How to do a simple unit test of kotlin-loggin

Question

I want to create a simple unit test of expected logging (level and message) in a try/catch scenario.

One exception should log a info-message with a specified message and other exceptions should log warning messages.

I am using the following dependency: implementation("io.github.oshai:kotlin-logging-jvm:7.0.0")

My first attempt of this is done by logging with an object which I can mock in a unit test:

private val logger = KotlinLogging.logger {}


object MyLogger {
    fun info(exception: Exception, msg: String) {
        logger.info(exception) { msg }
    }

    fun warn(exception: Exception, msg: String) {
        logger.warn(exception) { msg }
    }
}

I do not like this solution as code is introduced for the purpose of testing only. Is there a way where I can test invocations of the logger directly?

I am using testImplementation("io.mockk:mockk-jvm:1.13.11") for mocking purposes

Answer 1

I was able to achieve this in the following way:

1. Create a class like this (in the same or a new .kt file):

   import ch.qos.logback.classic.spi.ILoggingEvent
   import ch.qos.logback.core.AppenderBase
   
   class TestLogAppender : AppenderBase<ILoggingEvent>() {
       val logMessages = mutableListOf<ILoggingEvent>()
   
       override fun append(eventObject: ILoggingEvent) {
           logMessages.add(eventObject)
       }
   }
   

2. Update the test class to use the new TestLogAppender

       private lateinit var testLogAppender: TestLogAppender
       private val logger = LoggerFactory.getLogger("com.sap.pi.eureka.pandora.util") as Logger
   
       @BeforeEach
       fun setUp() {
           testLogAppender = TestLogAppender()
           testLogAppender.start()
           logger.addAppender(testLogAppender)
       }
   
       @AfterEach
       fun tearDown() {
           logger.detachAppender(testLogAppender)
       }
   

3. Use the testLogAppender in your @Test case. Example:

   val warningLogs = testLogAppender.logMessages.filter { it.level == Level.WARN }
   assertTrue(warningLogs.isNotEmpty())
   

4. (Optional) In case you want to assert on detailed log content including the stacktrace, I recommend introducing the following function

   fun ILoggingEvent.toFormattedString(): String {
       val s = "${this.level} - ${this.loggerName} - ${this.formattedMessage}"
       if (this.throwableProxy != null) {
           val throwable = (this.throwableProxy as ThrowableProxy).throwable
           return s + "\n ${throwable.stackTraceToString()}"
       }
       return s
   }
   

   Then you can do assertions like the following

   assertTrue(testLogAppender.logMessages.none { it.toFormattedString().contains("sensitiveValue") })
   

Hope that is useful to you!