apache/hadoop · error · IOException

can't find class

Error message

can't find class 

What it means

Error "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/mapred/MapTask.java:738

            return partitions - 1;
          }
        };
      }
    }

    @Override
    public void write(K key, V value) throws IOException, InterruptedException {
      collector.collect(key, value,
                        partitioner.getPartition(key, value, partitions));
    }

    @Override
    public void close(TaskAttemptContext context
                      ) throws IOException,InterruptedException {
      try {
        collector.flush();
      } catch (ClassNotFoundException cnf) {
        throw new IOException("can't find class ", cnf);
      }
      collector.close();
    }
  }

  @SuppressWarnings("unchecked")
  private <INKEY,INVALUE,OUTKEY,OUTVALUE>
  void runNewMapper(final JobConf job,
                    final TaskSplitIndex splitIndex,
                    final TaskUmbilicalProtocol umbilical,
                    TaskReporter reporter
                    ) throws IOException, ClassNotFoundException,
                             InterruptedException {
    // make a task context so we can get the classes
    org.apache.hadoop.mapreduce.TaskAttemptContext taskContext =
      new org.apache.hadoop.mapreduce.task.TaskAttemptContextImpl(job, 
                                                                  getTaskID(),
                                                                  reporter);

View on GitHub (pinned to 2add963021)

Solutions

1. Ensure the class being deserialized is on the task classpath; ship 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/mapred/MapTask.java:738 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/641281d40f422898. Report an issue: GitHub.