Sample Unit test illustrating how to test log4j2 loggers using a custom appender

FooTest.kt

package com.example

import com.example.appender.TestAppender
import org.apache.logging.log4j.LogManager
import org.apache.logging.log4j.core.LoggerContext
import org.apache.logging.log4j.core.config.Configuration
import org.junit.jupiter.api.AfterEach
import org.junit.jupiter.api.BeforeEach
import org.junit.jupiter.api.Test
import kotlin.test.assertEquals

internal class FooTest {
    private val foo = Foo()
    private lateinit var testAppender: TestAppender
    private lateinit var logConfig: Configuration

    @BeforeEach
    private fun setUp() {
        val ctx = LogManager.getContext(false) as LoggerContext
        logConfig = ctx.configuration
        // no need to output test log data to stdout
        logConfig.rootLogger.removeAppender("Console")
        testAppender = logConfig.appenders["TestAppender"] as TestAppender
    }

    @Test
    fun `bar should add a log message`() {
        foo.bar()
        assertEquals(1, testAppender.getMessages().size)
    }

    @AfterEach
    private fun teardown() {
        testAppender.clearMessages()
        logConfig.addAppender(logConfig.getAppender("Console"))
    }
}