【hadoop】在eclipse上运行WordCount的操作过程

序：本以为今天花点时间将WordCount例子完全理解到，但高估自己了，更别说我只是在大学选修一学期的java，之后再也没碰过java语言了

总的来说，从宏观上能理解具体的程序思路，但具体到每个代码有什么作用，什么原理，那还需要花点时间，毕竟需要一点java基础和hadoop的运行机制的知识

首先启动hadoop；

[hadoop@hadoop01 eclipse]$ cd ~/hadoop-3.2.0
[hadoop@hadoop01 hadoop-3.2.0]$ sbin/start-all.sh
WARNING: Attempting to start all Apache Hadoop daemons as hadoop in 10 seconds.
WARNING: This is not a recommended production deployment configuration.
WARNING: Use CTRL-C to abort.
Starting namenodes on [hadoop01]
Starting datanodes
Starting secondary namenodes [hadoop01]
Starting resourcemanager
Starting nodemanagers
[hadoop@hadoop01 hadoop-3.2.0]$ jps
8497 NameNode
9121 ResourceManager
8868 SecondaryNameNode
9268 NodeManager
9630 Jps

然后，进入root权限打开eclipse；

[hadoop@hadoop01 hadoop-3.2.0]$ su root
Password:
[root@hadoop01 hadoop-3.2.0]# cd ..
[root@hadoop01 hadoop]# cd eclipse
[root@hadoop01 eclipse]# ./eclipse

在eclipse的window里面show view打开terminal；

在eclipse中点击打开open a terminal，在终端中输入命令：gedit input.txt；

在文档中任意输入内容；

在终端中输入命令：hadoop fs -put /home/hadoop/input.txt /test/；

最后，file--new--project--MapReduce project并取项目名“Wordcount”，再从创建的文件下src中new--package并为包取名“com.hadoop”，又在src下new--class并为类取名“Wordcount”，然后将下面的代码粘贴进去。

然后可以run as hadoop，成功运行得到计算结果

注：若package下无log4j.properties，会报错，需在该文件下手动添加该文件。
内容 如下：

# Configure logging for testing: optionally with log file

#log4j.rootLogger=debug,appender
log4j.rootLogger=info,appender
#log4j.rootLogger=error,appender

#\u8F93\u51FA\u5230\u63A7\u5236\u53F0
log4j.appender.appender=org.apache.log4j.ConsoleAppender
#\u6837\u5F0F\u4E3ATTCCLayout
log4j.appender.appender.layout=org.apache.log4j.TTCCLayout

附代码：

/**
 * 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.examples;

import java.io.IOException;
import java.util.StringTokenizer;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.Mapper;
import org.apache.hadoop.mapreduce.Reducer;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
import org.apache.hadoop.util.GenericOptionsParser;

public class WordCount {

  public static class TokenizerMapper
       extends Mapper<Object, Text, Text, IntWritable>{

    private final static IntWritable one = new IntWritable(1);
    private Text word = new Text();

    public void map(Object key, Text value, Context context
                    ) throws IOException, InterruptedException {
      StringTokenizer itr = new StringTokenizer(value.toString());
      while (itr.hasMoreTokens()) {
        word.set(itr.nextToken());
        context.write(word, one);
      }
    }
  }

  public static class IntSumReducer
       extends Reducer<Text,IntWritable,Text,IntWritable> {
    private IntWritable result = new IntWritable();

    public void reduce(Text key, Iterable<IntWritable> values,
                       Context context
                       ) throws IOException, InterruptedException {
      int sum = 0;
      for (IntWritable val : values) {
        sum += val.get();
      }
      result.set(sum);
      context.write(key, result);
    }
  }

  public static void main(String[] args) throws Exception {
    Configuration conf = new Configuration();
    String[] otherArgs = new GenericOptionsParser(conf, args).getRemainingArgs();
    if (otherArgs.length < 2) {
      System.err.println("Usage: wordcount <in> [<in>...] <out>");
      System.exit(2);
    }
    Job job = Job.getInstance(conf, "word count");
    job.setJarByClass(WordCount.class);
    job.setMapperClass(TokenizerMapper.class);
    job.setCombinerClass(IntSumReducer.class);
    job.setReducerClass(IntSumReducer.class);
    job.setOutputKeyClass(Text.class);
    job.setOutputValueClass(IntWritable.class);
    for (int i = 0; i < otherArgs.length - 1; ++i) {
      FileInputFormat.addInputPath(job, new Path(otherArgs[i]));
    }
    FileOutputFormat.setOutputPath(job,
      new Path(otherArgs[otherArgs.length - 1]));
    System.exit(job.waitForCompletion(true) ? 0 : 1);
  }
}