MapReduce – Mapper

曾经终败给现在 2022-11-14 13:17 8阅读 0赞

Mapper有setup()，map()，cleanup()和run()四个方法。  
1.setup()一般是用来进行一些map()前的准备工作。  
2.map()则一般承担主要的处理工作。  
3.cleanup()则是收尾工作如关闭文件或者执行map()后的K-V分发等。  
4.run()方法提供了setup->map->cleanup()的执行模板。

>     //
>     // Source code recreated from a .class file by IntelliJ IDEA
>     // (powered by FernFlower decompiler)
>     //
>     
>     package org.apache.hadoop.mapreduce;
>     
>     import java.io.IOException;
>     import org.apache.hadoop.classification.InterfaceAudience.Public;
>     import org.apache.hadoop.classification.InterfaceStability.Stable;
> 
>     @Public
>     @Stable
>     public class Mapper<KEYIN, VALUEIN, KEYOUT, VALUEOUT> {
>         public Mapper() {
>         }
>     
>         /**
>          * 预处理，仅在map task启动时运行一次
>          * @param context
>          * @throws IOException
>          * @throws InterruptedException
>          */
>         protected void setup(Mapper<KEYIN, VALUEIN, KEYOUT, VALUEOUT>.Context context) throws IOException, InterruptedException {
>         }
>     
>         /**
>          * 对于InputSplit中的每一对<key, value>都会运行一次
>          * @param key
>          * @param value
>          * @param context
>          * @throws IOException
>          * @throws InterruptedException
>          */
>         protected void map(KEYIN key, VALUEIN value, Mapper<KEYIN, VALUEIN, KEYOUT, VALUEOUT>.Context context) throws IOException, InterruptedException {
>             context.write(key, value);
>         }
>     
>         /**
>          * 扫尾工作，比如关闭流等
>          * @param context
>          * @throws IOException
>          * @throws InterruptedException
>          */
>         protected void cleanup(Mapper<KEYIN, VALUEIN, KEYOUT, VALUEOUT>.Context context) throws IOException, InterruptedException {
>         }
>     
>         /**
>          * map task的驱动器
>          * @param context
>          * @throws IOException
>          * @throws InterruptedException
>          */
>         public void run(Mapper<KEYIN, VALUEIN, KEYOUT, VALUEOUT>.Context context) throws IOException, InterruptedException {
>             this.setup(context);
>     
>             try {
>                 while(context.nextKeyValue()) {
>                     this.map(context.getCurrentKey(), context.getCurrentValue(), context);
>                 }
>             } finally {
>                 this.cleanup(context);
>             }
>     
>         }
>     
>         public abstract class Context implements MapContext<KEYIN, VALUEIN, KEYOUT, VALUEOUT> {
>             public Context() {
>             }
>         }
>     }