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如果没有给Job设置Mapper和Reducer，则将会使用默认的Mapper和Reducer。默认的Mapper和Reducer什么都不会做，只是进行文件的快速复制，如【代码4-8】所示。此代码中，并没有设置Mapper和Reducer类，将会使用默认的Mapper和Reducer类。
【代码4-8】DefaultMR.java
1. package org.hadoop.dft;
2. public class DefaultMR extends Configured implements Tool {
3. @Override
4. public int run(String[] args) throws Exception {
5. if(args.length!=2){
6. System.out.println("usage : <in> <out>");
7. return -1;
8. }
9. Configuration conf = getConf();
10. Job job = Job.getInstance(conf,"DefaultMR");
11. job.setJarByClass(this.getClass());
12. FileSystem fs = FileSystem.get(conf);
13. if(fs.exists(new Path(args[1]))){
14. fs.delete(new Path(args[1]),true);
15. }
16. FileInputFormat.setInputPaths(job,new Path(args[0]));
17. FileOutputFormat.setOutputPath(job,new Path(args[1]));
18. int code = job.waitForCompletion(true)?0:1;
19. return code;
20. }
21. public static void main(String[] args) throws Exception {
22. int code = ToolRunner.run(new DefaultMR(),args);
23. System.exit(code);
24. }
25. }
26.