用户在使用Mapreduce时默认以part-＊命名，MultipleOutputs可以将不同的键值对输出到用户自定义的不同的文件中。
实现过程是在调用output.write(key, new IntWritable(total), key.toString());
方法时候第三个参数是 public void write(KEYOUT key, VALUEOUT value, String baseOutputPath) 指定了输出文件的命名前缀，那么我们可以通过对不同的key使用不同的baseOutputPath来使不同key对应的value输出到不同的文件中，比如将同一天的数据输出到以该日期命名的文件中
Hadoop技术内幕：深入解析MapReduce架构设计与实现原理 PDF高清扫描版 http://www.linuxidc.com/Linux/2014-06/103576.htm
测试数据：ip-to-hosts.txt
18.217.167.70 United States 206.96.54.107 United States 196.109.151.139 Mauritius 174.52.58.113 United States 142.111.216.8 Canada 162.100.49.185 United States 146.38.26.54 United States 36.35.107.36 China 95.214.95.13 Spain 2.96.191.111 United Kingdom 62.177.119.177 Czech Republic 21.165.189.3 United States 46.190.32.115 Greece 113.173.113.29 Vietnam 42.65.172.142 Taiwan 197.91.198.199 South Africa 68.165.71.27 United States 110.119.165.104 China 171.50.76.89 India 171.207.52.113 Singapore 40.174.30.170 United States 191.170.95.175 United States 17.81.129.101 United States 91.212.157.202 France 173.83.82.99 United States 129.75.56.220 United States 149.25.104.198 United States 103.110.22.19 Indonesia 204.188.117.122 United States 138.23.10.72 United States 172.50.15.32 United States 85.88.38.58 Belgium 49.15.14.6 India 19.84.175.5 United States 50.158.140.215 United States 161.114.120.34 United States 118.211.174.52 Australia 220.98.113.71 Japan 182.101.16.171 China 25.45.75.194 United Kingdom 168.16.162.99 United States 155.60.219.154 Australia 26.216.17.198 United States 68.34.157.157 United States 89.176.196.28 Czech Republic 173.11.51.134 United States 116.207.191.159 China 164.210.124.152 United States 168.17.158.38 United States 174.24.173.11 United States 143.64.173.176 United States 160.164.158.125 Italy 15.111.128.4 United States 22.71.176.163 United States 105.57.100.182 Morocco 111.147.83.42 China 137.157.65.89 Australia
该文件中每行数据有两个字段 分别是ip地址和该ip地址对应的国家，以t分隔
上代码
public static class IPCountryReducer extends Reducer<Text, IntWritable, Text, IntWritable> {
private MultipleOutputs output;
@Override protected void setup(Context context ) throws IOException, InterruptedException { output = new MultipleOutputs(context); }
@Override protected void reduce(Text key, Iterable<IntWritable> values, Context context ) throws IOException, InterruptedException { int total = 0; for(IntWritable value: values) { total = value.get(); } <span style="color:#FF0000;"> output.write(new Text("Output by MultipleOutputs"), NullWritable.get(), key.toString()); output.write(key, new IntWritable(total), key.toString());</span>
}
@Override protected void cleanup(Context context ) throws IOException, InterruptedException { output.close(); } }
在reduce的setup方法中
output = new MultipleOutputs(context);
然后在reduce中通过该output将内容输出到不同的文件中
private Configuration conf; public static final String NAME = "named_output";
public static void main(String[] args) throws Exception { args =new String[] {"hdfs://caozw:9100/user/hadoop/hadooprealword","hdfs://caozw:9100/user/hadoop/hadooprealword/output"}; ToolRunner.run(new Configuration(), new NamedCountryOutputJob(), args); }
public int run(String[] args) throws Exception { if(args.length != 2) { System.err.println("Usage: named_output <input> <output>"); System.exit(1); }
Job job = new Job(conf, "IP count by country to named files"); job.setInputFormatClass(TextInputFormat.class);
job.setMapperClass(IPCountryMapper.class); job.setReducerClass(IPCountryReducer.class);
job.setMapOutputKeyClass(Text.class); job.setMapOutputValueClass(IntWritable.class); job.setJarByClass(NamedCountryOutputJob.class);
FileInputFormat.addInputPath(job, new Path(args[0])); FileOutputFormat.setOutputPath(job, new Path(args[1]));
return job.waitForCompletion(true) ? 1 : 0;
}
public void setConf(Configuration conf) { this.conf = conf; }
public Configuration getConf() { return conf; }
public static class IPCountryMapper extends Mapper<LongWritable, Text, Text, IntWritable> {
private static final int country_pos = 1; private static final Pattern pattern = Pattern.compile("\t");
@Override protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException { String country = pattern.split(value.toString())[country_pos]; context.write(new Text(country), new IntWritable(1)); } }
测试结果：
