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。