How to intercept SLF4J (with logback) logging via a JUnit test?

Question

Is it possible to somehow intercept the logging (SLF4J + logback) and get an InputStream (or something else that is readable) via a JUnit test case...?

Answer 1

I had problems when testing logs line like: LOGGER.error(message, exception).

The solution described in http://projects.lidalia.org.uk/slf4j-test/ tries to assert as well on the exception and it is not easy (and in my opinion worthless) to recreate the stacktrace.

I resolved in this way:

import org.junit.Test;
import org.slf4j.Logger;
import uk.org.lidalia.slf4jext.LoggerFactory;
import uk.org.lidalia.slf4jtest.TestLogger;
import uk.org.lidalia.slf4jtest.TestLoggerFactory;

import static org.assertj.core.api.Assertions.assertThat;
import static org.assertj.core.groups.Tuple.tuple;
import static uk.org.lidalia.slf4jext.Level.ERROR;
import static uk.org.lidalia.slf4jext.Level.INFO;


public class Slf4jLoggerTest {

    private static final Logger LOGGER = LoggerFactory.getLogger(Slf4jLoggerTest.class);


    private void methodUnderTestInSomeClassInProductionCode() {
        LOGGER.info("info message");
        LOGGER.error("error message");
        LOGGER.error("error message with exception", new RuntimeException("this part is not tested"));
    }





    private static final TestLogger TEST_LOGGER = TestLoggerFactory.getTestLogger(Slf4jLoggerTest.class);

    @Test
    public void testForMethod() throws Exception {
        // when
        methodUnderTestInSomeClassInProductionCode();

        // then
        assertThat(TEST_LOGGER.getLoggingEvents()).extracting("level", "message").contains(
                tuple(INFO, "info message"),
                tuple(ERROR, "error message"),
                tuple(ERROR, "error message with exception")
        );
    }

}

This has as well the advantage to not having depend on Hamcrest matchers library.