# reactor/BlockHound

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

Repository: https://github.com/reactor/BlockHound
Canonical: https://ross.abutalabs.com/products/blockhound
Language: Java
License: Apache-2.0
License Family: permissive
Last push: 2026-08-24T07:54:38+00:00

## Health v2 (maintenance only)
Score: 87/100 (v2, computed 2026-09-03T02:20:16.233290+00:00)
- activity 99, release rhythm 63, longevity 100
- inputs: {"age_days": 2829, "days_push": 9, "days_rel": 86, "gap_med": 98, "n_releases_24m": 8}
- flags: none
- formula: round(0.45*activity + 0.35*rhythm + 0.20*longevity); archived -> min(score, 10)

## Adoption (not part of the score)
Stars 1506, forks 95 (observed 2026-08-28T04:04:55.114110+00:00)

## What it is
BlockHound is a Java agent that detects blocking calls (like IO or Thread.sleep) made from threads marked as non-blocking, such as Reactor's parallel scheduler threads. It transparently instruments JVM classes and throws an error pinpointing the exact location of the offending blocking call.

## Use cases
- detect blocking calls in reactive code
- find Thread.sleep in non-blocking threads
- debug Project Reactor applications for blocking operations
- catch accidental blocking IO in event loop threads
- verify non-blocking guarantees in tests
- locate source of BlockingOperationError in reactive pipelines

## When to choose
- you develop reactive applications with Project Reactor or RxJava
- you want to enforce non-blocking discipline in tests
- you need to pinpoint exactly where blocking calls happen in async code

## When to avoid
- your codebase is purely blocking/synchronous
- you need runtime production monitoring rather than detection during development or testing
- you work outside the JVM ecosystem

## Facets
- artifact type: library
- maturity: stable
- function: testing, developer-tools
- domain: developer-tools, testing, performance
- platform: jvm, cross-platform
- tags: java-agent, reactor, reactive-programming, non-blocking, blocking-detection, bytecode-instrumentation, instrumentation

## Member repositories
- reactor/BlockHound (main) score 87

## Provenance
- Observed fields: from GitHub, fetched 2026-08-28T04:04:55.114110+00:00.
- Health v2: computed from the inputs above; adoption is never an input.
- Inferred fields (summary, facets, guidance): AI-extracted, prompt v1, taxonomy v1, on 2026-08-30T04:32:40.709825+00:00, confidence not recorded.
  - readme: https://github.com/reactor/BlockHound (fetched 2026-08-28T04:04:55.114110+00:00, sha 532b8ed14032)
- Data as of 2026-08-30T08:39:29.467469+00:00.