apache/iceberg · error · UnsupportedOperationException

readFields is not supported.

Error message

readFields is not supported.

What it means

Container.readFields(DataInput) is intentionally unsupported: containers are populated directly by the Iceberg record reader rather than by Writable deserialization, so calling readFields always throws UnsupportedOperationException.

Solutions

1. Do not use Container as the value class for shuffle/intermediate output; use a serializable Writable type instead.
2. Read Iceberg data through the provided input format which populates Containers itself, and copy values out for downstream stages.
3. Switch to the mapreduce IcebergInputFormat if you need framework-managed serialization.

Example fix

// before
Container<Record> value = new Container<>();
value.readFields(in); // throws
// after
MyWritable value = new MyWritable();
value.readFields(in);

if (Container.class.equals(job.getOutputValueClass())) {
  throw new IllegalArgumentException("Container cannot be a reducer input value");
}

boolean supportsReadFields(Class<?> c) {
  return Writable.class.isAssignableFrom(c)
      && !org.apache.iceberg.mr.mapred.Container.class.isAssignableFrom(c);
}

try { value.readFields(in); }
catch (UnsupportedOperationException e) { throw new IOException("Container is read-only; use a writable value type", e); }

When it happens

Trigger: A MapReduce framework path that attempts to deserialize values of type Container from DataInput (e.g. using Container as reducer input value class or in a sequence file read).

Common situations: Pipelined MR jobs using Iceberg's Container as intermediate value type; frameworks that call readFields reflectively on Writable value classes.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• write is not supported.
• Cannot modify a static table
• Cannot set fields in a TypeProjection
• Cannot update InternalRecordWrapper
• Could not set a field in the RowDataWrapper because rowData…

AI-assisted analysis of apache/iceberg@86d9c8fc54 (2026-09-12). Data as JSON: /api/errors/88cc5cd58523f986. Report an issue: GitHub.

Appendix: source

Thrown at mr/src/main/java/org/apache/iceberg/mr/mapred/Container.java:49

  private T value;

  public T get() {
    return value;
  }

  public void set(T newValue) {
    this.value = newValue;
  }

  @Override
  public void write(DataOutput dataOutput) {
    throw new UnsupportedOperationException("write is not supported.");
  }

  @Override
  public void readFields(DataInput dataInput) {
    throw new UnsupportedOperationException("readFields is not supported.");
  }
}

View on GitHub (pinned to 86d9c8fc54)