使用自定义可写时为空的DataInput

如何解决使用自定义可写时为空的DataInput

运行作业时，我遇到了hadoop map reduce框架引发的怪异错误。我有一个自定义的可写密钥，这是读/写方法：

    @Override
    public void write(DataOutput out) throws IOException {
        if (_bytes == null) {
            throw new NullPointerException();
        }

        if (_bytes.length != 18) {
            throw new IllegalArgumentException("bytes length " + _bytes.length);
        }

        out.write(_bytes);
    }

    @Override
    public void readFields(DataInput in) throws IOException {
        _bytes = new byte[18];
        DataInputBuffer dis = (DataInputBuffer) in;
        int pos = dis.getPosition();
        int len = dis.getLength();
        int read = dis.read(_bytes);
        if (read != 18) {
            String msg =
                "couldn't read from buffer size " +
                dis.getData().length +
                "; pos=" +
                pos +
                "; len=" +
                len +
                "; read=" +
                read;
            throw new IOException(msg);
        }

我在日志中收到的错误消息是：

java.lang.RuntimeException: java.io.IOException: couldn't read from buffer len 419430400; pos=0; len=-1435735686; read=-1
    at org.apache.hadoop.io.WritableComparator.compare(WritableComparator.java:165)
    at org.apache.hadoop.mapred.MapTask$MapOutputBuffer.compare(MapTask.java:1269)
    at org.apache.hadoop.util.QuickSort.sortInternal(QuickSort.java:99)
    at org.apache.hadoop.util.QuickSort.sort(QuickSort.java:63)
    at org.apache.hadoop.mapred.MapTask$MapOutputBuffer.sortAndSpill(MapTask.java:1597)
    at org.apache.hadoop.mapred.MapTask$MapOutputBuffer.flush(MapTask.java:1486)
    at org.apache.hadoop.mapred.MapTask$NewOutputCollector.close(MapTask.java:723)
    at org.apache.hadoop.mapred.MapTask.closeQuietly(MapTask.java:2016)
    at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:797)
    at org.apache.hadoop.mapred.MapTask.run(MapTask.java:341)
    at org.apache.hadoop.mapred.YarnChild$2.run(YarnChild.java:164)
    at java.security.AccessController.doPrivileged(Native Method)
    at javax.security.auth.Subject.doAs(Subject.java:422)
    at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1693)
    at org.apache.hadoop.mapred.YarnChild.main(YarnChild.java:158)
Caused by: java.io.IOException: couldn't read from buffer size 419430400; pos=0; len=-1435735686; read=-1
    at my.custom.Writable.readFields(Writable.java:101)
    at org.apache.hadoop.io.WritableComparator.compare(WritableComparator.java:158)
    ... 14 more

对DataInputBuffer的强制转换纯粹是出于调试目的，我以前使用过in.readFully，但得到了java.io.EOFException。我不知所措。在我看来，我的代码只能写长度为byte[]的{​​{1}}，但是出于某种原因hadoop试图使用长度为负的DataInputBuffer调用readFields？我们正在使用CDH 5.8.0发行版，我相信它会映射到hadoop 2.6.0。