目标：统计文件中指定字符串出现的次数，如计算f1.txt中“voidxin”出现了几次。
1、maper类如下：

public class VXWordCountMapper extends Mapper<LongWritable,Text , Text, Text> {
    protected void map(LongWritable key, Text value, Context context)
            throws IOException, InterruptedException {
        String line = value.toString();
        String keyStr = key.toString();
        context.write(new Text(keyStr), new Text(line));
    }

}

2、reducer类的实现如下：

public class VXWordCountReducer extends Reducer<Text, Text, Text, IntWritable> {
    protected void reduce(Text key, Iterable<Text> values, Context context)
            throws IOException, InterruptedException {
        int count = 0;
        
        String targetStr = "voidxin";
        for (Text textWritable : values) {
            
            @SuppressWarnings("resource")
            Scanner scan = new Scanner(textWritable.toString()).useDelimiter(" |,|\\?|\\.|'");
            while(scan.hasNext()) {
                if(scan.next().equals(targetStr)) {
                    count ++;
                }
            }
            
        }

        context.write(key, new IntWritable(count));

    }

}

3、main函数实现如下：

public class VXWordCount {
    public static void main(String[] args) throws IOException, ClassNotFoundException, InterruptedException {
        Job job = new Job();
        job.setJarByClass(VXWordCount.class);
        job.setJobName("word count");

        FileInputFormat.addInputPath(job, new Path(args[0]));
        FileOutputFormat.setOutputPath(job, new Path(args[1]));
        
        job.setMapperClass(VXWordCountMapper.class);
        job.setMapOutputKeyClass(Text.class);
        job.setMapOutputValueClass(Text.class);

        job.setReducerClass(VXWordCountReducer.class);
       
        // 等待作业完成
        System.out.println(job.waitForCompletion(true));
    }
}

最后导出jar包，运行：
1:从mac本地拷贝到linux虚拟机系统指定的文件夹中，之后的命令输入都是基于在testfile目录下的

cp ***.jar /home/testfile 

2:编辑需要统计的文件，vi f1.txt

ni zhi dao wo shi shui ma ? my name is voidxin 
voidxin do you know?
wo shi shui,ni shishui a 
are you,voidxin'voidxin.
hahahahahahahahah
nishi

3:把f1.txt上传,

hadoop fs -put f1.txt  /home/input

4：运行(注意输出目录之前不能存在)

hadoop jar ***.jar /home/input/f1/txt  /home/wordcountDemo 

QQ20170616-2.png

5:结果如下、

QQ20170616-1.png

其中最要的核心代码应该就这几行：

for (Text textWritable : values) {
            
            @SuppressWarnings("resource")
            Scanner scan = new Scanner(textWritable.toString()).useDelimiter(" |,|\\?|\\.|'");
            while(scan.hasNext()) {
                if(scan.next().equals(targetStr)) {
                    count ++;
                }
            }
            
        }

demo地址请前往：https://github.com/voidxin/WordCountDemo