Java tutorial
/** * Licensed to the Apache Software Foundation (ASF) under one * or more contributor license agreements. See the NOTICE file * distributed with this work for additional information * regarding copyright ownership. The ASF licenses this file * to you under the Apache License, Version 2.0 (the * "License"); you may not use this file except in compliance * with the License. You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ package org.apache.hadoop.mapreduce.lib.input; import java.io.IOException; import java.util.ArrayList; import java.util.List; import org.apache.hadoop.classification.InterfaceAudience; import org.apache.hadoop.classification.InterfaceStability; import org.apache.hadoop.conf.Configuration; import org.apache.hadoop.fs.FSDataInputStream; import org.apache.hadoop.fs.FileStatus; import org.apache.hadoop.fs.FileSystem; import org.apache.hadoop.fs.Path; import org.apache.hadoop.io.LongWritable; import org.apache.hadoop.io.Text; import org.apache.hadoop.mapreduce.InputSplit; import org.apache.hadoop.mapreduce.Job; import org.apache.hadoop.mapreduce.JobContext; import org.apache.hadoop.mapreduce.RecordReader; import org.apache.hadoop.mapreduce.TaskAttemptContext; import org.apache.hadoop.util.LineReader; /** * NLineInputFormat which splits N lines of input as one split. * * In many "pleasantly" parallel applications, each process/mapper * processes the same input file (s), but with computations are * controlled by different parameters.(Referred to as "parameter sweeps"). * One way to achieve this, is to specify a set of parameters * (one set per line) as input in a control file * (which is the input path to the map-reduce application, * where as the input dataset is specified * via a config variable in JobConf.). * * The NLineInputFormat can be used in such applications, that splits * the input file such that by default, one line is fed as * a value to one map task, and key is the offset. * i.e. (k,v) is (LongWritable, Text). * The location hints will span the whole mapred cluster. */ @InterfaceAudience.Public @InterfaceStability.Stable public class NLineInputFormat extends FileInputFormat<LongWritable, Text> { public static final String LINES_PER_MAP = "mapreduce.input.lineinputformat.linespermap"; public RecordReader<LongWritable, Text> createRecordReader(InputSplit genericSplit, TaskAttemptContext context) throws IOException { context.setStatus(genericSplit.toString()); return new LineRecordReader(); } /** * Logically splits the set of input files for the job, splits N lines * of the input as one split. * * @see FileInputFormat#getSplits(JobContext) */ public List<InputSplit> getSplits(JobContext job) throws IOException { List<InputSplit> splits = new ArrayList<InputSplit>(); int numLinesPerSplit = getNumLinesPerSplit(job); for (FileStatus status : listStatus(job)) { splits.addAll(getSplitsForFile(status, job.getConfiguration(), numLinesPerSplit)); } return splits; } public static List<FileSplit> getSplitsForFile(FileStatus status, Configuration conf, int numLinesPerSplit) throws IOException { List<FileSplit> splits = new ArrayList<FileSplit>(); Path fileName = status.getPath(); if (status.isDirectory()) { throw new IOException("Not a file: " + fileName); } FileSystem fs = fileName.getFileSystem(conf); LineReader lr = null; try { FSDataInputStream in = fs.open(fileName); lr = new LineReader(in, conf); Text line = new Text(); int numLines = 0; long begin = 0; long length = 0; int num = -1; while ((num = lr.readLine(line)) > 0) { numLines++; length += num; if (numLines == numLinesPerSplit) { splits.add(createFileSplit(fileName, begin, length)); begin += length; length = 0; numLines = 0; } } if (numLines != 0) { splits.add(createFileSplit(fileName, begin, length)); } } finally { if (lr != null) { lr.close(); } } return splits; } /** * NLineInputFormat uses LineRecordReader, which always reads * (and consumes) at least one character out of its upper split * boundary. So to make sure that each mapper gets N lines, we * move back the upper split limits of each split * by one character here. * @param fileName Path of file * @param begin the position of the first byte in the file to process * @param length number of bytes in InputSplit * @return FileSplit */ protected static FileSplit createFileSplit(Path fileName, long begin, long length) { return (begin == 0) ? new FileSplit(fileName, begin, length - 1, new String[] {}) : new FileSplit(fileName, begin - 1, length, new String[] {}); } /** * Set the number of lines per split * @param job the job to modify * @param numLines the number of lines per split */ public static void setNumLinesPerSplit(Job job, int numLines) { job.getConfiguration().setInt(LINES_PER_MAP, numLines); } /** * Get the number of lines per split * @param job the job * @return the number of lines per split */ public static int getNumLinesPerSplit(JobContext job) { return job.getConfiguration().getInt(LINES_PER_MAP, 1); } }