Java agent to detect blocking calls from non-blocking threads.
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Java agent to detect blocking calls from non-blocking threads.

How it works

BlockHound will transparently instrument the JVM classes and intercept blocking calls (e.g. IO) if they are performed from threads marked as "non-blocking operations only" (ie. threads implementing Reactor's NonBlocking marker interface, like those started by Schedulers.parallel()). If and when this happens (but remember, this should never happen!😜), an error will be thrown. Here is an example:


    .doOnNext(it -> {
        try {
        catch (InterruptedException e) {
            throw new RuntimeException(e);

Will result in:

reactor.blockhound.BlockingOperationError: Blocking call! java.lang.Thread.sleep
	at java.base/java.lang.Thread.sleep(Native Method)
	at com.example.Example.lambda$exampleTest$0(

Note that it points to the exact place where the blocking call got triggered. In this example it was

Getting it

Download it from Maven Central repositories (stable releases only) or


repositories {
  // maven { url '' }
  // maven { url '' }

dependencies {
  testImplementation '$LATEST_RELEASE'
  // testImplementation '$LATEST_MILESTONE'
  // testImplementation '$LATEST_SNAPSHOT'





JDK13+ support

for JDK 13+, it is no longer allowed redefining native methods. So for the moment, as a temporary work around, please use the -XX:+AllowRedefinitionToAddDeleteMethods jvm argument:




    tasks.withType(Test).all {
        if (JavaVersion.current().isCompatibleWith(JavaVersion.VERSION_13)) {
            jvmArgs += [

Built-in integrations

Although BlockHound supports the SPI mechanism to integrate with, it comes with a few built-in integrations:

  1. Project Reactor
    Version 3.2.x is supported out of the box.
    Starting with reactor-core version 3.3.0, there is a built-in integration in Reactor itself that uses the SPI.
  2. RxJava 2 is supported.
    RxJava 3 and further versions of RxJava will require an SPI to be implemented, either by the framework or user. See this PR to RxJava with an example of the SPI's implementation.

Quick Start

See the docs.

Licensed under Apache Software License 2.0

Sponsored by Pivotal

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