apache/hadoop · error · RuntimeException

readObject can't find class

Error message

readObject can't find class

What it means

Error "readObject can't find class" thrown in apache/hadoop.

Source

Thrown at hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapreduce/lib/input/TaggedInputSplit.java:136

  @SuppressWarnings("unchecked")
  public void readFields(DataInput in) throws IOException {
    inputSplitClass = (Class<? extends InputSplit>) readClass(in);
    inputFormatClass = (Class<? extends InputFormat<?, ?>>) readClass(in);
    mapperClass = (Class<? extends Mapper<?, ?, ?, ?>>) readClass(in);
    inputSplit = (InputSplit) ReflectionUtils
       .newInstance(inputSplitClass, conf);
    SerializationFactory factory = new SerializationFactory(conf);
    Deserializer deserializer = factory.getDeserializer(inputSplitClass);
    deserializer.open((DataInputStream)in);
    inputSplit = (InputSplit)deserializer.deserialize(inputSplit);
  }

  private Class<?> readClass(DataInput in) throws IOException {
    String className = StringInterner.weakIntern(Text.readString(in));
    try {
      return conf.getClassByName(className);
    } catch (ClassNotFoundException e) {
      throw new RuntimeException("readObject can't find class", e);
    }
  }

  @SuppressWarnings("unchecked")
  public void write(DataOutput out) throws IOException {
    Text.writeString(out, inputSplitClass.getName());
    Text.writeString(out, inputFormatClass.getName());
    Text.writeString(out, mapperClass.getName());
    SerializationFactory factory = new SerializationFactory(conf);
    Serializer serializer = 
          factory.getSerializer(inputSplitClass);
    serializer.open((DataOutputStream)out);
    serializer.serialize(inputSplit);
  }

  public Configuration getConf() {
    return conf;
  }

View on GitHub (pinned to 2add963021)

Solutions

1. Ensure the InputSplit's stored classes are on the task classpath; ship the required jars via distributed cache or libjars.

When it happens

Trigger: Thrown at hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapreduce/lib/input/TaggedInputSplit.java:136 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/d278190c9a1f5dee. Report an issue: GitHub.