JUnit: How to simulate System.in testing?

I have a Java command-line program. I would like to create JUnit test case to be able to simulate System.in. Because when my program runs it will get into the while loop and waits for input from users. How do I simulate that in JUnit?

Thanks

It is technically possible to switch System.in, but in general, it would be more robust not to call it directly in your code, but add a layer of indirection so the input source is controlled from one point in your application. Exactly how you do that is an implementation detail - the suggestions of dependency injection are fine, but you don't necessarily need to introduce 3rd party frameworks; you could pass round an I/O context from the calling code, for example.

How to switch System.in:

String data = "Hello, World!\r\n";
InputStream stdin = System.in;
try {
  System.setIn(new ByteArrayInputStream(data.getBytes()));
  Scanner scanner = new Scanner(System.in);
  System.out.println(scanner.nextLine());
} finally {
  System.setIn(stdin);
}

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:

setUpOutput(), because of the @Before annotation
provideInput(String data), called from testCase1()
getOutput(), called from testCase1()
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.

There are a few ways to approach this. The most complete way is to pass in an InputStream while running the class under test which is a fake InputStream which passes simulated data to your class. You can look at a dependency injection framework (such as Google Guice) if you need to do this a lot in your code, but the simple way is:

 public class MyClass {
     private InputStream systemIn;

     public MyClass() {
         this(System.in);
     }

     public MyClass(InputStream in) {
         systemIn = in;
     }
 }

Under test you would call the constructor that takes the input stream. You cloud even make that constructor package private and put the test in the same package, so that other code would not generally consider using it.

Try to refactor your code to use dependency injection. Instead of having your a method that uses System.in directly, have the method accept an InputStream as an argument. Then in your junit test, you'll be able to pass a test InputStream implementation in place of System.in.

JUnit: How to simulate System.in testing?

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