How to Test System.out and System.err Output in JUnit?

Testing System Output in JUnit

In legacy systems, applications may print error messages or debug information to the standard output instead of returning errors explicitly. Testing such output can be challenging with JUnit.

Problem:

Unit tests need a way to assert the content of the console output to catch cases where System.out.println() is used instead of proper error handling.

Solution:

Using ByteArrayOutputStream and System.setXXX() allows us to redirect the system output and capture it for testing.

Implementation:

private final ByteArrayOutputStream outContent = new ByteArrayOutputStream();
private final ByteArrayOutputStream errContent = new ByteArrayOutputStream();
private final PrintStream originalOut = System.out;
private final PrintStream originalErr = System.err;

@Before
public void setUpStreams() {
    System.setOut(new PrintStream(outContent));
    System.setErr(new PrintStream(errContent));
}

@After
public void restoreStreams() {
    System.setOut(originalOut);
    System.setErr(originalErr);
}

Test Cases:

@Test
public void out() {
    System.out.print("hello");
    assertEquals("hello", outContent.toString());
}

@Test
public void err() {
    System.err.print("hello again");
    assertEquals("hello again", errContent.toString());
}

Note:

Prior versions of this answer called System.setOut(null) after the tests. This is the cause of NullPointerExceptions commenters referred to.

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