本blog介绍如何读取Hbase中的数据并写入到HDFS分布式文件系统中。读取数据比较简单，我们借用上一篇【HBase入门基础教程】6、HBase之读取MapReduce数据写入HBase的hbase数据输出wordcount表作为本篇数据源的输入，编写Mapper函数，读取wordcount表中的数据填充到< key,value>，通过Reduce函数直接输出得到的结果即可。
开发环境硬件环境：CentOS 6.5 服务器4台（一台为Master节点，三台为Slave节点）
软件环境：Java 1.7.0_45、Eclipse Juno Service Release 2、Hadoop-1.2.1、hbase-0.94.20。
1）输入数据源：
上一篇【HBase入门基础教程】6、HBase之读取MapReduce数据写入HBase实现了读取MapReduce数据写入到Hbase表wordcount中，在本篇blog中，我们将wordcount表作为输入数据源。
2）输出目标：
HDFS分布式文件系统中的文件。
2、 Mapper函数实现WordCountHbaseReaderMapper类继承了TableMapper< Text,Text>抽象类，TableMapper类专门用于完成MapReduce中Map过程与Hbase表之间的操作。此时的map(ImmutableBytesWritable key,Result value,Context context)方法，第一个参数key为Hbase表的rowkey主键，第二个参数value为key主键对应的记录集合，此处的map核心实现是遍历key主键对应的记录集合value，将其组合成一条记录通过contentx.write(key,value)填充到< key,value>键值对中。
详细源码请参考：WordCountHbaseReader\src\com\zonesion\hbase\WordCountHbaseReader.java
此处的WordCountHbaseReaderReduce实现了直接输出Map输出的< key,value>键值对，没有对其做任何处理。详细源码请参考：WordCountHbaseReader\src\com\zonesion\hbase\WordCountHbaseReader.java
public static class WordCountHbaseReaderReduce extends Reducer<Text,Text,Text,Text>{ private Text result = new Text(); @Override protected void reduce(Text key, Iterable<Text> values,Context context) throws IOException, InterruptedException { for(Text val:values){ result.set(val); context.write(key, result); } } } 4、 驱动函数实现与WordCount的驱动类不同，在Job配置的时候没有配置job.setMapperClass()，而是用以下方法执行Mapper类： TableMapReduceUtil.initTableMapperJob(tablename,scan,WordCountHbaseReaderMapper.class, Text.class, Text.class, job);
该方法指明了在执行job的Map过程时，数据输入源是hbase的tablename表，通过扫描读入对象scan对表进行全表扫描，为Map过程提供数据源输入，通过WordCountHbaseReaderMapper.class执行Map过程，Map过程的输出key/value类型是 Text.class与Text.class，最后一个参数是作业对象。特别注意：这里声明的是一个最简单的扫描读入对象scan，进行表扫描读取数据，其中scan可以配置参数，这里为了例子简单不再详述，用户可自行尝试。
详细源码请参考：WordCountHbaseReader\src\com\zonesion\hbase\WordCountHbaseReader.java
… 你可以直接 下载 WordCountHbaseReader
------------------------------------------分割线------------------------------------------
FTP地址：ftp://ftp1.linuxidc.com
用户名：ftp1.linuxidc.com
密码：
在 2015年LinuxIDC.com\3月\HBase入门基础教程
------------------------------------------分割线------------------------------------------
3）修改配置文件a）查看hbase核心配置文件hbase-site.xml的hbase.zookeeper.quorum属性
参考“【HBase入门基础教程】5、HBase API访问 3、部署运行 3）修改配置文件”查看hbase核心配置文件hbase-site.xml的hbase.zookeeper.quorum属性；
b）修改项目WordCountHbaseWriter/src/config.properties属性文件
将项目WordCountHbaseWriter/src/config.properties属性文件的hbase.zookeeper.quorum属性值修改为上一步查询到的属性值，保持config.properties文件的hbase.zookeeper.quorum属性值与hbase-site.xml文件的hbase.zookeeper.quorum属性值一致；
