mapreduce: InputFormat详解 -- RecordReader篇

InputFormat是MapReduce中一个很常用的概念，它在程序的运行中到底起到了什么作用呢？

InputFormat其实是一个接口，包含了两个方法：

public interface InputFormat<K, V> {
  InputSplit[] getSplits(JobConf job, int numSplits) throws IOException;
  RecordReader<K, V> getRecordReader(InputSplit split,
                                     JobConf job,
                                     Reporter reporter) throws IOException;
}

这两个方法有分别完成着以下工作：

      方法getSplits将输入数据切分成splits，splits的个数即为map tasks的个数，splits的大小默认为块大小，即64M

     方法getSplits将每个split解析成records, 再依次将record解析成<K,V>对

也就是说InputFormat完成以下工作：

 InputFile --> splits --> <K,V>

 

系统常用的 InputFormat 又有哪些呢？

                      

其中TextInputFormat便是最常用的，它的<K,V>就代表<行偏移,该行内容>

 

然而系统所提供的这几种固定的将 InputFile转换为<K,V>的方式有时候并不能满足我们的需求：

此时需要我们自定义 InputFormat ，从而使Hadoop框架按照我们预设的方式来将

InputFile解析为<K,V>

在领会自定义 InputFormat 之前，需要弄懂一下几个抽象类、接口及其之间的关系：

 

InputFormat(interface), FileInputFormat(abstract class), TextInputFormat(class),

RecordReader(interface), LineRecordReader(class)的关系

     FileInputFormat implements InputFormat

     TextInputFormat extends FileInputFormat

     TextInputFormat.getRecordReader calls LineRecordReader

     LineRecordReader implements RecordReader

 

对于InputFormat接口，上面已经有详细的描述

再看看FileInputFormat，它实现了InputFormat接口中的getSplits方法，而将getRecordReader与isSplitable留给具体类(如TextInputFormat)实现，isSplitable方法通常不用修改，所以只需要在自定义的InputFormat中实现

getRecordReader方法即可，而该方法的核心是调用LineRecordReader(即由LineRecorderReader类来实现 "将每个split解析成records, 再依次将record解析成<K,V>对")，该方法实现了接口RecordReader

 

  public interface RecordReader<K, V> {

boolean next(K key, V value) throws IOException;
  K createKey();
  V createValue();
  long getPos() throws IOException;
  public void close() throws IOException;
  float getProgress() throws IOException;
}

 

     因此自定义InputFormat的核心是自定义一个实现接口RecordReader类似于LineRecordReader的类，该类的核心也正是重写接口RecordReader中的几大方法，

     定义一个InputFormat的核心是定义一个类似于LineRecordReader的，自己的RecordReader

     示例，数据每一行为 “物体，x坐标，y坐标，z坐标”

   ball 3.5,12.7,9.0

   car 15,23.76,42.23

   device 0.0,12.4,-67.1

每一行将要被解析为<Text, Point3D>（Point3D是我们在上一篇日志中自定义的数据类型）

 

方式一，自定义的RecordReader使用中LineRecordReader，

 

  public class ObjectPositionInputFormat extends
     FileInputFormat<Text, Point3D> {

   public RecordReader<Text, Point3D> getRecordReader(
       InputSplit input, JobConf job, Reporter reporter)
       throws IOException {

     reporter.setStatus(input.toString());
     return new ObjPosRecordReader(job, (FileSplit)input);
   }
 }
 class ObjPosRecordReader implements RecordReader<Text, Point3D> {
   private LineRecordReader lineReader;
   private LongWritable lineKey;
   private Text lineValue;

   public ObjPosRecordReader(JobConf job, FileSplit split) throws IOException {
     lineReader = new LineRecordReader(job, split);

     lineKey = lineReader.createKey();
     lineValue = lineReader.createValue();
   }

   public boolean next(Text key, Point3D value) throws IOException {
     // get the next line
     if (!lineReader.next(lineKey, lineValue)) {
       return false;
     }

