How can I unit test whether my app logs as expected with log4j2?

Question

I tried using reflection, but in my case sl4fj creates a final logger. This means I can't customize it.

Any advice?

Answer 1

if you're using log4j2 with spring-boot, you can do the following:

test code:

import org.springframework.boot.test.system.CapturedOutput;
import org.springframework.boot.test.system.OutputCaptureExtension;
import static org.assertj.core.api.Assertions.assertThat;

@ExtendWith(OutputCaptureExtension.class)
 class MyClassTest {
    @Test
    void myFunction_whenIDoAThing_thenItDoesNotLog(CapturedOutput output) {
        // given

        ...
        // when
        when(myDependency.doSomething()).thenReturn(something);

        myClass.doSomething();

        // then
        assertThat(output).doesNotContain(AN_EXPECTED_ERROR_MESSAGE);

        verify(myDependency).doSomething();
    }
}

the key is adding the follow attribute and sending the logs to system out

https://docs.spring.io/spring-boot/api/java/org/springframework/boot/test/system/OutputCaptureExtension.html https://logging.apache.org/log4j/2.x/manual/appenders.html#ConsoleAppender

log4j2 configuration:

❯ cat spring-rest/src/test/resources/log4j2-test.xml
<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="warn">
  <Appenders>
    <Console name="STDOUT" target="SYSTEM_OUT" follow="true">
      <PatternLayout pattern="%m%n"/>
    </Console>
  </Appenders>
  <Loggers>
    <Root level="warn">
      <AppenderRef ref="STDOUT"/>
    </Root>
  </Loggers>
</Configuration>