我希望使用parquet mr库以parquet文件格式编写mapreduce输出，如下所示：

job.setInputFormatClass(TextInputFormat.class); 
        job.setOutputFormatClass(ParquetOutputFormat.class);

        FileInputFormat.addInputPath(job, new Path(args[1]));
        ParquetOutputFormat.setOutputPath(job, new Path(args[2]));
        ParquetOutputFormat.setCompression(job, CompressionCodecName.GZIP);

        SkipBadRecords.setMapperMaxSkipRecords(conf, Long.MAX_VALUE);
        SkipBadRecords.setAttemptsToStartSkipping(conf, 0);

        job.submit();

但是，我经常会遇到这样的错误：

2018-02-23 09:32:58,325 WARN [main] org.apache.hadoop.mapred.YarnChild: Exception running child : java.lang.NullPointerException: writeSupportClass should not be null
    at org.apache.parquet.Preconditions.checkNotNull(Preconditions.java:38)
    at org.apache.parquet.hadoop.ParquetOutputFormat.getWriteSupport(ParquetOutputFormat.java:350)
    at org.apache.parquet.hadoop.ParquetOutputFormat.getRecordWriter(ParquetOutputFormat.java:293)
    at org.apache.parquet.hadoop.ParquetOutputFormat.getRecordWriter(ParquetOutputFormat.java:283)
    at org.apache.hadoop.mapred.ReduceTask$NewTrackingRecordWriter.<init>(ReduceTask.java:548)
    at org.apache.hadoop.mapred.ReduceTask.runNewReducer(ReduceTask.java:622)
    at org.apache.hadoop.mapred.ReduceTask.run(ReduceTask.java:390)
    at org.apache.hadoop.mapred.YarnChild$2.run(YarnChild.java:164)

我知道writesupportclass需要像这样传递/设置

ParquetOutputFormat.setWriteSupportClass(job, ProtoWriteSupport.class);

但是我能问一下如何指定schema，实现protowritesupport或者其他writesupport类吗？我需要实现什么方法？有没有正确实现的例子？
如果有帮助的话，我的mr job的输出应该看起来像&以Parquet格式存储：

Text      INTWRITABLE
 a            100