hadoop - 如何从 Hadoop 序列文件中提取数据？

Question

Hadoop序列文件真是奇怪。我将图像打包到序列文件中，无法恢复图像。我做了一些简单的测试。而且我发现使用序列文件前后字节大小甚至不一样。

Configuration confHadoop = new Configuration();
        FileSystem fs = FileSystem.get(confHadoop);

        String fileName = args[0];
        Path file = new Path(fs.getUri().toString() + "/" + fileName);
        Path seqFile = new Path("/temp.seq");
        SequenceFile.Writer writer = null;
        FSDataInputStream in = null;
        try{
            writer = SequenceFile.createWriter(confHadoop,Writer.file(seqFile), Writer.keyClass(Text.class),
                    Writer.valueClass(BytesWritable.class));

            in = fs.open(file);
            byte buffer[] = IOUtils.toByteArray(in);


            System.out.println("original size ---->  " + String.valueOf(buffer.length));
            writer.append(new Text(fileName), new BytesWritable(buffer));
            System.out.println(calculateMd5(buffer));
            writer.close();

        }finally{
            IOUtils.closeQuietly(in);
        }

        SequenceFile.Reader reader = new SequenceFile.Reader(confHadoop, Reader.file(seqFile));

        Text key = new Text();
        BytesWritable val = new BytesWritable();

        while (reader.next(key, val)) {
            System.out.println("size get from sequence file --->" + String.valueOf(val.getLength()));
            String md5 = calculateMd5(val.getBytes());
            Path readSeq=new Path("/write back.png");  
            FSDataOutputStream out = null;
            out = fs.create(readSeq);
            out.write(val.getBytes());  //YES! GOT THE ORIGIANL IAMGE
            out.close();
            System.out.println(md5);
            .............
}

输出显示我得到了相同数量的字节，并且在我将图像写回本地磁盘后，我确定我得到了原始图像。但是为什么MD5值不一样呢？

我在这里做错了什么？

14/04/22 16:21:35 INFO compress.CodecPool: Got brand-new compressor [.deflate]
original size ---->  485709
c413e36fd864b27d4c8927956298edbb
14/04/22 16:21:35 INFO compress.CodecPool: Got brand-new decompressor [.deflate]
size get from sequence file --->485709
322cce20b732126bcb8876c4fcd925cb

Answer 1

终于解决了这个奇怪的问题，不得不分享一下。首先，我将向您展示从序列中获取字节的错误方法。

Configuration conf = new Configuration();
FileSystem fs = FileSystem.get(conf);
Path input = new Path(inPath);
Reader reader = new SequenceFile.Reader(conf, Reader.file(input));
Text key = new Text();

BytesWritable val = new BytesWritable();
    while (reader.next(key, val)) {
    fileName = key.toString();
    byte[] data = val.getBytes(); //don't think you have got the data!
}

原因是 getBytes() 没有返回原始数据的准确大小。我把数据放在using中

FSDataInputStream in = null;
in = fs.open(input);
byte[] buffer = IOUtils.toByteArray(in);

Writer writer = SequenceFile.createWriter(conf,
Writer.file(output), Writer.keyClass(Text.class),
Writer.valueClass(BytesWritable.class));

writer.append(new Text(inPath), new BytesWritable(buffer));
writer.close();

我检查了输出序列文件的大小，它是原始大小加上头部，我不确定 getBytes() 给我的字节数比原来多的原因。但是让我们看看如何正确获取数据。

选项 #1，复制您需要的数据大小。

byte[] rawdata = val.getBytes();
length = val.getLength(); //exactly size of original data
byte[] data = Arrays.copyOfRange(rawdata,  0, length); this is corrent

Option #2

byte[] data = val.copyBytes();

这个更甜。 :)终于搞定了。