1 概述
Hadoop内置的输入文件格式类有：
1）FileInputFormat<K,V>这个是基本的父类，自定义就直接使用它作为父类。
2）TextInputFormat<LongWritable,Text>这个是默认的数据格式类。key代表当前行数据距离文件开始的距离，value代码当前行字符串。
3）SequenceFileInputFormat<K,V>这个是序列文件输入格式，使用序列文件可以提高效率，但是不利于查看结果，建议在过程中使用序列文件，最后展示可以使用可视化输出。
4）KeyValueTextInputFormat<Text,Text>这个是读取以Tab（也即是\t）分隔的数据，每行数据如果以\t分隔，那么使用这个读入，就可以自动把\t前面的当做key，后面的当做value。
5）CombineFileInputFormat<K,V>合并大量小数据是使用。
6）MultipleInputs，多种输入，可以为每个输入指定逻辑处理的Mapper。
2 运行轨迹
2.1 Mapper
public void run(Context context) throws IOException, InterruptedException {
setup(context);
try {
while (context.nextKeyValue()) {
map(context.getCurrentKey(), context.getCurrentValue(), context);
}
} finally {
cleanup(context);
}
}进入context.nextKeyValue()方法，从而进入WrappedMapper类。
2.2 WrappedMapper
public boolean nextKeyValue() throws IOException, InterruptedException{
return mapContext.nextKeyValue();
}进入该方法的nextKeyValue()，从而进入MapContextImpl类。
2.3 MapContextImpl
public boolean nextKeyValue() throws IOException, InterruptedException {
return reader.nextKeyValue();
}现希望知道reader具体类型是什么，先看reader的申明和赋值。
public class MapContextImpl<KEYIN,VALUEIN,KEYOUT,VALUEOUT>
extends TaskInputOutputContextImpl<KEYIN,VALUEIN,KEYOUT,VALUEOUT>
implements MapContext<KEYIN, VALUEIN, KEYOUT, VALUEOUT> {
private RecordReader<KEYIN,VALUEIN> reader;
private InputSplit split;
public MapContextImpl(Configuration conf, TaskAttemptID taskid,
RecordReader<KEYIN,VALUEIN> reader,
RecordWriter<KEYOUT,VALUEOUT> writer,
OutputCommitter committer,
StatusReporter reporter,
InputSplit split) {
super(conf, taskid, writer, committer, reporter);
this.reader = reader;
this.split = split;
}
}此处看到是调用MapContextImpl构造方法进行赋值的，那么继续跟进看何处调用了MapContextImpl方法。右击MapContextImpl > open call Hierarchy。跟进一个方法叫runNewMapper可以看到，一步步看变量申明，就可以看到inputFormat就是我们代码中设置的InputFormat.class类型。
3 自定义InputFormat
基于文件的FileInputFormat的设计思想是：
A 由公共基类FileInputFormat采用统一的方法，对文件进行切分成InputSplit（如按照统一的大小）。getSplit方法。
B 由各个派生类根据自己的需求，解析InputSplit。即各个子类实现的createRecordReader方法。
那么Input只需实现自定义createRecordReader方法即可。
3.1 MyInputFormat
import java.io.IOException;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.InputSplit;
import org.apache.hadoop.mapreduce.RecordReader;
import org.apache.hadoop.mapreduce.TaskAttemptContext;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;;
public class MyInputFormat extends FileInputFormat<Text, Text> {
@Override
public RecordReader<Text, Text> createRecordReader(InputSplit split,
TaskAttemptContext context) throws IOException,
InterruptedException {
return new MyRecordReader();
}
}3.2 MyRecordReader
import java.io.IOException;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FSDataInputStream;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.InputSplit;
import org.apache.hadoop.mapreduce.RecordReader;
import org.apache.hadoop.mapreduce.TaskAttemptContext;
import org.apache.hadoop.mapreduce.lib.input.FileSplit;
import org.apache.hadoop.util.LineReader;
public class MyRecordReader extends RecordReader<Text, Text> {
private LineReader lr;
private Text key = new Text();
private Text value = new Text();
private long start;
private long end;
private long currentPos;
private Text line = new Text();
@Override
public void initialize(InputSplit inputSplit, TaskAttemptContext cxt)
throws IOException, InterruptedException {
FileSplit split = (FileSplit) inputSplit;
Configuration conf = cxt.getConfiguration();
// 获取分片文件对应的完整文件
Path path = split.getPath();
FileSystem fs = path.getFileSystem(conf);
FSDataInputStream is = fs.open(path);
lr = new LineReader(is, conf);
// 获取分片文件的启始位置
start = split.getStart();
// 获取分片文件的结束位置
end = start + split.getLength();
is.seek(start);
if (start != 0) {
start += lr.readLine(new Text(), 0,
(int) Math.min(Integer.MAX_VALUE, end - start));
}
currentPos = start;
}
// 针对每行数据进行处理
@Override
public boolean nextKeyValue() throws IOException, InterruptedException{
if (currentPos > end) {
return false;
}
currentPos += lr.readLine(line);
if (line.getLength() == 0) {
return false;
}
// 若是需要被忽略的行，直接读下一行
if (line.toString().startsWith("ignore")) {
currentPos += lr.readLine(line);
}
String[] words = line.toString().split(",");
if (words.length < 2) {
System.err.println("line:" + line.toString() + ".");
return false;
}
key.set(words[0]);
value.set(words[1]);
return true;
}
@Override
public Text getCurrentKey() throws IOException, InterruptedException {
return key;
}
@Override
public Text getCurrentValue() throws IOException, InterruptedException{
return value;
}
@Override
public float getProgress() throws IOException, InterruptedException {
if (start == end) {
return 0.0f;
} else {
return Math.min(1.0f, (currentPos - start) / (float) (end - start));
}
}
@Override
public void close() throws IOException {
lr.close();
}
} 3.3 TestFormat
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.conf.Configured;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.Mapper;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
import org.apache.hadoop.mapreduce.lib.output.TextOutputFormat;
import org.apache.hadoop.util.Tool;
import org.apache.hadoop.util.ToolRunner;
public class TestFormat extends Configured implements Tool {
public static void main(String[] args) throws Exception {
ToolRunner.run(new Configuration(), new TestFormat(), args);
}
@Override
public int run(String[] args) throws Exception {
Configuration conf = getConf();
String inPath = "hdfs://192.XXX.XXX.XXX:9000/test/bigFile.txt";
String outPath = "hdfs://192.XXX.XXX.XXX:9000/test/out/";
Path in = new Path(inPath);
Path out = new Path(outPath);
out.getFileSystem(conf).delete(out, true);
Job job = Job.getInstance(conf, "fileintputformat test job");
job.setJarByClass(getClass());
job.setInputFormatClass(MyInputFormat.class);
job.setOutputFormatClass(TextOutputFormat.class);
job.setMapperClass(Mapper.class);
job.setMapOutputKeyClass(Text.class);
job.setMapOutputValueClass(Text.class);
job.setNumReduceTasks(0);
FileInputFormat.setInputPaths(job, in);
FileOutputFormat.setOutputPath(job, out);
return job.waitForCompletion(true) ? 0 : -1;
}
} 参考地址：http://www.cnblogs.com/hyl8218/p/5198030.html
