apache/hadoop · error · IOException

wrong key class: {} is not {}

Error message

wrong key class: {} is not {}

What it means

Error "wrong key class: {} is not {}" thrown in apache/hadoop.

Source

Thrown at hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapred/IFile.java:192

      }

      compressedBytesWritten = rawOut.getPos() - start;

      if (compressOutput) {
        // Return back the compressor
        CodecPool.returnCompressor(compressor);
        compressor = null;
      }

      out = null;
      if(writtenRecordsCounter != null) {
        writtenRecordsCounter.increment(numRecordsWritten);
      }
    }

    public void append(K key, V value) throws IOException {
      if (key.getClass() != keyClass)
        throw new IOException("wrong key class: "+ key.getClass()
                              +" is not "+ keyClass);
      if (value.getClass() != valueClass)
        throw new IOException("wrong value class: "+ value.getClass()
                              +" is not "+ valueClass);

      // Append the 'key'
      keySerializer.serialize(key);
      int keyLength = buffer.getLength();
      if (keyLength < 0) {
        throw new IOException("Negative key-length not allowed: " + keyLength + 
                              " for " + key);
      }

      // Append the 'value'
      valueSerializer.serialize(value);
      int valueLength = buffer.getLength() - keyLength;
      if (valueLength < 0) {
        throw new IOException("Negative value-length not allowed: " + 

View on GitHub (pinned to 2add963021)

Solutions

1. Configure the map output key class (mapreduce.map.output.key.class) to match the key class the IFile writer/reader was created with.
2. Check that the SerializationFactory accepts the configured key class.

When it happens

Trigger: Thrown at hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapred/IFile.java:192 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of apache/hadoop@2add963021 (2026-08-22). Data as JSON: /api/errors/af8696a0754b61bc. Report an issue: GitHub.