Unit test log4j2 log output

Sometimes you want to test if certain log output gets generated when certain events happen in your application. Here is how I unit test that using log4j2 (version 2.11.0).

Use LoggerContextRule to get to your ListAppender quickly

If you are using JUnit 4, then the quickest solution would be one that is used by log4j2 itself:

import org.apache.logging.log4j.junit.LoggerContextRule;
/* other imports */

public class LogEventTest {
    private static ListAppender appender;

    @ClassRule
    public static LoggerContextRule init = new LoggerContextRule("log4j2-test.yaml");

    @BeforeClass
    public static void setupLogging() {
        appender = init.getListAppender("List");
    }

    @Before
    public void clearAppender() {
        appender.clear();
    }

    @Test
    public void someMethodShouldLogAnError() {
        // setup test and invoke logic
        List<LogEvent> logEvents = appender.getEvents();
        List<String> errors = logEvents.stream()
                .filter(event -> event.getLevel().equals(Level.ERROR))
                .map(event -> event.getMessage().getFormattedMessage())
                .collect(Collectors.toList());

        // we logged at least one event of level error
        assertThat(errors.size(), is(greaterThanOrEqualTo(1)));

        // log event message should contain "wrong" for example
        assertThat(errors, everyItem(containsString("wrong")));
    }
}

The LoggerContextRule provides methods that come in handy while testing. Here we use the getListAppender(...) method to get access to an appender that uses a list to store all log events. Before each test, we clear the list, so we have a clean slate for new log events. The test invokes the code-under-test, requests the log events from the appender and filters them so that we only have the error log events left. Then we that at least one error log message was captured and that it contains the word “wrong”.

Use the LoggerContext

Instead of using the class rule (which lets you conveniently pass it the file name of the configuration), you could also use the LoggerContext:

@BeforeClass
public static void setupLogging() {
    LoggerContext context = LoggerContext.getContext(false);
    Logger logger = context.getLogger("com.relentlesscoding");
    appender = (ListAppender) logger.getAppenders().get("List");
}

This might be your only option if you are working with JUnit 5 (and eventually you will want to migrate to that). In JUnit 5 we can’t use LoggerContextRule anymore, because @Rules don’t longer exist (they were replaced with an extension mechanism that works differently and log4j2 doesn’t provide such an extension currently).

Create a working configuration

To get the examples working, we need to define an appender called “List” and a logger in our log4j2 configuration.

log4j2-test.yaml

Configuration:
  status: warn
  name: TestConfig
  appenders:
    Console:
      name: STDOUT
    List:
      name: List

  Loggers:
    logger:
      -
        name: com.relentlesscoding
        AppenderRef:
          ref: List
    Root:
      level: info AppenderRef:
        ref: STDOUT

This configuration will send log events occurring in the package com.relentlesscoding and sub-packages to the appender with the name “List” (which is of type ListAppender). This configuration is defined in YAML, but you can use XML, JSON or the properties format as well.

Maven dependencies

To get the LoggerContextRule in JUnit 4 working, you need the following:

<dependency>
    <groupId>org.apache.logging.log4j</groupId>
    <artifactId>log4j-core</artifactId>
    <version>2.11.0</version>
    <type>test-jar</type>
</dependency>

To get the YAML log4j2 configuration working, you need:

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.9.5</version>
</dependency>
<dependency>
    <groupId>com.fasterxml.jackson.dataformat</groupId>
    <artifactId>jackson-dataformat-yaml</artifactId>
    <version>2.9.5</version>
</dependency>

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