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

The Slf4j API doesn't provide such a way but Logback provides a simple solution.

You can use ListAppender : a whitebox logback appender where log entries are added in a public List field that we could use to make our assertions.

Here is a simple example.

Foo class :

import org.slf4j.Logger; import org.slf4j.LoggerFactory;  public class Foo {      static final Logger LOGGER = LoggerFactory.getLogger(Foo .class);      public void doThat() {         logger.info("start");         //...         logger.info("finish");     } } 

FooTest class :

import org.slf4j.LoggerFactory; import ch.qos.logback.classic.Level; import ch.qos.logback.classic.Logger; import ch.qos.logback.classic.spi.ILoggingEvent; import ch.qos.logback.core.read.ListAppender;  public class FooTest {      @Test     void doThat() throws Exception {         // get Logback Logger          Logger fooLogger = (Logger) LoggerFactory.getLogger(Foo.class);          // create and start a ListAppender         ListAppender<ILoggingEvent> listAppender = new ListAppender<>();         listAppender.start();          // add the appender to the logger         fooLogger.addAppender(listAppender);          // call method under test         Foo foo = new Foo();         foo.doThat();          // JUnit assertions         List<ILoggingEvent> logsList = listAppender.list;         assertEquals("start", logsList.get(0)                                       .getMessage());         assertEquals(Level.INFO, logsList.get(0)                                          .getLevel());          assertEquals("finish", logsList.get(1)                                        .getMessage());         assertEquals(Level.INFO, logsList.get(1)                                          .getLevel());     } } 

You can also use Matcher/assertion libraries as AssertJ or Hamcrest.

With AssertJ it would be :

import org.assertj.core.api.Assertions;  Assertions.assertThat(listAppender.list)           .extracting(ILoggingEvent::getFormattedMessage, ILoggingEvent::getLevel)           .containsExactly(Tuple.tuple("start", Level.INFO), Tuple.tuple("finish", Level.INFO)); 

Answer 2

You can create a custom appender

public class TestAppender extends AppenderBase<LoggingEvent> {     static List<LoggingEvent> events = new ArrayList<>();          @Override     protected void append(LoggingEvent e) {         events.add(e);     } } 

and configure logback-test.xml to use it. Now we can check logging events from our test:

@Test public void test() {     ...     Assert.assertEquals(1, TestAppender.events.size());     ... } 

NOTE: Use ILoggingEvent if you do not get any output - see the comment section for the reasoning.