How to Test Console Output in JUnit?

Testing Console Output in JUnit

When testing legacy applications with poor design, you may encounter code that writes error messages to standard output. To assert such console output in JUnit, you need a way to redirect and capture the output.

Redirecting Console Output

The Java system can be configured to send console output to specified streams. We can leverage this mechanism to capture and test the output.

Capturing Output with ByteArrayOutputStream

A ByteArrayOutputStream can be used to capture the console output. To do this, we replace the standard output streams with new streams that write to the ByteArrayOutputStream.

Example:

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);
}

Testing Console Output

Once the output is captured, we can assert its content using JUnit's assertEquals() method.

Example 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());
}

Conclusion

By redirecting console output to ByteArrayOutputStreams and using System.setXXX() to configure the system streams, you can effectively test and assert console output in your JUnit tests.

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