     // parse the lineValue which is in the format:
     // objName, x, y, z
     String [] pieces = lineValue.toString().split(",");
     if (pieces.length != 4) {
       throw new IOException("Invalid record received");
     }

     // try to parse floating point components of value
     float fx, fy, fz;
     try {
       fx = Float.parseFloat(pieces[1].trim());
       fy = Float.parseFloat(pieces[2].trim());
       fz = Float.parseFloat(pieces[3].trim());
     } catch (NumberFormatException nfe) {
       throw new IOException("Error parsing floating point value in record");
     }
    // now that we know we'll succeed, overwrite the output objects
     key.set(pieces[0].trim()); // objName is the output key.
     value.x = fx;
     value.y = fy;
     value.z = fz;
     return true;
   }
   public Text createKey() {
     return new Text("");
   }
   public Point3D createValue() {
     return new Point3D();
   }
   public long getPos() throws IOException {
     return lineReader.getPos();
   }
   public void close() throws IOException {
     lineReader.close();
   }
 public float getProgress() throws IOException {
     return lineReader.getProgress();
   }
 }

 

方式二：自定义的RecordReader中使用LineReader，

 public class ObjectPositionInputFormat extends FileInputFormat<Text, Point3D> {
     @Override
     protected boolean isSplitable(JobContext context, Path filename) {
         // TODO Auto-generated method stub
         return false;
     }
     @Override
     public RecordReader<Text, Point3D> createRecordReader(InputSplit inputsplit,
             TaskAttemptContext context) throws IOException, InterruptedException {
         // TODO Auto-generated method stub
         return new objPosRecordReader();
     }
     public static class objPosRecordReader extends RecordReader<Text,Point3D>{

         public LineReader in;
         public Text lineKey;
         public Point3D lineValue;
         public StringTokenizer token=null;
         public Text line;
         @Override
         public void close() throws IOException {
             // TODO Auto-generated method stub
         }
      @Override
         public Text getCurrentKey() throws IOException, InterruptedException {
             // TODO Auto-generated method stub
             System.out.println("key");
             //lineKey.set(token.nextToken());
             System.out.println("hello");
             return lineKey;
         }
         @Override
         public Point3D getCurrentValue() throws IOException,
                 InterruptedException {
             // TODO Auto-generated method stub
             return lineValue;
         }
         @Override
         public float getProgress() throws IOException, InterruptedException {
             // TODO Auto-generated method stub
             return 0;
         }
         @Override
         public void initialize(InputSplit input, TaskAttemptContext context)
                 throws IOException, InterruptedException {
             // TODO Auto-generated method stub
             FileSplit split=(FileSplit)input;
             Configuration job=context.getConfiguration();
             Path file=split.getPath();
             FileSystem fs=file.getFileSystem(job);

             FSDataInputStream filein=fs.open(file);
             in=new LineReader(filein,job);

             line=new Text();
             lineKey=new Text();
             lineValue=new Point3D();
         }
         @Override
         public boolean nextKeyValue() throws IOException, InterruptedException {
             // TODO Auto-generated method stub
             int linesize=in.readLine(line);
             if(linesize==0)
                 return false;
                token=new StringTokenizer(line.toString());
             String []temp=new String[2];
             if(token.hasMoreElements()){
                 temp[0]=token.nextToken();
                 if(token.hasMoreElements()){
                     temp[1]=token.nextToken();
                 }
             }
             System.out.println(temp[0]);
             System.out.println(temp[1]);
             String []points=temp[1].split(",");
             System.out.println(points[0]);
             System.out.println(points[1]);
             System.out.println(points[2]);
             lineKey.set(temp[0]);
             lineValue.set(Float.parseFloat(points[0]),Float.parseFloat(points[1]), Float.parseFloat(points[2]));
             System.out.println("pp");
             return true;
         }
     }
 }

 

     从以上可以看出，自定义一个InputFormat的核心是定义一个类似于LineRecordReader的，自己的RecordReader，而在其中可能会到LineReader/LineRecordReader/KeyValueLineRecordReader类

     因此，要自定义InputFormat，这三个类的源码就必须很熟悉~