#切换工作目录 [hadoop@K-Master ~]$ cd /usr/hadoop/workspace/Hbase/ WordCountHbaseReader #修改属性值 [hadoop@K-Master WordCountHbaseReader]$ vim src/config.properties hbase.zookeeper.quorum=K-Master #拷贝src/config.properties文件到bin/文件夹 [hadoop@K-Master WordCountHbaseReader]$ cp src/config.properties bin/ 4）编译文件 #切换工作目录 [hadoop@K-Master ~]$ cd /usr/hadoop/workspace/Hbase/WordCountHbaseReader #执行编译 [hadoop@K-Master WordCountHbaseReader]$ javac -classpath /usr/hadoop/hadoop-core-1.2.1.jar:/usr/hadoop/lib/commons-cli-1.2.jar:lib/zookeeper-3.4.5.jar:lib/hbase-0.94.20.jar -d bin/ src/com/zonesion/hbase/WordCountHbaseReader.java #查看编译文件 [hadoop@K-Master WordCountHbaseReader]$ ls bin/com/zonesion/hbase/ -la total 20 drwxrwxr-x 2 hadoop hadoop 4096 Dec 29 10:36 . drwxrwxr-x 3 hadoop hadoop 4096 Dec 29 10:36 .. -rw-rw-r-- 1 hadoop hadoop 2166 Dec 29 14:31 WordCountHbaseReader.class -rw-rw-r-- 1 hadoop hadoop 2460 Dec 29 14:31 WordCountHbaseReader$WordCountHbaseReaderMapper.class -rw-rw-r-- 1 hadoop hadoop 1738 Dec 29 14:31 WordCountHbaseReader$WordCountHbaseReaderReduce.class 5）打包Jar文件 #拷贝lib文件夹到bin文件夹 [hadoop@K-Master WordCountHbaseReader]$ cp -r lib/ bin/ #打包Jar文件 [hadoop@K-Master WordCountHbaseReader]$ jar -cvf WordCountHbaseReader.jar -C bin/ . added manifest adding: lib/(in = 0) (out= 0)(stored 0%) adding: lib/zookeeper-3.4.5.jar(in = 779974) (out= 721150)(deflated 7%) adding: lib/guava-11.0.2.jar(in = 1648200) (out= 1465342)(deflated 11%) adding: lib/protobuf-java-2.4.0a.jar(in = 449818) (out= 420864)(deflated 6%) adding: lib/hbase-0.94.20.jar(in = 5475284) (out= 5038635)(deflated 7%) adding: com/(in = 0) (out= 0)(stored 0%) adding: com/zonesion/(in = 0) (out= 0)(stored 0%) adding: com/zonesion/hbase/(in = 0) (out= 0)(stored 0%) adding: com/zonesion/hbase/PropertiesHelper.class(in = 4480) (out= 1926)(deflated 57%) adding: com/zonesion/hbase/WordCountHbaseReader.class(in = 2702) (out= 1226)(deflated 54%) adding: com/zonesion/hbase/WordCountHbaseReader$WordCountHbaseReaderMapper.class(in = 3250) (out= 1275)(deflated 60%) adding: com/zonesion/hbase/WordCountHbaseReader$WordCountHbaseReaderReduce.class(in = 2308) (out= 872)(deflated 62%) adding: config.properties(in = 32) (out= 34)(deflated -6%) 6）运行实例 [hadoop@K-Master WordCountHbase]$ hadoop jar WordCountHbaseReader.jar WordCountHbaseReader /user/hadoop/WordCountHbaseReader/output/ ...................省略............. 14/12/30 17:51:58 INFO mapred.JobClient: Running job: job_201412161748_0035 14/12/30 17:51:59 INFO mapred.JobClient: map 0% reduce 0% 14/12/30 17:52:13 INFO mapred.JobClient: map 100% reduce 0% 14/12/30 17:52:26 INFO mapred.JobClient: map 100% reduce 100% 14/12/30 17:52:27 INFO mapred.JobClient: Job complete: job_201412161748_0035 14/12/30 17:52:27 INFO mapred.JobClient: Counters: 39 14/12/30 17:52:27 INFO mapred.JobClient: Job Counters 14/12/30 17:52:27 INFO mapred.JobClient: Launched reduce tasks=1 14/12/30 17:52:27 INFO mapred.JobClient: SLOTS_MILLIS_MAPS=4913 14/12/30 17:52:27 INFO mapred.JobClient: Total time spent by all reduces waiting after reserving slots (ms)=0 14/12/30 17:52:27 INFO mapred.JobClient: Total time spent by all maps waiting after reserving slots (ms)=0 14/12/30 17:52:27 INFO mapred.JobClient: Rack-local map tasks=1 14/12/30 17:52:27 INFO mapred.JobClient: Launched map tasks=1 14/12/30 17:52:27 INFO mapred.JobClient: SLOTS_MILLIS_REDUCES=13035 14/12/30 17:52:27 INFO