apache/druid · error · ParseException

String cannot be deserialized as JSON to a Spectator…

Error message

String cannot be deserialized as JSON to a Spectator Histogram

What it means

SpectatorHistogram.deserialize received a String it could not interpret as a SpectatorHistogram. The string is first attempted to be parsed as JSON into a HashMap of counts; if that fails (or the shape is wrong) the deserializer rejects it, because only byte[] serialized histograms, JSON maps, or existing SpectatorHistogram objects are accepted.

Solutions

1. Check the wrapped JSON parse exception for why the string failed to deserialize.
2. Ensure histogram values are produced and serialized consistently by SpectatorHistogram/its serializer — do not hand-write serialized values.
3. Verify no intermediary (e.g. a store or cache) converted the byte[] histogram to a plain string.
4. If reading from a segment written by an older/incompatible version, re-ingest or upgrade the extension.

When it happens

Trigger: Thrown at extensions-contrib/spectator-histogram/src/main/java/org/apache/druid/spectator/histogram/SpectatorHistogram.java:174 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/7f1129e9703a6780. Report an issue: GitHub.

Appendix: source

Thrown at extensions-contrib/spectator-histogram/src/main/java/org/apache/druid/spectator/histogram/SpectatorHistogram.java:174

    }
    if (serializedHistogram instanceof byte[]) {
      return fromByteBuffer(ByteBuffer.wrap((byte[]) serializedHistogram));
    }
    if (serializedHistogram instanceof SpectatorHistogram) {
      return (SpectatorHistogram) serializedHistogram;
    }
    if (serializedHistogram instanceof String) {
      // Try parse as JSON into HashMap
      try {
        HashMap<String, Long> map = JSON_MAPPER.readerFor(HashMap.class).readValue((String) serializedHistogram);
        SpectatorHistogram histogram = new SpectatorHistogram();
        for (Map.Entry<String, Long> entry : map.entrySet()) {
          histogram.add(entry.getKey(), entry.getValue());
        }
        return histogram;
      }
      catch (JsonProcessingException e) {
        throw new ParseException((String) serializedHistogram, e, "String cannot be deserialized as JSON to a Spectator Histogram");
      }
    }
    if (serializedHistogram instanceof HashMap) {
      SpectatorHistogram histogram = new SpectatorHistogram();
      for (Map.Entry<?, ?> entry : ((HashMap<?, ?>) serializedHistogram).entrySet()) {
        histogram.add(entry.getKey(), (Number) entry.getValue());
      }
      return histogram;
    }
    throw new ParseException(
        null,
        "Object cannot be deserialized to a Spectator Histogram "
        + serializedHistogram.getClass()
    );
  }

  @Nullable
  static SpectatorHistogram fromByteBuffer(ByteBuffer buffer)

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