hadoop SequenceFile 读取例程注释

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 import java.io.IOException;
 import java.net.URI;

 import org.apache.hadoop.conf.Configuration;
 import org.apache.hadoop.fs.FileSystem;
 import org.apache.hadoop.fs.Path;
 import org.apache.hadoop.io.IOUtils;
 import org.apache.hadoop.io.SequenceFile;
 import org.apache.hadoop.io.Writable;
 import org.apache.hadoop.util.ReflectionUtils;

 // vv SequenceFileReadDemo
 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);//返回 SequenceFile.Reader 对象
       Writable key = (Writable)
         ReflectionUtils.newInstance(reader.getKeyClass(), conf);//getKeyClass()获得Sequence中使用的类型
       Writable value = (Writable)//ReflectionUtils.newInstace 得到常见的键值的实例
         ReflectionUtils.newInstance(reader.getValueClass(), conf);//同上
       long position = reader.getPosition();
       while (reader.next(key, value)) { //next（）方法迭代读取记录 直到读完返回false
         String syncSeen = reader.syncSeen() ? "*" : "";//替换特殊字符 同步
         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);
     }
   }
 }