`surefire.forkNumber` collision when running Maven with parallel modules

[vladilm]

Affected version

3.5.3

Bug description

When running Maven with parallel execution enabled (-T 2 verify), multiple modules are built and tested concurrently. We are using the maven-surefire-plugin with multiple forks configured:

<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-surefire-plugin</artifactId>
  <version>3.5.3</version>
  <configuration>
    <forkCount>4</forkCount>
    <argLine>
      -ea -Xmx1500m -XX:+HeapDumpOnOutOfMemoryError
      -Dsurefire.forkNumber=${surefire.forkNumber}
    </argLine>
    <reuseForks>false</reuseForks>
  </configuration>
</plugin>

The project consists of multiple modules, but test forks appear to clash across modules. Specifically, different modules start test forks with the same fork identifier (e.g., fork-1) at overlapping times.

Wed Dec 17 11:08:39 UTC 2025 -------------Surefire TestDB fork-1 in `server-tests` started--------------
Wed Dec 17 11:38:00 UTC 2025 -------------Surefire TestDB fork-1 in `token-tests` started--------------
Wed Dec 17 11:38:21 UTC 2025 -------------Surefire TestDB fork-1 in `token-test` finished--------------
Wed Dec 17 11:39:49 UTC 2025 -------------Surefire TestDB fork-1 in `server-tests` finished--------------

Is this behavior expected?

[vladilm]

Also, does the use of TestNG suite XML files affect this behavior in any way?

[Tibor17]

It should be used with TestNG as well!
What the documentation says?
Don't use -Dsurefire.forkNumber in the argLine.

[vladilm]

https://maven.apache.org/surefire/maven-surefire-plugin/examples/fork-options-and-parallel-execution.html
This is what it says

You can use the placeholder ${surefire.forkNumber} within argLine, environmentVariables (since maven-surefire-plugin:3.2.0), or within the system properties (both those specified via mvn test -D... and via systemPropertyVariables).

[olamy]

Are server-tests and token-tests separate Maven modules?
If yes in this case, each module is runned by Maven in parallel in the same jvm (because of -T2) BUT each surefire run for each each Maven module will start his own JVM and it's not reused by other Maven modules (so this can explain the same fork-1).
You need to check if it's the same JVM (not sure what is your fork-1 coming from? Maybe check the pid?)

[vladilm]

Yes, server-tests and token-tests are separate Maven modules, both part of the reactor.

I pass the fork value as an argLine parameter to each separate JVM using
-Dsurefire.forkNumber=${surefire.forkNumber}.
In the test code, I read this value via System.getProperty("surefire.forkNumber") and print it to the test output.
In practice, there may be up to eight JVMs running in parallel (for example, -T2 * 4 = 8).

[vladilm]

If I’m reading the code correctly, each fork should obtain a value from ForkNumberBucket, and while that value is in use, no other fork whether in the same module or not, since it’s static should be able to use it.
However, it appears that the same number may be returned to the bucket twice, and after a quick investigation, I’m unable to determine whether this is actually happening.