Skip to content


Switch branches/tags

Name already in use

A tag already exists with the provided branch name. Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. Are you sure you want to create this branch?

Java Concurrency Stress (jcstress)

The Java Concurrency Stress (jcstress) is the experimental harness and a suite of tests to aid the research in the correctness of concurrency support in the JVM, class libraries, and hardware.



In order to understand jcstress tests and maybe write your own, it might be useful to work through the jcstress-samples. The samples come in three groups:

  • APISample target to explain the jcstress API;
  • JMMSample target to explain the basics of Java Memory Model;
  • ConcurrencySample show the interesting concurrent behaviors of standard library.

See the test comments for run instructions. Most tests can be run like this:

 $ mvn clean verify -pl jcstress-samples -am
 $ java -jar jcstress-samples/target/jcstress.jar -t <test-name>

Running The Existing Tests

The quickest way to start running jcstress is to use a prebuilt JAR, for example from here.

 $ java -jar jcstress.jar

Run the JAR with -h to see available options.

Otherwise, you can build the entire test suite yourself:

 $ mvn clean verify
 $ java -jar tests-all/target/jcstress.jar

The project requires JDK 11+ to build. It can reference the APIs from the future releases, as the jcstress harness will fail gracefully on API mismatches, and the mismatched tests will be just skipped.

Extending The Tests

Please consider contributing the interesting tests back.

If you want to develop a test, you are encouraged to get familiar with existing set of tests first. You will have to have a class annotated with jcstress annotations, see the harness API. Read up their Javadocs to understand the conditions that are guaranteed for those tests. If you need some other test interface/harness support, please don't hesitate to raise the issue and describe the scenario you want to test.

You are encouraged to provide the thorough explanation why particular test outcome is acceptable/forbidden/special. Even though harness will print the debug output into the console if no description is given.

You should have Mercurial and Maven installed to check out and build the tests. You will need JDK 11+ to compile all the tests. Most tests are runnable on JDK 8+ afterwards.

The vast majority of jcstress tests are auto-generated. The custom/hand-written tests usually go to tests-custom. This also allows building the smaller subset of tests:

$ mvn clean verify -pl tests-custom -am
$ java -jar tests-custom/target/jcstress.jar

Using jcstress As Separate Dependency

If you want to use jcstress as separate dependency in your project, then you are recommended to create the submodule with the jcstress tests, which would use jcstress libraries and build steps.

Maven Central contains the latest releases of jcstress libraries. Using jcstress as the library requires special build configuration. The easiest way to bootstrap the project with jcstress is to use the archetype:

$ mvn archetype:generate \
 -DinteractiveMode=false \
 -DarchetypeGroupId=org.openjdk.jcstress \
 -DarchetypeArtifactId=jcstress-java-test-archetype \
 -DgroupId=org.sample \
 -DartifactId=test \

Then you can build and use it:

$ cd test
$ mvn clean verify
$ java -jar target/jcstress.jar

Interpreting The Results

The tests are arranged so that a few threads are executing the test concurrently, sometimes rendezvous'ing over the shared state. There are multiple state objects generated per each run. Threads then either mutate observe that state object. Test harness is collecting statistics on the observed states. In many cases this is enough to catch the reorderings or contract violations for concurrent code.

The console output can be used to track progress and debugging. Ordinary users should use generated HTML report, which has the full interpretation of the results.

Most of the tests are probabilistic, and require substantial time to catch all the cases. It is highly recommended to run tests longer to get reliable results. Since the tests are time-bound, the faster CPUs the machine has the more samples jcstress collects. There is a tradeoff between the number of samples harness collects and the suite run time. There are a few preset modes that set sensible test durations, see -m. Many CIs run jcstress with -m quick for quicker turnaround.

Test failure does not immediately mean the implementation bug. The usual suspects are the bugs in test infrastructure, test grading error, bugs in hardware, or something else. Share your results, discuss them, we will figure out what's wrong. Discuss the result on the relevant mailing lists first.

Two usual options are:

Reporting Harness and Test Bugs

If you have the access to JDK Bug System, please submit the bug there:

  • Project: CODETOOLS
  • Component: tools
  • Sub-component: jcstress

If you don't have the access to JDK Bug System, submit the bug report at "Issues" here, and wait for maintainers to pick that up.


jcstress project accepts pull requests, like other OpenJDK projects. If you have never contributed to OpenJDK before, then bots would require you to sign OCA first. Normally, you don't need to post patches anywhere else, or post to mailing lists, etc. If you do want to have a wider discussion about jcstress, please refer to jcstress-dev.

Compile and run internal tests:

$ mvn clean install

Run the quick tests:

$ java -jar tests-all/target/jcstress.jar -m quick

GitHub workflow "JCStress Pre-Integration Tests" should pass on the changes. It would be triggered for PRs. You can also trigger it manually for your branch.