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1. Overview

When unit testing we may occasionally want to test the messages that we write to standard output via System.out.println().

Although we'd generally preferlogging framework over direct interaction with standard output, sometimes this isn't possible.

In this quick tutorial, we'll take a look at a couple of ways we can unit test System.out.println() using JUnit.

2. A Simple Print Method

Throughout this tutorial, the focus of our tests will be a simple method that writes to the standard output stream:

private void print(String output) {
    System.out.println(output);
}

A quick reminder that the out variable is a public static final PrintStream object which represents the standard output stream intended for system-wide usage.

3. Working With Core Java

Now let's see how we can write a unit test to check the content of what we send to the println method. However, before we write our actual unit test, we'll need to provide some initialization in our test:

private final PrintStream standardOut = System.out;
private final ByteArrayOutputStream outputStreamCaptor = new ByteArrayOutputStream();

@BeforeEach
public void setUp() {
    System.setOut(new PrintStream(outputStreamCaptor));
}

In the setUp method, we reassign the standard output stream to a new PrintStream with a ByteArrayOutputStream. As we're going to see this output stream is where the values will now be printed:

@Test
void givenSystemOutRedirection_whenInvokePrintln_thenOutputCaptorSuccess() {
    print("Hello Baeldung Readers!!");
        
    Assert.assertEquals("Hello Baeldung Readers!!", outputStreamCaptor.toString()
      .trim());
}

After we call the print method with the chosen text, we can then verify that the outputStreamCaptor contains the content we were expecting. We call the trim method to remove the new line that System.out.println() adds.

As the standard output stream is a shared static resource used by other parts of the system, we should take care of restoring it to its original state when our test terminates:

@BeforeEach
public void tearDown() {
    System.setOut(standardOut);
}

This ensures we don't get any unwanted side effects later on in other tests.

4. Using System Rules

In this section, we'll take a look at a neat external library called System Rules which provides a set of JUnit rules for testing code that uses the System class.

Let's start by adding the dependency to our pom.xml:

<dependency>
    <groupId>com.github.stefanbirkner</groupId>
    <artifactId>system-rules</artifactId>
    <version>1.19.0</version>
    <scope>test</scope>
</dependency>

Now, we can go ahead and write a test using the SystemOutRule the library provides:

@Rule
public final SystemOutRule systemOutRule = new SystemOutRule().enableLog();

@Test
public void givenSystemOutRule_whenInvokePrintln_thenLogSuccess() {
    print("Hello Baeldung Readers!!");

    Assert.assertEquals("Hello Baeldung Readers!!", systemOutRule.getLog()
      .trim());
}

Pretty cool! Using the SystemOutRule, we can intercept the writes to System.out. First, we start logging everything written to System.out by calling the enableLog method on our rule. Then we simply call getLog to get the text written to System.out since we called enableLog.

This rule also includes a handy method that returns a log that always has the line separator as \n

Assert.assertEquals("Hello Baeldung Readers!!\n", systemOutRule.getLogWithNormalizedLineSeparator());

5. Using System Rules with JUnit5 and Lambdas

In JUnit5, the rules model was replaced by extensions. Luckily, the System Rules library presented in the last section has a variation prepared to work with JUnit5.

System Lambda is available from Maven Central. So we can go ahead and add it to our pom.xml:

<dependency>
    <groupId>com.github.stefanbirkner</groupId>
    <artifactId>system-lambda</artifactId>
    <version>1.0.0</version>
    <scope>test</scope>
</dependency>

Now let's implement our test using this version of the library:

@Test
void givenTapSystemOut_whenInvokePrintln_thenOutputIsReturnedSuccessfully() throws Exception {

    String text = tapSystemOut(() -> {
        print("Hello Baeldung Readers!!");
    });

    Assert.assertEquals("Hello Baeldung Readers!!", text.trim());
}

In this version, we make use of the tapSystemOut method, which executes the statement and lets us capture the content passed to System.out.

6. Conclusion

In this tutorial, we've learned about a couple of approaches for testing System.out.println. In the first approach, we saw how to redirect where we write the standard output stream using core Java.

Then we saw how to use a promising external library called System Rules using, first, JUnit 4 style rules and then later working with lambdas.

As always, the full source code of the article is available over on GitHub.

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