最近感受了hive的udf函式的強大威力了，不僅可以使用很多已經有的udf函式，還可以自己定義符合業務場景的udf函式，下面就說一下如何寫udf/udaf/udtf函式，算是一個入門介紹吧。

First, you need to create a new class that extends UDF, with one or more methods named evaluate.

package com.example.hive.udf;

import org.apache.hadoop.hive.ql.exec.UDF;

import org.apache.hadoop.io.Text;

public final class Lower extends UDF {

public Text evaluate(final Text s) {

if (s == null) { return null; }

return new Text(s.toString().toLowerCase());

}

}

After compiling your code to a jar, you need to add this to the hive classpath.

add jar my_jar.jar;

Once hive is started up with your jars in the classpath, the final step is to register your function

create temporary function my_lower as "com.example.hive.udf.Lower";