Cannot load Dropwizard metrics; is…

Error message

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

What it means

IcebergStreamWriterMetrics optionally exposes Dropwizard histograms for write latency. It loads org.apache.flink.dropwizard.metrics.DropwizardHistogramWrapper reflectively; when flink-metrics-dropwizard is not on the classpath the class can't be loaded, the metrics are disabled with this warning, and the writer continues without them.

Solutions

Add org.apache.flink:flink-metrics-dropwizard (matching your Flink version) to the job jar or Flink lib/ directory.
Ignore the warning if the Dropwriter-based latency metrics are not needed — functionality is unaffected.
Verify the dependency isn't lost by shading exclusions (check the shaded uber-jar contents).

Example fix

// Maven: before (missing)
<!-- no flink-metrics-dropwizard -->
// after
<dependency>
  <groupId>org.apache.flink</groupId>
  <artifactId>flink-metrics-dropwizard</artifactId>
  <version>${flink.version}</version>
</dependency>

Defensive patterns

Strategy: fallback

Validate before calling

try {
  Class.forName("org.apache.flink.dropwizard.metrics.DropwizardHistogramWrapper");
} catch (ClassNotFoundException e) {
  LOG.warn("flink-metrics-dropwizard missing; latency histograms disabled");
}

Try / catch

// Warning is non-fatal; if you need metrics, verify the jar at deploy time
try {
  Class.forName("org.apache.flink.dropwizard.metrics.DropwizardHistogramWrapper");
} catch (ClassNotFoundException e) {
  throw new IllegalStateException("Add flink-metrics-dropwizard to the classpath", e);
}

Prevention

Bundle flink-metrics-dropwizard in the job jar or Flink lib/.
Match its version to your Flink version.
Check shaded uber-jar for excluded optional dependencies.

When it happens

Trigger: Running a Flink job with the Iceberg stream writer where the flink-metrics-dropwizard jar is absent from the job's classpath (lib/, uber-jar, or user code loader).

Common situations: Deploying the Iceberg Flink runtime jar standalone without the metrics reporter dependency; using a minimal Flink distribution; shading the job and excluding optional dependencies.

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/a9106cfa07e83798. Report an issue: GitHub.

Appendix: source

Thrown at flink/v1.20/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) {}
}

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