How to test logging in junit5?

Question

I am transitioning from junit4 to junit5 on a project and trying to figure out how to test logs. Previously, I used

@Rule
OutputCapture outputCapture = new OutputCapture();

and would then write an assertion using outputCapture.toString(), for example

assertThat(outputCapture.toString(),containsString("status=200"));

Since @Rule annotation hasn't been implemented yet in junit5, I can't use outputCapture. Any ideas what to do instead? Thanks!

score 15 · Answer 1 · answered Apr 12 '21 at 09:00

15

There's an extension provided for the same and you can use it as below:

@ExtendWith(OutputCaptureExtension.class)
public class MyTestClass {

    @Test
    void myTestCase(CapturedOutput capturedOutput) {
        assertTrue(capturedOutput.getOut().contains("expected string"));
        assertTrue(capturedOutput.getErr().contains("expected string"));
    }
}

answered Apr 12 '21 at 09:00

VaibS

1,627
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import org.springframework.boot.test.system.CapturedOutput; import org.springframework.boot.test.system.OutputCaptureExtension; – AnthonyW Sep 27 '22 at 13:25

score 6 · Answer 2 · answered Mar 15 '19 at 10:07

We stumbled upon the same issue during our JUnit5 migration. After some research, I found a technical solution but it didn't seem like someone made a testing library out of it just yet. So that's what I did. It's published to Maven Central so you can use it right away:

https://github.com/netmikey/logunit

You can use it as follows:

public class MyModuleTest {

    @RegisterExtension
    LogCapturer logs = LogCapturer.create().captureForType(MyModule.class);

    @Test
    public void testLogging() {
        // ... do the testing ...

        // Run assertions on the logged messages
        logs.assertContains("Some message");
    }
}

(see the project's README for more examples)

implemented your library, excellent addition to my project!. Nice work! — Paul, Jun 25 '20 at 14:33

score 2 · Answer 3 · answered Mar 13 '17 at 17:35

The Migration Tip from the JUnit5 documentation clearly states that -

@Rule and @ClassRule no longer exist; superseded by @ExtendWith; see the following section for partial rule support.

For the purpose of using the existing @Rule support from JUnit 4, there is though a way suggested for method or class level annotations.

As in JUnit 4, Rule-annotated fields as well as methods are supported. By using these class-level extensions on a test class such Rule implementations in legacy codebases can be left unchanged including the JUnit 4 rule import statements.

This limited form of Rule support can be switched on by the class-level annotation org.junit.jupiter.migrationsupport.rules.EnableRuleMigrationSupport

A better option would still be redesigning your test suite to use the Extension Model from JUnit5 if you're using it.

score 2 · Answer 4 · answered Oct 21 '20 at 06:54

You can also easily test the log output written to System.out by quickly implementing a solution on your own as follows:

// Configure System.out to be written to a buffer from which we can read
PrintStream realSysOut = System.out;
BufferedOutputStream sysOutBuffer = new ByteArrayOutputStream();
System.setOut(new PrintStream(sysOutBuffer));
...
// Perform some action which logs something to System.out
System.out.println("Some random content written to System.out");
...
// Assert that a given string was written in the meantime to System.out
assertThat(new String(buffer.toByteArray()), containsString("random content"));
...
// Don't forget to bring back the real System.out at the end of the test
System.setOut(realSysOut);

In the case of checking log output written to System.err, you can equivalently implement it by replacing System.setOut(...) with System.setErr(...) and System.out with System.err in the example above.

How to test logging in junit5?

4 Answers4