Hive 中SerDe概述

一、背景

1、当进程在进行远程通信时，彼此可以发送各种类型的数据，无论是什么类型的数据都会以二进制序列的形式在网络上传送。发送方需要把对象转化为字节序列才可在网络上传输，称为对象序列化；接收方则需要把字节序列恢复为对象，称为对象的反序列化。

2、Hive的反序列化是对key/value反序列化成hive table的每个列的值。

3、Hive可以方便的将数据加载到表中而不需要对数据进行转换，这样在处理海量数据时可以节省大量的时间。

二、技术细节

1、SerDe是Serialize/Deserilize的简称，目的是用于序列化和反序列化。

2、用户在建表时可以用自定义的SerDe或使用Hive自带的SerDe，SerDe能为表指定列，且对列指定相应的数据。

CREATE [EXTERNAL] TABLE [IF NOT EXISTS] table_name

[(col_name data_type [COMMENT col_comment], ...)]

[COMMENT table_comment]

[PARTITIONED BY (col_name data_type

[COMMENT col_comment], ...)]

[CLUSTERED BY (col_name, col_name, ...)

[SORTED BY (col_name [ASC|DESC], ...)]

INTO num_buckets BUCKETS]

[ROW FORMAT row_format]

[STORED AS file_format]

[LOCATION hdfs_path]

创建指定SerDe表时，使用row format row_format参数，例如：

a、添加jar包。在hive客户端输入：hive>add jar /run/serde_test.jar;

或者在linux shell端执行命令：${HIVE_HOME}/bin/hive -auxpath /run/serde_test.jar

b、建表：create table serde_table row format serde 'hive.connect.TestDeserializer';

3、编写序列化类TestDeserializer。实现Deserializer接口的三个函数：

a）初始化：initialize(Configuration conf, Properties tb1)。

b）反序列化Writable类型返回Object:deserialize(Writable blob)。

c）获取deserialize(Writable blob)返回值Object的inspector:getObjectInspector()。

public interface Deserializer {

/**

* Initialize the HiveDeserializer.

* @param conf System properties

* @param tbl table properties

* @throws SerDeException

*/

public void initialize(Configuration conf, Properties tbl) throws SerDeException;

/**

* Deserialize an object out of a Writable blob.

* In most cases, the return value of this function will be constant since the function

* will reuse the returned object.

* If the client wants to keep a copy of the object, the client needs to clone the

* returned value by calling ObjectInspectorUtils.getStandardObject().

* @param blob The Writable object containing a serialized object

* @return A Java object representing the contents in the blob.

*/

public Object deserialize(Writable blob) throws SerDeException;

/**

* Get the object inspector that can be used to navigate through the internal

* structure of the Object returned from deserialize(...).

*/

public ObjectInspector getObjectInspector() throws SerDeException;

}

实现一行数据划分成hive表的time,userid,host,path四个字段的反序列化类。例如：

package hive.connect;

import java.net.MalformedURLException;

import java.net.URL;

import java.util.ArrayList;

import java.util.List;

import java.util.Properties;

import org.apache.hadoop.conf.Configuration;

import org.apache.hadoop.hive.serde2.Deserializer;

import org.apache.hadoop.hive.serde2.SerDeException;

import org.apache.hadoop.hive.serde2.objectinspector.ObjectInspector;

import org.apache.hadoop.hive.serde2.objectinspector.ObjectInspectorFactory;

import org.apache.hadoop.hive.serde2.objectinspector.-

ObjectInspectorFactory.ObjectInspectorOptions;

import org.apache.hadoop.io.Text;

import org.apache.hadoop.io.Writable;

public class TestDeserializer implements Deserializer {

private static List FieldNames = new ArrayList();

private static List FieldNamesObjectInspectors = new ArrayList();

static {

FieldNames.add("time");

FieldNamesObjectInspectors.add(ObjectInspectorFactory

.getReflectionObjectInspector(Long.class,

ObjectInspectorOptions.JAVA));

FieldNames.add("userid");

FieldNamesObjectInspectors.add(ObjectInspectorFactory

.getReflectionObjectInspector(Integer.class,

ObjectInspectorOptions.JAVA));

FieldNames.add("host");

FieldNamesObjectInspectors.add(ObjectInspectorFactory

.getReflectionObjectInspector(String.class,

ObjectInspectorOptions.JAVA));

FieldNames.add("path");

FieldNamesObjectInspectors.add(ObjectInspectorFactory

.getReflectionObjectInspector(String.class,

ObjectInspectorOptions.JAVA));

}

@Override

public Object deserialize(Writable blob) {

try {

if (blob instanceof Text) {

String line = ((Text) blob).toString();

if (line == null)

return null;

String[] field = line.split("\t");

if (field.length != 3) {

return null;

}

List