emre Zorlu · Accepted Answer · 2025-11-29 22:04:54Z

0

you can use DumbTerminal seems like its designed for non-interactive use.

public final class TestCode {
    private TestCode() { }
    
    public static void main(String[] args) throws IOException {
        String testInputString = "test" + System.lineSeparator() + "test2";
        ByteArrayInputStream input = 
            new ByteArrayInputStream(testInputString.getBytes(StandardCharsets.UTF_8));
        ByteArrayOutputStream output = new ByteArrayOutputStream();

        // Use DumbTerminal for automated input
        Terminal terminal = new DumbTerminal(input, output);
        
        LineReader lineReader = LineReaderBuilder.builder()
                .terminal(terminal)
                .build();

        System.out.println("Read: " + lineReader.readLine());
        System.out.println("Read: " + lineReader.readLine());

        terminal.close();
    }
}

or Piped input & output stream:

    public static void main(String[] args) throws IOException {
        PipedInputStream in = new PipedInputStream();
        PipedOutputStream out = new PipedOutputStream(in);
        PrintWriter writer = new PrintWriter(out, true);

        // Write test input to the pipe
        writer.println("test");
        writer.println("test2");

        Terminal terminal = TerminalBuilder.builder()
                .streams(in, System.out)
                .build();

        LineReader lineReader = LineReaderBuilder.builder()
                .terminal(terminal)
                .build();

        System.out.println("Read: " + lineReader.readLine());
        System.out.println("Read: " + lineReader.readLine());

        writer.close();
        terminal.close();
    }

answered 14 hours ago

Emre Zorlu

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1 Comment

Lucy 5 hours ago

Thank you! Although the first seems to give an end of file exception: ``` Exception in thread "main" org.jline.reader.EndOfFileException at org.jline.reader.impl.LineReaderImpl.readLine(LineReaderImpl.java:691) at org.jline.reader.impl.LineReaderImpl.readLine(LineReaderImpl.java:489) at edu.ntnu.idi.idat.TestCode.main(TestCode.java:35) ```

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