How to prevent JUnit5 from printing stack traces? - kotlin

Question

If I understand it correctly, the only way to fail a JUnit test is by throwing an uncaught exception somewhere in a method annotated with #Test. There is a fail method, but it just throws an exception.
JUnit5 somehow always finds a way to print the stack trace of this exception. I would like to avoid it because it makes it hard for me to read the console output.
I tried to redirect stderr to a file:
val file = File("./logs/stderr.txt")
val fos = FileOutputStream(file)
val ps = PrintStream(fos)
System.setErr(ps)
but the stack trace is nevertheless written to console in my IDE and it is shown in the tab 'Failed Tests' in the HTML output.
This indicates that the stack trace is printed to a different stream than stderr or stdout. Could someone explain what is in fact happening?
I also tried to implement my own TestExecutionExceptionHandler
class DisableJunitStacktracePrint : TestExecutionExceptionHandler
{
#Throws(Throwable::class)
override fun handleTestExecutionException(context: ExtensionContext, throwable: Throwable)
{
return // this makes all test succeed.
}
}
But the only way to tell the framework that a test is failing is to re-throw the exception, which puts me back where I started.

Answer 1

I'm not sure if this is the best you can do, but one idea is to strip the stack trace out of the exception so that only the exception name itself is printed out:
class DisableJunitStacktracePrint : TestExecutionExceptionHandler
{
override fun handleTestExecutionException(context: ExtensionContext, throwable: Throwable) {
throw throwable.apply { stackTrace = arrayOf() }
}
}
Unfortunately, this also means that you won't be able to see where your error occurred. So you might prefer to settle for a compromise, where you limit the depth of the stack trace so that you don't get inundated with too many lines. To do that, start the JVM with -XX:MaxJavaStackTraceDepth=10 (or whatever depth you prefer).
However, if you want to keep a full stack trace for your application code, but just limit the stack trace depth of your failed assertions, you can do this:
class DisableJunitStacktracePrint : TestExecutionExceptionHandler
{
override fun handleTestExecutionException(context: ExtensionContext, throwable: Throwable) {
throw throwable.apply { stackTrace = stackTrace.drop(1).take(10).toTypedArray() }
}
}
(The drop(1) is there because the top of the stack is the fail function itself and therefore not useful.)