Fray: General-Purpose Concurrency Testing

Fray is a concurrency testing tool for Java that can help you find and debug tricky race conditions that manifest as assertion violations, run-time exceptions, or deadlocks. It performs controlled concurrency testing using state-of-the-art techniques such as probabilistic concurrency testing or partial order sampling. Fray also provides deterministic replay capabilities for debugging specific thread interleavings. Fray is designed to be easy to use and can be integrated into existing testing frameworks.

Quick Start

JUnit 5

If you are using JUnit 5, you can use the @ConcurrencyTest annotation to mark a test as a concurrency test. You also need to add the @ExtendWith(FrayTestExtension.class) annotation to the test class.

import org.pastalab.fray.junit.junit5.FrayTestExtension;
import org.pastalab.fray.junit.junit5.annotations.ConcurrencyTest;

@ExtendWith(FrayTestExtension.class)
public class SimpleTest {
    @ConcurrencyTest
    public void concurrencyTest() {
        ... // some multithreaded code
        assert(...);
    }
}

Other Testing Frameworks

Fray can be used with other testing frameworks as well. You may use the FrayInTestLauncher

import org.pastalab.fray.junit.plain.FrayInTestLauncher;

public void test() {
    FrayInTestLauncher.INSTANCE.launchFrayTest(() -> {
        ... // some multithreaded code
        assert(...);
    });
}

Gradle

To use Fray with Gradle, add the following plugin to your build.gradle file:

plugins {
    id("org.pastalab.fray.gradle") version "0.7.3"
}

Maven

• First please add Fray plugin to your project

<plugin>
    <groupId>org.pastalab.fray.maven</groupId>
    <artifactId>fray-plugins-maven</artifactId>
    <version>0.7.3</version>
    <executions>
        <execution>
            <id>prepare-fray</id>
            <goals>
                <goal>prepare-fray</goal>
            </goals>
        </execution>
    </executions>
</plugin>

• Next, please add the fray-junit dependency

<dependency>
    <groupId>org.pastalab.fray</groupId>
    <artifactId>fray-junit</artifactId>
    <version>0.7.3</version>
    <scope>test</scope>
</dependency>

Documentation

• Technical Report
• Usage Guide
• IDE Settings
• Debug Fray
• Articles about Fray
• Bugs Found by Fray

Contributing

Contributions to Fray are both welcomed and appreciated! Please see our contributing guide for more information on how to contribute to Fray.

Publications and Media

Fray's design and evaluation is described in the following research paper (please cite this if you are referencing Fray from any other work):

Ao Li, Byeongjee Kang, Vasudev Vikram, Isabella Laybourn, Samvid Dharanikota, Shrey Tiwari, and Rohan Padhye. 2025. Fray: An Efficient General-Purpose Concurrency Testing Platform for the JVM. Proc. ACM Program. Lang. 9, OOPSLA2, Article 417 (October 2025), 28 pages. https://doi.org/10.1145/3764119

An extended version of the paper with detailed formalizations and proofs is available at: https://arxiv.org/abs/2501.12618

Talks about Fray have been given at the following venues:

• SPLASH 2025 by Ao Li
• DC Systems Meetup by Rohan Padhye

Acknowledgements

This material is based upon work supported in part by the National Science Foundation and by an Amazon Research Award.

Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of any sponsor.