How can I test logs of Spring Boot application? - spring

Question

I have an application that is a mix of Spring Boot, Jersey, and Camel applications. It starts as a Spring Boot app. I am writing integration tests, and I need to make asserts on logs?
For instance, I need to assert that the Camel route read a message from source A. How can I make reliable asserts on logs? Is there any industry standard for this?
NOTE: I tried finding any solution, but at the moment, I neither understand how to solve it nor can find ready solutions.
UPDATE 1: The detail that I underestimated, but it seems important. I use Kotlin, NOT Java. I tried applying answer, but it isn't one to one transferable to Kotlin.
UPDATE 2:
This is a conversion from Java to Kotlin. ListAppender doesn't have enough information to resolve the type in Kotlin.
class LoggerExtension : BeforeEachCallback, AfterEachCallback {
private val listAppender: ListAppender<ILoggingEvent> = ListAppender<ILoggingEvent>()
private val logger: Logger = LoggerFactory.getLogger(ROOT_LOGGER_NAME) as Logger
override fun afterEach(extensionContext: ExtensionContext) {
listAppender.stop()
listAppender.list.clear()
logger.detachAppender(listAppender)
}
override fun beforeEach(extensionContext: ExtensionContext) {
logger.addAppender(listAppender)
listAppender.start()
}
val messages: List<String>
get() = listAppender.list.stream().map { e -> e.getMessage() }.collect(Collectors.toList())
val formattedMessages: List<String>
get() = listAppender.list.stream().map { e -> e.getFormattedMessage() }.collect(Collectors.toList())
}
Kotlin: Not enough information to infer type variable A
Not an error, but I have a feeling that it will fail in runtime:
private val logger: Logger = LoggerFactory.getLogger(ROOT_LOGGER_NAME) as Logger

Answer 1

Spring Boot comes with OutputCapture rule for JUnit 4 and OutputCaptureExtension for JUnit 5, that let you assert on text sent to standard output.
public class MyTest {
#Rule
public OutputCaptureRule output = new OutputCaptureRule();
#Test
public void test() {
// test code
assertThat(output).contains("ok");
}
}