Hadoop HDFS SequenceFile

SequenceFile 是一个由二进制序列化过的 key/value 的字节流组成的文本存储文件，它可以在map/reduce过程中的input/output 的format时被使用。在map/reduce过程中，map处理文件的临时输出就是使用SequenceFile处理过的。 所以一般的SequenceFile均是在FileSystem中生成，供map调用的原始文件。

SequenceFile 也可用于处理小文件：如果为key小文件名，value为文件内容，则可以将大批小文件合并成一个大文件。

public   class   SequenceFileWriteDemo {

        public   static   void  main(String[] args)  throws  IOException {

               //  TODO  Auto-generated method stub

            String uri =  "file:///Repositories/workspace/Demo/hadoop1/target/seq.sq"   ;

            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());

                  key.set(1);

                  value.set(uri);

                  writer.append(key, value);

            }  finally {

                  IOUtils. closeStream(writer);

            }

      }

}

public   class   SequenceFileReadDemo {

        public   static   void  main(String[] args)  throws  IOException {

               //  TODO  Auto-generated method stub

            String uri =  "file:///Repositories/workspace/Demo/hadoop1/target/seq.sq"   ;

            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);

            }

      }

}

压缩类型， 由SequenceFile类的内部枚举类CompressionType来表示，定义了三种方式

不采用压缩: CompressionType.NONE

记录级别的压缩: CompressionType.RECORD

块级别的压缩: CompressionType.BLOCK

使用时可以通过参数: io.seqfile.compression.type=[NONE|RECORD|BLOCK] 来指定具体的压缩方式  

可以通过下面命令查看生成的文件内容：

hadoop fs -text  numbers.seq

sequence 文件由一个head 和多个 record 组成。

Head头3个字节为SEQ，跟着1个字节表示版本。然后包括