apache/hadoop · error · IOException

wrong value class: is not

Error message

wrong value class: {} is not {}

What it means

The value-side twin of the key check in Writer.append: val.getClass() must be exactly (==) the declared valClass or append throws this IOException naming both classes. The check is per record, so a single bad value stops the write before the record header is emitted.

Solutions

1. Match Writer.valueClass(...) to the concrete appended type
2. Convert or copy subclass values into the exact declared type before append
3. Add a guard: val.getClass() == writer.getValueClass() before each append in generic plumbing

Example fix

// before
Writer w = SequenceFile.createWriter(conf, Writer.file(p),
    Writer.keyClass(Text.class), Writer.valueClass(IntWritable.class));
w.append(new Text("k"), new Text("not-an-int")); // wrong value class

// after
w.append(new Text("k"), new IntWritable(7));

if (val.getClass() != w.getValueClass()) {
  throw new IllegalArgumentException("value " + val.getClass() + " != declared " + w.getValueClass());
}

static boolean isExactValueClass(SequenceFile.Writer w, Object val) {
  return val != null && val.getClass() == w.getValueClass();
}

try {
  w.append(key, val);
} catch (IOException e) {
  if (e.getMessage() != null && e.getMessage().startsWith("wrong value class")) {
    // value type drift: convert to the declared class or recreate the writer
  } else { throw e; }
}

When it happens

Trigger: Appending a value whose concrete class differs from Writer.valueClass(...): a subclass instance, a type from another library version, or null-ish placeholders with the wrong type after refactoring.

Common situations: Value schema changed in code but not in the writer options; mixed collections feeding append with heterogeneous elements; copy-paste writers reused across data types.

Related errors

• wrong key class: is not
• A record version mismatch occurred. Expecting v
• Compression option provided does not match the file
• Key/value class provided does not match the file
• negative length keys not allowed

AI-assisted analysis of apache/hadoop@2add963021 (2026-08-22). Data as JSON: /api/errors/f1461fcfedcbafb4. Report an issue: GitHub.

Appendix: source

Thrown at hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/io/SequenceFile.java:1476

    public void append(Writable key, Writable val)
      throws IOException {
      append((Object) key, (Object) val);
    }

    /**
     * Append a key/value pair.
     * @param key input Object key.
     * @param val input Object val.
     * @throws IOException raised on errors performing I/O.
     */
    @SuppressWarnings("unchecked")
    public synchronized void append(Object key, Object val)
      throws IOException {
      if (key.getClass() != keyClass)
        throw new IOException("wrong key class: "+key.getClass().getName()
                              +" is not "+keyClass);
      if (val.getClass() != valClass)
        throw new IOException("wrong value class: "+val.getClass().getName()
                              +" is not "+valClass);

      buffer.reset();

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

      // Append the 'value'
      if (compress == CompressionType.RECORD) {
        deflateFilter.resetState();
        compressedValSerializer.serialize(val);
        deflateOut.flush();
        deflateFilter.finish();
      } else {
        uncompressedValSerializer.serialize(val);

View on GitHub (pinned to 2add963021)