How to mock log4j2 logger object using slf4j in Junit - spring-boot

Question

I have below code where I am creating different log4j2 loggers object using slf4j factory and I need to write to specific log file based on input param.
#Service
class WriterService {
val logger1: Logger = LoggerFactory.getLogger("t_logger")
val logger2: Logger = LoggerFactory.getLogger("b_logger")
val labelKey = "Label"
fun writeLog(payload: Map<String, Any>){
if(payload.containsKey(labelKey)) {
val label = payload[labelKey].toString().toLowerCase()
if (label == "t") {
logger1.info("{}", payload)
} else if (label == "b") {
logger2.info("{}", payload)
}
}
}
I am wondering how can I mock Logger objects using MockitoJUnitRunner so that I can mock which logger object was invoked ?

Answer 1

I found solution using Appender and ArgumentCaptor...
Solution below for someone who might be interested...
import org.apache.logging.log4j.Level
import org.apache.logging.log4j.LogManager
import org.apache.logging.log4j.core.Appender
import org.apache.logging.log4j.core.LogEvent
import org.apache.logging.log4j.core.Logger
import org.junit.After
import org.junit.Assert.assertEquals
import org.junit.Before
import org.junit.Test
import org.junit.runner.RunWith
import org.mockito.ArgumentCaptor
import org.mockito.Captor
import org.mockito.InjectMocks
import org.mockito.Mock
import org.mockito.Mockito.*
import org.mockito.runners.MockitoJUnitRunner
import java.lang.StringBuilder
#RunWith(MockitoJUnitRunner::class)
class ServiceTest {
#InjectMocks
var service: WriterService
#Mock
private val mockAppender: Appender? = null
#Captor
private val captorLoggingEvent: ArgumentCaptor<LogEvent>? = null
private var logger1: Logger? = null
private var logger2: Logger? = null
#Before
fun setup() {
`when`(mockAppender!!.name).thenReturn("MockAppender")
`when`(mockAppender.isStarted).thenReturn(true)
`when`(mockAppender.isStopped).thenReturn(false)
logger1 = LogManager.getLogger("t_logger") as Logger
logger1!!.addAppender(mockAppender)
logger1!!.level = Level.INFO
logger2 = LogManager.getLogger("b_logger") as Logger
logger2!!.addAppender(mockAppender)
logger2!!.level = Level.INFO
}
#After
fun tearDown() {
logger1!!.removeAppender(mockAppender!!)
logger2!!.removeAppender(mockAppender!!)
}
#Test
fun writeLog_shouldWriteTransactionLogWhenLabelIs_T() {
val payload = hashMapOf("Label" to "t")
service.writeLog(payload)
verify(mockAppender, times(1))!!.append(captorLoggingEvent!!.capture())
assertEquals(captorLoggingEvent.value.level, Level.INFO)
assertEquals(captorLoggingEvent.value.loggerName, "t_logger")
assertEquals(captorLoggingEvent.value.message.formattedMessage, "{Label=t}")
}
#Test
fun writeLog_shouldWriteBackendLogWhenLabelIs_B() {
val payload = hashMapOf("Label" to "b")
service.writeLog(payload)
verify(mockAppender, times(1))!!.append(captorLoggingEvent!!.capture())
assertEquals(captorLoggingEvent.value.level, Level.INFO)
assertEquals(captorLoggingEvent.value.loggerName, "b_logger")
assertEquals(captorLoggingEvent.value.message.formattedMessage, "{Label=b}")
}
}