📅 最后修改于: 2020-12-03 01:44:10 🧑 作者: Mango
在本节中，我们重点介绍MapReduce API。在这里，我们了解MapReduce编程中使用的类和方法。
在MapReduce中，Mapper类的作用是将输入键值对映射到一组中间键值对。它将输入记录转换为中间记录。
这些中间记录与给定的输出键相关联，并传递给Reducer作为最终输出。
| void cleanup(Context context) | This method called only once at the end of the task. |
| void map(KEYIN key, VALUEIN value, Context context) | This method can be called only once for each key-value in the input split. |
| void run(Context context) | This method can be override to control the execution of the Mapper. |
| void setup(Context context) | This method called only once at the beginning of the task. |
在MapReduce中，Reducer类的作用是减少中间值的集合。它的实现可以通过JobContext.getConfiguration()方法访问作业的Configuration。
| void cleanup(Context context) | This method called only once at the end of the task. |
| void map(KEYIN key, Iterable |
This method called only once for each key. |
| void run(Context context) | This method can be used to control the tasks of the Reducer. |
| void setup(Context context) | This method called only once at the beginning of the task. |
Job类用于配置作业并提交。它还控制执行和查询状态。提交作业后，set方法将引发IllegalStateException。
| Methods | Description |
|---|---|
| Counters getCounters() | This method is used to get the counters for the job. |
| long getFinishTime() | This method is used to get the finish time for the job. |
| Job getInstance() | This method is used to generate a new Job without any cluster. |
| Job getInstance(Configuration conf) | This method is used to generate a new Job without any cluster and provided configuration. |
| Job getInstance(Configuration conf, String jobName) | This method is used to generate a new Job without any cluster and provided configuration and job name. |
| String getJobFile() | This method is used to get the path of the submitted job configuration. |
| String getJobName() | This method is used to get the user-specified job name. |
| JobPriority getPriority() | This method is used to get the scheduling function of the job. |
| void setJarByClass(Class c) | This method is used to set the jar by providing the class name with .class extension. |
| void setJobName(String name) | This method is used to set the user-specified job name. |
| void setMapOutputKeyClass(Class class) | This method is used to set the key class for the map output data. |
| void setMapOutputValueClass(Class class) | This method is used to set the value class for the map output data. |
| void setMapperClass(Class class) | This method is used to set the Mapper for the job. |
| void setNumReduceTasks(int tasks) | This method is used to set the number of reduce tasks for the job |
| void setReducerClass(Class class) | This method is used to set the Reducer for the job. |