apache/hadoop · error · IllegalArgumentException

map cannot be copied: {}

Error message

map cannot be copied: {}

What it means

AbstractMapWritable.copy(Writable) implements copying by serializing 'other' into a buffer and readFields-ing it back into 'this'. Any IOException from that write/read round-trip is rethrown as IllegalArgumentException('map cannot be copied: <io message>'). Note only the underlying message survives - the IOException's stack trace is dropped, which makes diagnosis harder.

Source

Thrown at hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/io/AbstractMapWritable.java:135

  protected byte getId(Class<?> clazz) {
    return classToIdMap.containsKey(clazz) ? classToIdMap.get(clazz) : -1;
  }

  /**
   * Used by child copy constructors.
   * @param other other.
   */
  protected synchronized void copy(Writable other) {
    if (other != null) {
      try {
        DataOutputBuffer out = new DataOutputBuffer();
        other.write(out);
        DataInputBuffer in = new DataInputBuffer();
        in.reset(out.getData(), out.getLength());
        readFields(in);

      } catch (IOException e) {
        throw new IllegalArgumentException("map cannot be copied: " +
            e.getMessage());
      }

    } else {
      throw new IllegalArgumentException("source map cannot be null");
    }
  }

  /** constructor. */
  protected AbstractMapWritable() {
    this.conf = new AtomicReference<Configuration>();

    addToMap(ArrayWritable.class, (byte)-127);
    addToMap(BooleanWritable.class, (byte)-126);
    addToMap(BytesWritable.class, (byte)-125);
    addToMap(FloatWritable.class, (byte)-124);
    addToMap(IntWritable.class, (byte)-123);
    addToMap(LongWritable.class, (byte)-122);

View on GitHub (pinned to 2add963021)

Solutions

1. Check the appended message text - it is the original IOException's message and usually names the real problem (often a class-not-registered or id conflict)
2. Ensure source and target maps share the same registration table (same subclass/version on both sides)
3. Test other.write(new DataOutputBuffer()) in isolation to identify the failing entry

// verify the source serializes cleanly before copying
try {
  DataOutputBuffer buf = new DataOutputBuffer();
  other.write(buf);
} catch (IOException e) {
  throw new IllegalArgumentException("source map not serializable", e);
}

try {
  target.copyFrom(source);
} catch (IllegalArgumentException e) {
  if (e.getMessage().startsWith("map cannot be copied")) {
    // the suffix is the original IOException message; fix that underlying issue
  }
}

When it happens

Trigger: new MapWritable(otherMap) or copyFrom(other) where an entry's write() fails: a value that is not Writable-serializable in this context, inconsistent registration tables between the two maps (see the class/id conflicts), or a value whose write throws.

Common situations: Copy constructors fed maps produced by a different version of the subclass; entries whose classes are not visible to this map's registrations.

Related errors

• Class {} already registered but maps to {} and not {}
• Id {} exists but maps to {} and not {}
• adding an additional class would exceed the maximum number a
• source map cannot be null
• Exception while get content summary

AI-assisted analysis of apache/hadoop@2add963021 (2026-08-22). Data as JSON: /api/errors/fa2098e3e42246c0. Report an issue: GitHub.