原理
我们知道在创建一个mapreduce任务的时候,需要指定Inputformat的类型,我们就从这里入手了解一个mr任务中,是如何读取数据的,首先来看一个具体的inputformat的子类源码(hadoop2.6版本源码)
#segment_1
package org.apache.hadoop.mapreduce.lib.input;
import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.io.compress.CompressionCodec;
import org.apache.hadoop.io.compress.CompressionCodecFactory;
import org.apache.hadoop.io.compress.SplittableCompressionCodec;
import org.apache.hadoop.mapreduce.InputFormat;
import org.apache.hadoop.mapreduce.InputSplit;
import org.apache.hadoop.mapreduce.JobContext;
import org.apache.hadoop.mapreduce.RecordReader;
import org.apache.hadoop.mapreduce.TaskAttemptContext;
import com.google.common.base.Charsets;
/** An {@link InputFormat} for plain text files. Files are broken into lines.
* Either linefeed or carriage-return are used to signal end of line. Keys are
* the position in the file, and values are the line of text.. */
@InterfaceAudience.Public
@InterfaceStability.Stable
public class TextInputFormat extends FileInputFormat<LongWritable, Text> {
@Override
public RecordReader<LongWritable, Text>
createRecordReader(InputSplit split,
TaskAttemptContext context) {
String delimiter = context.getConfiguration().get(
"textinputformat.record.delimiter");
byte[] recordDelimiterBytes = null;
if (null != delimiter)
recordDelimiterBytes = delimiter.getBytes(Charsets.UTF_8);
//注意这里,返回一个recordReader对象,但此时并不用InputSplit对象来初始化它,
return new LineRecordReader(recordDelimiterBytes);
}
}
那么了解LineRecordReader则是最关键的,首先看原理图:
上图中,split是对hdfs文件的分片,里面封装了关于分片的大小,以及超始信息,路径信息等;
block就是hdfs中的文件分块;而readReader则是上面介绍的recordReader,它根据split中的分片信息读取数据;
所以分析的流程就是:生成分片信息—->处理分片信息;
生成分片信息
segment_1源码处,TextInputFormat 继承了FileInputFormat,FileInputFormat里面实现了对split分片信息的获取,源码如下:
package org.apache.hadoop.mapreduce.lib.input;
public abstract class FileInputFormat<K, V> extends InputFormat<K, V> {
//配置分片最小量
public static final String SPLIT_MINSIZ