Cannot load Dropwizard metrics; is…

Error message

Cannot load Dropwizard metrics; is org.apache.flink:flink-metrics-dropwizard on the classpath?

What it means

A warning emitted when the Iceberg writer's Dropwizard-based histogram metrics cannot be initialized because the flink-metrics-dropwizard module (or its Codahale/Dropwizard metrics classes) is not on the classpath. Metrics are disabled (returns null) but the write itself continues normally.

Solutions

Add org.apache.flink:flink-metrics-dropwizard to the job's classpath (drop it in Flink's lib/ or ship it with the user jar)
If metrics are not needed, ignore the warning — the writer works without the histogram
Check the Flink version's package location for DropwizardHistogramWrapper if the dependency is present but still failing

Example fix

// build.gradle
// before: (no metrics dep)
// after
implementation("org.apache.flink:flink-metrics-dropwizard:1.18.1")

Defensive patterns

Strategy: fallback

Validate before calling

try {
  Class.forName("org.apache.flink.dropwizard.metrics.DropwizardHistogramWrapper");
} catch (ClassNotFoundException e) {
  // dropwizard metrics unavailable; proceed without histograms
}

Prevention

Ship flink-metrics-dropwizard with deployment artifacts
Verify optional metrics plugins in the base Flink image
Document that writer histograms are optional

When it happens

Trigger: Running a Flink job whose IcebergStreamWriterMetrics tries to construct org.apache.flink.dropwizard.metrics.DropwizardHistogramWrapper via DynConstructors, throwing ClassNotFoundException/NoSuchMethodException because the flink-metrics-dropwizard jar is absent.

Common situations: Deploying to a slim Flink distribution without optional metrics plugins; fat jars that exclude optional dependencies; Flink versions where the Dropwizard wrapper package moved.

Understand the failure class

Background: "X is not installed. Please install it with pip install Y": missing optional dependency errors — ImportError/ValueError raised when a library's optional extra was never installed — this error's family across 22 libraries.

Related errors

AI-assisted analysis of apache/iceberg@86d9c8fc54 (2026-09-12). Data as JSON: /api/errors/a2021d960dd299d5. Report an issue: GitHub.

Appendix: source

Thrown at flink/v2.3/flink/src/main/java/org/apache/iceberg/flink/sink/IcebergStreamWriterMetrics.java:150

    try {
      Class<?> reservoirInterface =
          DynClasses.builder().impl("com.codahale.metrics.Reservoir").buildChecked();
      Class<?> codahaleHistogramClass =
          DynClasses.builder().impl("com.codahale.metrics.Histogram").buildChecked();
      return new DropwizardCtors(
          DynConstructors.builder()
              .impl("com.codahale.metrics.SlidingWindowReservoir", int.class)
              .buildChecked(),
          DynConstructors.builder()
              .impl("com.codahale.metrics.Histogram", reservoirInterface)
              .buildChecked(),
          DynConstructors.builder(Histogram.class)
              .impl(
                  "org.apache.flink.dropwizard.metrics.DropwizardHistogramWrapper",
                  codahaleHistogramClass)
              .buildChecked());
    } catch (ClassNotFoundException | NoSuchMethodException e) {
      LOG.warn(
          "Cannot load Dropwizard metrics; is org.apache.flink:flink-metrics-dropwizard on the classpath?",
          e);
      return null;
    }
  }

  private record DropwizardCtors(
      DynConstructors.Ctor<?> reservoirCtor,
      DynConstructors.Ctor<?> histogramCtor,
      DynConstructors.Ctor<Histogram> wrapperCtor) {}
}

View on GitHub (pinned to 86d9c8fc54)