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Unit testing : Asserting that a line was logged by Logback

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I encountered a tricky problem today, I was writing a test for which the assertion was onto some function called or not. I couldn’t mock that function as it was part of an external API for which I couldn’t change the real Object to a mocked Object.

There was a solution, checking that this function was logging a particular event or not. So in order to assert that this specific line was logged or not, I added a mocked Appender. (Note: The Appender is the class that will actually do the logging, e.g. ConsoleAppender to log to the console)

I’m using two great libraries here: Mockito and Logback, please refer to my other post explaining how to configure Logback.

Here is the code:

package com.jsoft.test;

import ch.qos.logback.classic.spi.LoggingEvent;
import ch.qos.logback.core.Appender;
import org.mockito.ArgumentMatcher;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.junit.Test;

import static org.mockito.Matchers.argThat;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.verify;
import static org.mockito.Mockito.when;

public class MyTest {
  private static Logger logger = LoggerFactory.getLogger(MyTest.class);

    public void testSomething() {
    ch.qos.logback.classic.Logger root = (ch.qos.logback.classic.Logger) LoggerFactory.getLogger(ch.qos.logback.classic.Logger.ROOT_LOGGER_NAME);
    final Appender mockAppender = mock(Appender.class);

    //... do whatever you need to trigger the log

    verify(mockAppender).doAppend(argThat(new ArgumentMatcher() {
      public boolean matches(final Object argument) {
        return ((LoggingEvent)argument).getFormattedMessage().contains("Hey this is the message I want to see");


From http://jsoftbiz.wordpress.com/2011/11/29/unit-testing-asserting-that-a-line-was-logged-by-logback/


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