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Version: 5.6


The Kotest BlockHound extension activates BlockHound support for coroutines. It helps to detect blocking code on non-blocking coroutine threads, e.g. when accidentally calling a blocking I/O library function on a UI thread.


To use this extension add the io.kotest.extensions:kotest-extensions-blockhound module to your test compile path.

Getting Started

Register the BlockHound extension in your test class:

class BlockHoundSpecTest : FunSpec({

test("detects for spec") {

The BlockHound extension can also be registered per test case or at the project level.


This code is sensitive to concurrency. There can only be one instance of this extension running at a time as it will take effect globally.

You cannot register the BlockHound extension multiple times at different levels.

Use @DoNotParallelize for BlockHound-enabled tests.


Blocking calls will be detected in coroutine threads which are expected not to block. Such threads are created by the default dispatcher as this example demonstrates:

private suspend fun blockInNonBlockingContext() {
withContext(Dispatchers.Default) {

The BlockHound extension will by default produce an exception like this whenever it detects a blocking call:

reactor.blockhound.BlockingOperationError: Blocking call! java.lang.Thread.sleep
at io.kotest.extensions.blockhound.KotestBlockHoundIntegration.applyTo$lambda-2$lambda-1(KotestBlockHoundIntegration.kt:27)
at reactor.blockhound.BlockHound$Builder.lambda$install$8(
at reactor.blockhound.BlockHoundRuntime.checkBlocking(
at java.base/java.lang.Thread.sleep(
at io.kotest.extensions.blockhound.BlockHoundTestKt$blockInNonBlockingContext$2.invokeSuspend(BlockHoundTest.kt:17)
at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
at kotlinx.coroutines.scheduling.CoroutineScheduler.runSafely(CoroutineScheduler.kt:570)
at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.executeTask(CoroutineScheduler.kt:750)
at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.runWorker(CoroutineScheduler.kt:677)
at kotlinx.coroutines.scheduling.CoroutineScheduler$

By invoking it as BlockHound(BlockHoundMode.PRINT), it will print detected calls and continue the test without interruption.

Whenever a blocking call is detected, you can

  • replace the call with a non-blocking one (using a coroutine-aware library), or
  • schedule the calling coroutine to run on a separate I/O thread (e.g. via Dispatchers.IO), or
  • add an exception if the blocking is harmless (see below).


To customize BlockHound, familiarize yourself with the BlockHound documentation.

Exceptions for blocking calls considered harmless can be added via a separate BlockHoundIntegration class like this:

import reactor.blockhound.BlockHound
import reactor.blockhound.integration.BlockHoundIntegration

class MyBlockHoundIntegration : BlockHoundIntegration {
override fun applyTo(builder: BlockHound.Builder): Unit = with(builder) {
allowBlockingCallsInside("org.slf4j.LoggerFactory", "performInitialization")

In order to allow BlockHound to auto-detect and load the integration, add its fully qualified class name to a service provider configuration file resources/META-INF/services/reactor.blockhound.integration.BlockHoundIntegration.