Reducer类阅读

Reducer类中主要有setup,run,reduce,cleanup这几个函数。

/**
   * Called once at the start of the task.
   */
  protected void setup(Context context
                       ) throws IOException, InterruptedException {
    // NOTHING
  }

/**
   * This method is called once for each key. Most applications will define
   * their reduce class by overriding this method. The default implementation
   * is an identity function.
   */
  @SuppressWarnings("unchecked")
  protected void reduce(KEYIN key, Iterable<VALUEIN> values, Context context
                        ) throws IOException, InterruptedException {
    for(VALUEIN value: values) {
      context.write((KEYOUT) key, (VALUEOUT) value);
    }
  }

      /**
       * Called once at the end of the task.
       */
      protected void cleanup(Context context
                             ) throws IOException, InterruptedException {
        // NOTHING
      }

在用户的应用程序中调用到reducer时，会直接调用reducer里面的run函数，其代码如下：

    /*
       * control how the reduce task works.
       */
      @SuppressWarnings("unchecked")
      public void run(Context context) throws IOException, InterruptedException {
        setup(context);
        while (context.nextKey()) {
          reduce(context.getCurrentKey(), context.getValues(), context);
          // If a back up store is used, reset it
          ((ReduceContext.ValueIterator)
              (context.getValues().iterator())).resetBackupStore();
        }
        cleanup(context);
      }
    }

从上边的程序可以看出在执行的时候先执行setup函数，最后执行cleanup函数。