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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;
}
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> {
booleannext(K key, V value) throws IOException;KcreateKey();
VcreateValue();
longgetPos() throws IOException;
public voidclose() throws IOException;
floatgetProgress() 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~
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