hive自定义函数分为三类：

• UDF（User-Defined-Function）普通函数， 一进一出

• UDAF（User-Defined Aggregation Function）聚合函数，多进一出

• UDTF（User-Defined Table-Generating Functions）表生成函数， 一进多出

UDF实例

• 新建子模块hive_demo

• 配置pom.xml文件

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>cn.hive</groupId>
    <artifactId>hive-udf</artifactId>
    <version>1.0</version>
    <dependencies>
        <dependency>
            <groupId>org.apache.hive</groupId>
            <artifactId>hive-exec</artifactId>
            <version>1.2.1</version>
        </dependency>
        <dependency>
            <groupId>org.apache.hadoop</groupId>
            <artifactId>hadoop-common</artifactId>
            <version>2.7.4</version>
        </dependency>
    </dependencies>
    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-shade-plugin</artifactId>
                <version>2.2</version>
                <executions>
                    <execution>
                        <phase>package</phase>
                        <goals>
                            <goal>shade</goal>
                        </goals>
                        <configuration>
                                <filters>
                                    <filter>
                                        <artifact>*:*</artifact>
                                        <excludes>
                                            <exclude>META-INF/*.SF</exclude>
                                            <exclude>META-INF/*.DSA</exclude>
                                            <exclude>META-INF/*.RSA</exclude>
                                        </excludes>
                                    </filter>
                                </filters>
                        </configuration>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>
</project>

编写一个java类，继承UDF，并重载evaluate方法。

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

public class Lower extends UDF{
    public Text   evaluate(Text s){
        if(s==null){
            return null;
        }
        return new Text(s.toString().toLowerCase());
    }
}   

• 打成jar包上传到服务器

• 将jar包添加到hive的classpath

hive>add JAR /home/hadoop/udf.jar;

• 创建临时函数与开发好的java class关联

create temporary function tolowercase as 'cn.itcast.cloud.hive.demo.udf.CustomUDF';

• 即可在hql中使用自定义的函数tolowercase ip

Select tolowercase(name),age from t_test;