bazelbuild/bazel · error · RuntimeException

Could not create custom sharding strategy class ${strategy}

Error message

Could not create custom sharding strategy class ${strategy}

What it means

Thrown by ShardingFilters.getShardingFilterFactory when the system property test.sharding.strategy is neither a value of the ShardingStrategy enum (compared case-insensitively) nor a loadable class name. The code first tries valueOf(), then falls back to reflection: load the class via the context classloader, narrow it to ShardingFilterFactory, and instantiate via a no-arg constructor. Any ReflectiveOperationException in that fallback is wrapped in this RuntimeException.

Source

Thrown at src/java_tools/junitrunner/java/com/google/testing/junit/runner/sharding/ShardingFilters.java:104

        shardingEnvironment.getTotalShards());
  }

  private ShardingFilterFactory getShardingFilterFactory() {
    String strategy = shardingEnvironment.getTestShardingStrategy();
    if (strategy == null) {
      return defaultShardingStrategy;
    }
    ShardingFilterFactory shardingFilterFactory;
    try {
      shardingFilterFactory = ShardingStrategy.valueOf(strategy.toUpperCase(Locale.ENGLISH));
    } catch (IllegalArgumentException e) {
      try {
        ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
        Class<? extends ShardingFilterFactory> strategyClass =
            classLoader.loadClass(strategy).asSubclass(ShardingFilterFactory.class);
        shardingFilterFactory = strategyClass.getConstructor().newInstance();
      } catch (ReflectiveOperationException | IllegalArgumentException e2) {
        throw new RuntimeException(
            "Could not create custom sharding strategy class " + strategy, e2);
      }
    }
    return shardingFilterFactory;
  }
}

View on GitHub (pinned to e6e199d060)

Solutions

Use a built-in enum value of ShardingStrategy (e.g. DEFAULT) spelled exactly, case-insensitively.
For a custom factory: ensure the class is on the test runtime classpath (deps of the java_test / runner config).
Verify the class is public, implements com.google.testing.junit.runner.sharding.ShardingFilterFactory, and has a public no-arg constructor that does not throw.
Read the suppressed cause (e2) from the stack trace: ClassNotFoundException vs ClassCastException vs InvocationTargetException tells you which link broke.

Example fix

// before
public final class MySharding implements ShardingFilterFactory {
  MySharding(ShardIndex idx) {...}  // package-private, no no-arg ctor
}
// after
public final class MySharding implements ShardingFilterFactory {
  public MySharding() {}  // public no-arg constructor required
}

Defensive patterns

Strategy: try-catch

Validate before calling

// verify a custom strategy before setting the property
String strategy = "com.mycorp.MyShardingFactory";
if (!ShardingFilters.ShardingStrategy.VALID_NAMES.contains(strategy.toUpperCase(Locale.ROOT))) {
  Class<?> c = Class.forName(strategy); // ClassNotFoundException surfaces early
  if (!ShardingFilterFactory.class.isAssignableFrom(c)
      || c.getConstructor() == null) {
    throw new IllegalArgumentException("Bad sharding strategy: " + strategy);
  }
}

Type guard

static boolean isValidShardingStrategy(String s) {
  try {
    ShardingFilters.ShardingStrategy.valueOf(s.toUpperCase(Locale.ENGLISH));
    return true;
  } catch (IllegalArgumentException e) {
    try {
      return ShardingFilterFactory.class
          .isAssignableFrom(Thread.currentThread().getContextClassLoader().loadClass(s));
    } catch (ClassNotFoundException cnf) { return false; }
  }
}

Try / catch

try {
  ShardingFilters.createShardingFilterFactory(...);
} catch (RuntimeException e) {
  Throwable cause = e.getCause(); // ReflectiveOperationException tells which step failed
  log.config("Invalid test.sharding.strategy, falling back to DEFAULT: " + cause);
}

Prevention

Keep custom ShardingFilterFactory implementations public with a public no-arg constructor.
Add the factory jar as a runtime dep of every test that uses the strategy.
Pin the strategy property in a checked-in bazelrc so typos surface at config review.

When it happens

Trigger: Setting -Dtest.sharding.strategy=custom.factory (or a typo like "defualt") where the class is missing from the test classpath, does not implement ShardingFilterFactory, is not public, lacks a public no-arg constructor, or its constructor throws.

Common situations: Teams experimenting with custom sharding plug-ins who forget to add the factory jar to the test's deps; typos in the strategy property; refactoring that renamed or removed the factory class without updating the flag.

Related errors

AI-assisted analysis of bazelbuild/bazel@e6e199d060 (2026-08-14). Data as JSON: /api/errors/4485368943b1c839. Report an issue: GitHub.