Configure Maven for Parallel Test Execution

Configure Maven for Parallel Test Execution | Generated by AI

Home 2025.08

To enable parallel test execution in your Maven project, particularly for the provided MavenTest.java class, you can configure the Maven Surefire Plugin to run tests in parallel. The MavenTest.java class uses JUnit 4 with dynamic tests to execute Maven commands for different problems, and the stack trace from your original error indicates that tests are already running in parallel (evidenced by ForkJoinPool in the stack trace). However, you can explicitly control parallel execution by configuring the Surefire Plugin in your pom.xml file. Below, I’ll explain how to set up parallel test execution and address the use of a -DParallel flag as requested.

Why Parallel Execution Is Happening

Your test output shows ForkJoinPool in the stack trace, indicating that JUnit or Maven is already using a thread pool for parallel execution. The MavenTest class uses @TestFactory with DynamicTest, and the tests are likely running in parallel due to JUnit’s default behavior or an existing Surefire configuration. The goal now is to explicitly configure parallel execution and allow control via a command-line flag like -DParallel.

Steps to Configure Parallel Test Execution

Update the pom.xml to Configure Maven Surefire Plugin: The Maven Surefire Plugin supports parallel test execution for JUnit 4.7+ (which your project uses, as it’s compatible with DynamicTest). You can specify the level of parallelism (e.g., classes, methods, or both) and the number of threads. To enable control via -DParallel, you can use a Maven property to toggle or configure parallelism.

Add or update the Surefire Plugin configuration in your pom.xml:
```
<project>
    
    <properties>
        
        <parallel.mode>none</parallel.mode>
        <thread.count>4</thread.count>
    </properties>
    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-surefire-plugin</artifactId>
                <version>3.5.3</version>
                <configuration>
                    <parallel>${parallel.mode}</parallel>
                    <threadCount>${thread.count}</threadCount>
                    <perCoreThreadCount>false</perCoreThreadCount>
                    
                    <parallelTestsTimeoutInSeconds>10</parallelTestsTimeoutInSeconds>
                    
                    <forkCount>1</forkCount>
                    <reuseForks>true</reuseForks>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>
```
- Explanation:
  - <parallel>: Specifies the level of parallelism. Valid values for JUnit 4.7+ are methods, classes, both, suites, suitesAndClasses, suitesAndMethods, classesAndMethods, or all. For your MavenTest class, classes is suitable since each DynamicTest represents a problem, and you want to run tests for different problems in parallel.
  - <threadCount>: Sets the number of threads (e.g., 4 for four concurrent tests). You can override this via -Dthread.count=<number>.
  - <perCoreThreadCount>false</perCoreThreadCount>: Ensures threadCount is a fixed number, not scaled by CPU cores.
  - <parallelTestsTimeoutInSeconds>: Sets a timeout for parallel tests to prevent hanging (matches your TEST_TIMEOUT of 10 seconds in MavenTest.java).
  - <forkCount>1</forkCount>: Runs tests in a separate JVM process to isolate them, reducing shared state issues. <reuseForks>true</reuseForks> allows reusing the JVM for efficiency.
  - <parallel.mode> and <thread.count>: Maven properties that can be overridden via command-line flags (e.g., -Dparallel.mode=classes).

Running Tests with -DParallel: To use a -DParallel flag to control parallel execution, you can map it to the parallel.mode property. For example, run:

mvn test -Dparallel.mode=classes -Dthread.count=4

If -Dparallel.mode is not specified, the default value (none) disables parallel execution.
You can also use a simpler flag like -DParallel=true to enable parallelism with a predefined mode (e.g., classes). To support this, update the pom.xml to interpret -DParallel:

<project>
    <!-- Other configurations -->
    <properties>
        <parallel.mode>${Parallel ? 'classes' : 'none'}</parallel.mode>
        <thread.count>4</thread.count>
    </properties>
    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-surefire-plugin</artifactId>
                <version>3.5.3</version>
                <configuration>
                    <parallel>${parallel.mode}</parallel>
                    <threadCount>${thread.count}</threadCount>
                    <perCoreThreadCount>false</perCoreThreadCount>
                    <parallelTestsTimeoutInSeconds>10</parallelTestsTimeoutInSeconds>
                    <forkCount>1</forkCount>
                    <reuseForks>true</reuseForks>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

Now, you can run tests with:

mvn test -DParallel=true

-DParallel=true: Enables parallel execution with parallel=classes and threadCount=4.
-DParallel=false or omitting -DParallel: Disables parallel execution (parallel=none).
You can override the thread count with -Dthread.count=<number> (e.g., -Dthread.count=8).