mapred.JobClient: HBase Counters 14/12/30 17:52:27 INFO mapred.JobClient: REMOTE_RPC_CALLS=8 14/12/30 17:52:27 INFO mapred.JobClient: RPC_CALLS=8 14/12/30 17:52:27 INFO mapred.JobClient: RPC_RETRIES=0 14/12/30 17:52:27 INFO mapred.JobClient: NOT_SERVING_REGION_EXCEPTION=0 14/12/30 17:52:27 INFO mapred.JobClient: NUM_SCANNER_RESTARTS=0 14/12/30 17:52:27 INFO mapred.JobClient: MILLIS_BETWEEN_NEXTS=9 14/12/30 17:52:27 INFO mapred.JobClient: BYTES_IN_RESULTS=216 14/12/30 17:52:27 INFO mapred.JobClient: BYTES_IN_REMOTE_RESULTS=216 14/12/30 17:52:27 INFO mapred.JobClient: REGIONS_SCANNED=1 14/12/30 17:52:27 INFO mapred.JobClient: REMOTE_RPC_RETRIES=0 14/12/30 17:52:27 INFO mapred.JobClient: File Output Format Counters 14/12/30 17:52:27 INFO mapred.JobClient: Bytes Written=76 14/12/30 17:52:27 INFO mapred.JobClient: FileSystemCounters 14/12/30 17:52:27 INFO mapred.JobClient: FILE_BYTES_READ=92 14/12/30 17:52:27 INFO mapred.JobClient: HDFS_BYTES_READ=68 14/12/30 17:52:27 INFO mapred.JobClient: FILE_BYTES_WRITTEN=159978 14/12/30 17:52:27 INFO mapred.JobClient: HDFS_BYTES_WRITTEN=76 14/12/30 17:52:27 INFO mapred.JobClient: File Input Format Counters 14/12/30 17:52:27 INFO mapred.JobClient: Bytes Read=0 14/12/30 17:52:27 INFO mapred.JobClient: Map-Reduce Framework 14/12/30 17:52:27 INFO mapred.JobClient: Map output materialized bytes=92 14/12/30 17:52:27 INFO mapred.JobClient: Map input records=5 14/12/30 17:52:27 INFO mapred.JobClient: Reduce shuffle bytes=92 14/12/30 17:52:27 INFO mapred.JobClient: Spilled Records=10 14/12/30 17:52:27 INFO mapred.JobClient: Map output bytes=76 14/12/30 17:52:27 INFO mapred.JobClient: Total committed heap usage (bytes)=211025920 14/12/30 17:52:27 INFO mapred.JobClient: CPU time spent (ms)=2160 14/12/30 17:52:27 INFO mapred.JobClient: Combine input records=0 14/12/30 17:52:27 INFO mapred.JobClient: SPLIT_RAW_BYTES=68 14/12/30 17:52:27 INFO mapred.JobClient: Reduce input records=5 14/12/30 17:52:27 INFO mapred.JobClient: Reduce input groups=5 14/12/30 17:52:27 INFO mapred.JobClient: Combine output records=0 14/12/30 17:52:27 INFO mapred.JobClient: Physical memory (bytes) snapshot=263798784 14/12/30 17:52:27 INFO mapred.JobClient: Reduce output records=5 14/12/30 17:52:27 INFO mapred.JobClient: Virtual memory (bytes) snapshot=1491795968 14/12/30 17:52:27 INFO mapred.JobClient: Map output records=5 7）查看运行结果 [hadoop@K-Master WordCountHbaseReader]$ hadoop fs -ls /user/hadoop/WordCountHbaseReader/output/ Found 3 items -rw-r--r-- 1 hadoop supergroup 0 2014-07-28 18:04 /user/hadoop/WordCountHbaseReader/output/_SUCCESS drwxr-xr-x - hadoop supergroup 0 2014-07-28 18:04 /user/hadoop/WordCountHbaseReader/output/_logs -rw-r--r-- 1 hadoop supergroup 76 2014-07-28 18:04 /user/hadoop/WordCountHbaseReader/output/part-r-00000 [hadoop@K-Master WordCountHbaseReader]$ hadoop fs -cat /user/hadoop/WordCountHbaseReader/output/part-r-00000 Bye count:1 Goodbye count:1 Hadoope count:2 Hellope count:2 Worldpe count:2