Hadoop学习记录（五、hadoop IO操作）

1.压缩从标准输入读取的数据，然后将其写到标准输出

通过GzipCodec的StreamCompressor对象对字符串“Text”进行压缩，再使用gunzip从标准输出中对它进行读取并解压缩

public class StreamCompressor {
    public static void main(String[] args) throws Exception {
        String codecClassname = args[0];
        Class<?> codecClass = Class.forName(codecClassname);
        Configuration conf = new Configuration();
        CompressionCodec codec = (CompressionCodec)
                ReflectionUtils.newInstance(codecClass, conf);

        CompressionOutputStream out = codec.createOutputStream(System.out);
        IOUtils.copyBytes(System.in, out, 4096, false);
        out.finish();
    }
}

2.根据文件扩展名选取codec解压缩文件 

public class FileDecompressor {

  public static void main(String[] args) throws Exception {
    String uri = args[0];
    Configuration conf = new Configuration();
    FileSystem fs = FileSystem.get(URI.create(uri), conf);
    
    Path inputPath = new Path(uri);
    CompressionCodecFactory factory = new CompressionCodecFactory(conf);
    CompressionCodec codec = factory.getCodec(inputPath);
    if (codec == null) {
      System.err.println("No codec found for " + uri);
      System.exit(1);
    }

    String outputUri =
      CompressionCodecFactory.removeSuffix(uri, codec.getDefaultExtension());

    InputStream in = null;
    OutputStream out = null;
    try {
      in = codec.createInputStream(fs.open(inputPath));
      out = fs.create(new Path(outputUri));
      IOUtils.copyBytes(in, out, conf);
    } finally {
      IOUtils.closeStream(in);
      IOUtils.closeStream(out);
    }
  }
}

3. 使用压缩池对读取自标准输入数据进行压缩，然后将其写在标准输出里

public class PooledStreamCompressor {
    public static void main(String[] args) throws Exception {
        String codecClassname = args[0];
        Class<?> codecClass = Class.forName(codecClassname);
        Configuration conf = new Configuration();
        CompressionCodec codec = (CompressionCodec)
                ReflectionUtils.newInstance(codecClass, conf);
        Compressor compressor = null;
        try {
            compressor = CodecPool.getCompressor(codec);
            CompressionOutputStream out =
                    codec.createOutputStream(System.out, compressor);
            IOUtils.copyBytes(System.in, out, 4096, false);
            out.finish();
        } finally {
            CodecPool.returnCompressor(compressor);
        }
    }
}

4.在MapReduce中使用压缩

对输出进行压缩

./hadoop jar /tmp/hadoop-1.0-SNAPSHOT.jar HotSearch /input/IAMSinger.txt /output/

5.使用SequenceFile对小文件进行读取 

public class SequenceFileWriteDemo {
  private static final String[] DATA = {
    "One, two, buckle my shoe",
    "Three, four, shut the door",
    "Five, six, pick up sticks",
    "Seven, eight, lay them straight",
    "Nine, ten, a big fat hen"
  };
  
  public static void main(String[] args) throws IOException {
    String uri = args[0];
    Configuration conf = new Configuration();
    FileSystem fs = FileSystem.get(URI.create(uri), conf);
    Path path = new Path(uri);

    IntWritable key = new IntWritable();
    Text value = new Text();
    SequenceFile.Writer writer = null;
    try {
      writer = SequenceFile.createWriter(fs, conf, path,
          key.getClass(), value.getClass());
      
      for (int i = 0; i < 100; i++) {
        key.set(100 - i);
        value.set(DATA[i % DATA.length]);
        System.out.printf("[%s]\t%s\t%s\n", writer.getLength(), key, value);
        writer.append(key, value);
      }
    } finally {
      IOUtils.closeStream(writer);
    }
  }
}

public class SequenceFileReadDemo {
  public static void main(String[] args) throws IOException {
    String uri = args[0];
    Configuration conf = new Configuration();
    FileSystem fs = FileSystem.get(URI.create(uri), conf);
    Path path = new Path(uri);

    SequenceFile.Reader reader = null;
    try {
      reader = new SequenceFile.Reader(fs, path, conf);
      Writable key = (Writable)
        ReflectionUtils.newInstance(reader.getKeyClass(), conf);
      Writable value = (Writable)
        ReflectionUtils.newInstance(reader.getValueClass(), conf);
      long position = reader.getPosition();
      while (reader.next(key, value)) {
        String syncSeen = reader.syncSeen() ? "*" : "";
        System.out.printf("[%s%s]\t%s\t%s\n", position, syncSeen, key, value);
        position = reader.getPosition(); // beginning of next record
      }
    } finally {
      IOUtils.closeStream(reader);
    }
  }
}