Ensuring Thread Safety: Your MavenTest class executes Maven commands (mvn exec:exec -Dproblem=<problem>) in parallel, which spawns separate processes. This is inherently thread-safe since each process has its own memory space, avoiding shared state issues. However, ensure that:
- The com.lzw.solutions.uva.<problem>.Main classes don’t share resources (e.g., files or databases) that could cause conflicts.
- Input/output files or resources used by each problem (e.g., test data for p10009) are isolated to avoid race conditions.
- If shared resources are used, consider using @NotThreadSafe on specific test classes or synchronizing access to shared resources.
Handling the Skip List with Parallel Execution: Your MavenTest.java already includes a SKIP_PROBLEMS set to skip problems like p10009. This works well with parallel execution, as skipped problems are excluded before tests are scheduled. Ensure the skip list is updated as needed:
```
private static final Set<String> SKIP_PROBLEMS = new HashSet<>(Arrays.asList(
    "p10009", // Skips p10009 due to NumberFormatException
    "p10037"  // Add other problematic problems here
));
```
Running the Tests: To run tests in parallel with the skip list and the -DParallel flag:
```
mvn test -DParallel=true -Dthread.count=4
```
- This runs up to four problem tests concurrently, skipping p10009 and any other problems in SKIP_PROBLEMS.
- If you want to disable parallelism for debugging:
```
mvn test -DParallel=false
```
Addressing the NumberFormatException for p10009: The NumberFormatException in p10009 (from your original error) indicates a null string being parsed. While skipping p10009 avoids the issue, you may want to fix it for completeness. Check com.lzw.solutions.uva.p10009.Main at line 17 (readInt method) and add null-checks:
```
public int readInt() {
    String input = readInput(); // Hypothetical input reading method
    if (input == null || input.trim().isEmpty()) {
        throw new IllegalArgumentException("Input cannot be null or empty");
    }
    return Integer.parseInt(input);
}
```
Alternatively, keep p10009 in the skip list until the issue is resolved.

Notes on Parallel Execution

Performance: Parallel execution with parallel=classes is suitable for your MavenTest class, as each DynamicTest represents a distinct problem. This minimizes overhead compared to methods or both.
Resource Usage: Parallel execution increases CPU and memory usage. Monitor your system to ensure threadCount (e.g., 4) doesn’t overwhelm your hardware. Use forkCount to isolate tests in separate JVMs if memory issues arise.
Timeouts: The parallelTestsTimeoutInSeconds setting ensures tests don’t hang indefinitely, aligning with your TEST_TIMEOUT of 10 seconds in MavenTest.java.
Thread Safety: Since your tests execute mvn exec:exec commands, which run in separate processes, thread safety is less of a concern. However, ensure that test inputs/outputs (e.g., files) are isolated per problem.
Debugging: If tests fail unexpectedly in parallel mode, run them sequentially (-DParallel=false) to isolate issues.

Example Full Command

To run tests in parallel, skipping p10009, with four threads:

mvn test -DParallel=true -Dthread.count=4

To debug a specific problem (e.g., p10009) without parallelism, temporarily remove it from SKIP_PROBLEMS and run:

mvn test -DParallel=false -Dproblem=p10009

Additional Considerations

JUnit 4 Limitations: Your project uses JUnit 4 (based on the org.junit.jupiter.api imports and DynamicTest). JUnit 4.7+ supports parallel execution via Surefire, but JUnit 5 offers more flexible parallelization options. Consider upgrading to JUnit 5 if you need advanced features like per-class parallelism or custom strategies.
Maven Version: Ensure you’re using a recent version of Maven (e.g., 3.9.10, as in your error output) and Surefire (e.g., 3.5.3) for optimal parallel execution support.

Error Handling: To make tests more robust, consider catching exceptions in runMavenExecTests (as suggested in the previous response) to log failures without stopping the test suite:

 @TestFactory
 Collection<DynamicTest> runMavenExecTests() {
     return PROBLEMS.stream()
             .map(problem -> DynamicTest.dynamicTest("Test problem: " + problem, () -> {
                 String command = String.format("mvn exec:exec -Dproblem=%s", problem);
                 System.out.println("Executing command for " + problem + ": " + command);
                 try {
                     Process process = Runtime.getRuntime().exec(command);
                     // ... (rest of the process handling code)
                     int exitCode = process.exitValue();
                     Assertions.assertTrue(
                             exitCode == 0,
                             "Maven command failed for problem: " + problem + "\nError output:\n" + errorOutput);
                 } catch (Exception e) {
                     System.err.println("Exception for problem " + problem + ": " + e.getMessage());
                     // Optionally continue instead of failing
                 }
             }))
             .collect(Collectors.toList());
 }

This setup allows you to control parallel execution with -DParallel=true, skip problematic tests like p10009, and maintain robust test execution. Let me know if you need further customization, such as dynamic skip lists via command-line arguments or advanced JUnit 5 configurations!

Back Donate