Mapper与Reducer的链接

书上说的不清晰透彻，下面是在StackOverflow上的一个方案，我觉得很好：
 
(1) Cascading jobs
 
Create the JobConf object "job1" for the first job and set all the parameters with "input" as inputdirectory and "temp" as output directory. Execute this job:

 

  

   
JobClient.run(job1);
 
Immediately below it, create the JobConf object "job2" for the second job and set all the parameters with "temp" as inputdirectory and "output" as output directory. Execute this job: 
 

  

   
JobClient.run(job2);
 
 
(2) Create two JobConf objects and set all the parameters in them just like (1) except that you don't use JobClient.run.
 
Then create two Job objects with jobconfs as parameters:
 

  

   
Job job1=new Job(jobconf1); 
Job job2=new Job(jobconf2);
 
 
Using the jobControl object, you specify the job dependencies and then run the jobs: 
 

  

   
JobControl jbcntrl=new JobControl("jbcntrl"); 
jbcntrl.addJob(job1); 
jbcntrl.addJob(job2); 
job2.addDependingJob(job1); 
jbcntrl.run();
 
 
(3) If you need a structure somewhat like Map+ | Reduce | Map*, you can use the ChainMapper and ChainReducer classes that come with Hadoop version 0.19 and onwards. Note that in this case, u can use only one reducer but any number of mappers before or after it.
 
下面对这段话进行一定的展开。
 
（注：以下标为原文是为了在日记中进行突出显示，非原文字句，请作者及读者见谅，如果存在版权问题请指出~）
 

  MapReduce的Map与Reduce进行链接可能存在多重关系，按照本书的说明，主要考虑四种：A、顺序级联，类似与Unix/Linux管道：Map1/Reduce1 -> Map2/Reduce2 -> ...B、具有比较复杂的依赖关系，如：Map1/Reduce1 && Map2/Reduce2 -> Map3/Reduce3，倒立的树形结构；C、与预处理及后阶段处理相关的组合/链接：Map1-> Map2-> Map3 -> Reduce -> Map4 -> Map5D、
 
 
对于A，这个简单，因为Hadoop已经提供了JobClient这个类，我们只需要构建多个Job就行，注意使用JobClient的runJob(JobConf)方法，别一不小心用了submitJob(JobConf)，文档的说明是：
 
runJob(JobConf) : submits the job and returns only after the job has completed.
 
submitJob(JobConf) : only submits the job, then poll the returned handle to the RunningJob to query status and make scheduling decisions.
 
runJob会等到job执行结束才返回。于是可以很简洁的写出类似下面的代码：
 

  

   
/* some configuration code goes here */
Job job1=new Job(jobconf1); 
JobClient.run(job1);
// ...
Job job2=new Job(jobconf2);
JobClient.run(job2);
 
 
对于B，先贴张图说明一下到底是啥：
 

  
  

  
 
(说明：本图片来自http://www.cnblogs.com/xuqiang/archive/2011/06/05/2073155.html，非常感谢)
 
这种纠结的问题就是JobControl的用武之处了，请见下面代码：
 

  

   
Job job1=new Job(jobconf1); 
Job job2=new Job(jobconf2);
Job job3=new Job(jobconf3); 
Job job4=new Job(jobconf4);
Job job5=new Job(jobconf5); 
JobControl jbcntrl=new JobControl("MyJobCtrl"); // 传入一个字符串作为名字
jbcntrl.addJob(job1); 
jbcntrl.addJob(job2); 
jbcntrl.addJob(job3); 
jbcntrl.addJob(job4); 
jbcntrl.addJob(job5); 
job2.addDependingJob(job1); // 表示job2依赖于job1，job1没有完成，job2就不会启动
job4.addDependingJob(job3);
job5.addDependingJob(job2);
job5.addDependingJob(job4);
jbcntrl.run();