JUnit: How to simulate System.in testing?

Based on @McDowell's answer and another answer that shows how to test System.out, I would like to share my solution to give an input to a program and test its output.

As a reference, I use JUnit 4.12.

Let's say we have this program that simply replicates input to output:

import java.util.Scanner;

public class SimpleProgram {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print(scanner.next());
        scanner.close();
    }
}

To test it, we can use the following class:

import static org.junit.Assert.*;

import java.io.*;

import org.junit.*;

public class SimpleProgramTest {
    private final InputStream systemIn = System.in;
    private final PrintStream systemOut = System.out;

    private ByteArrayInputStream testIn;
    private ByteArrayOutputStream testOut;

    @Before
    public void setUpOutput() {
        testOut = new ByteArrayOutputStream();
        System.setOut(new PrintStream(testOut));
    }

    private void provideInput(String data) {
        testIn = new ByteArrayInputStream(data.getBytes());
        System.setIn(testIn);
    }

    private String getOutput() {
        return testOut.toString();
    }

    @After
    public void restoreSystemInputOutput() {
        System.setIn(systemIn);
        System.setOut(systemOut);
    }

    @Test
    public void testCase1() {
        final String testString = "Hello!";
        provideInput(testString);

        SimpleProgram.main(new String[0]);

        assertEquals(testString, getOutput());
    }
}

I won't explain much, because I believe the code is readable and I cited my sources.

When JUnit runs testCase1(), it is going to call the helper methods in the order they appear:

1. setUpOutput(), because of the @Before annotation
2. provideInput(String data), called from testCase1()
3. getOutput(), called from testCase1()
4. restoreSystemInputOutput(), because of the @After annotation

I didn't test System.err because I didn't need it, but it should be easy to implement, similar to testing System.out.

@Stefan Birkner, Thanks!

1. Modify Pom.xml

Ref:
https://stackoverflow.com/a/66127606/8317677
https://github.com/stefanbirkner/system-lambda/blob/master/pom.xml
https://github.com/stefanbirkner/system-lambda/blob/master/src/test/java/com/github/stefanbirkner/systemlambda/WithTextFromSystemInTest.java

  <properties>
      <system-lambda.version>1.2.1</system-lambda.version>
  </properties>
  <dependencies>
    <dependency>
      <groupId>com.github.stefanbirkner</groupId>
      <artifactId>system-lambda</artifactId>
      <version>${system-lambda.version}</version>
      <scope>test</scope>
    </dependency>
  </dependencies>

2. Add function code

import java.io.BufferedReader;
import java.io.InputStreamReader;

public class SimpleProgram003 {
    public static void main(String[] args) {
        try{
            String c;
            BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
            do{
                c = in.readLine();
                System.out.println(c);
                String d = c;
            }while(!c.equals("q"));
        }catch(Exception e){
            System.out.println("catch Exception");
        }
    }
}

3. Add test code

import static com.github.stefanbirkner.systemlambda.SystemLambda.*;
import static org.junit.Assert.*;

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.InputStream;
import java.io.PrintStream;

import org.junit.After;
import org.junit.Before;
import org.junit.Test;

/**
 * Unit test for simple App. JUnit 4.x.
 */
public class SimpleProgram003Test {
    private final InputStream systemIn = System.in;
    private final PrintStream systemOut = System.out;

    private ByteArrayInputStream testIn;
    private ByteArrayOutputStream testOut;

    @Before
    public void setUpOutput() {
        testOut = new ByteArrayOutputStream();
        System.setOut(new PrintStream(testOut));
    }

    private void setInput(String data) {
        testIn = new ByteArrayInputStream(data.getBytes());
        System.setIn(testIn);
    }

    private String getOutput() {
        return testOut.toString();
    }

    @After
    public void restoreSystemInputOutput() {
        System.setIn(systemIn);
        System.setOut(systemOut);
    }

    @Test
    public void testCase1() {
        final String testString = "Hello 1\nq\n";
        setInput(testString);

        SimpleProgram003.main(new String[0]);
        // String a = getOutput();
        assertEquals("Hello 1\r\nq\r\n", getOutput());
    }

    @Test // Multiply inputs
    public void testCase2() throws Exception {
        withTextFromSystemIn(
            "Input1",
            "Input2",
            "q",
            "Input3"
        ).execute(() -> {
            SimpleProgram003.main(new String[0]);
            // String a = getOutput();
            assertEquals("Input1\r\nInput2\r\nq\r\n", getOutput());
        });
    }
}