Hadoop-0.19.2的代码可以到Apache上下载,使用的Linux机器是RHEL 5，Linux上安装的Java版本为1.6.0_16，并且JAVA_HOME=/usr/java/jdk1.6.0_16
实践过程
1、ssh无密码验证登陆localhost
保证Linux系统的ssh服务已经启动，并保证能够通过无密码验证登陆本机Linux系统。如果不能保证，可以按照如下的步骤去做：
（1）启动命令行窗口，执行命令行：
$ ssh-keygen -t dsa -P '' -f ~/.ssh/id_dsa
$ cat ~/.ssh/id_dsa.pub >> ~/.ssh/authorized_keys
（2）ssh登陆localhost，执行命令行：
$ ssh localhost
第一次登录，会提示你无法建立到127.0.0.1的连接，是否要建立，输入yes即可，下面是能够通过无密码验证登陆的信息：
[root@localhost hadoop-0.19.2]# ssh localhost
Last login: Sun Aug 1 18:35:37 2010 from 192.168.0.104
[root@localhost ~]#
2、Hadoop-0.19.0配置
下载hadoop-0.19.0.tar.gz，大约是40.3M，解压缩到Linux系统指定目录，这里我的是/root/hadoop-0.19.2目录下。
下面按照有序的步骤来说明配置过程：
（1）修改hadoop-env.sh配置
将Java环境的配置进行修改后，并取消注释“#”，修改后的行为：
export JAVA_HOME=/usr/java/jdk1.6.0_16
（2）修改hadoop-site.xml配置
在<configuration>与</configuration>加上3个属性的配置，修改后的配置文件内容为：
<?xml version="1.0"?>
<?xml-stylesheet type="text/xsl" href="https://www.linuxidc.com/configuration.xsl"?>
<!-- Put site-specific property overrides in this file. -->
<configuration>
<property>
<name>fs.default.name</name>
<value>hdfs://localhost:9000</value>
</property>
<property>
<name>mapred.job.tracker</name>
<value>localhost:9001</value>
</property>
<property>
<name>dfs.replication</name>
<value>1</value>
</property>
</configuration>
3运行wordcount实例
wordcount例子是hadoop发行包中自带的实例，通过运行实例可以感受并尝试理解hadoop在执行MapReduce任务时的执行过程。按照官方的“Hadoop Quick Start”教程基本可以容易地实现，下面简单说一下我的练习过程。
导航到hadoop目录下面，我的是/root/hadoop-0.19.0。
（1）格式化HDFS
执行格式化HDFS的命令行：
[root@localhost hadoop-0.19.2]# bin/hadoop namenode -format
格式化执行信息如下所示：
10/08/01 19:04:02 INFO namenode.NameNode: STARTUP_MSG:
Re-format filesystem in /tmp/hadoop-root/dfs/name ? (Y or N) y
Format aborted in /tmp/hadoop-root/dfs/name
10/08/01 19:04:05 INFO namenode.NameNode: SHUTDOWN_MSG:
（2）启动Hadoop相关后台进程
执行命令行：
[root@localhost hadoop-0.19.2]# bin/start-all.sh
启动执行信息如下所示：
starting namenode, logging to /root/hadoop-0.19.0/bin/../logs/hadoop-root-namenode-localhost.out
localhost: starting datanode, logging to /root/hadoop-0.19.0/bin/../logs/hadoop-root-datanode-localhost.out
localhost: starting secondarynamenode, logging to /root/hadoop-0.19.0/bin/../logs/hadoop-root-secondarynamenode-localhost.out
starting jobtracker, logging to /root/hadoop-0.19.0/bin/../logs/hadoop-root-jobtracker-localhost.out
localhost: starting tasktracker, logging to /root/hadoop-0.19.0/bin/../logs/hadoop-root-tasktracker-localhost.out
（3）准备执行wordcount任务的数据
首先，这里在本地创建了一个数据目录input，并拷贝一些文件到该目录下面，如下所示：
[root@localhost hadoop-0.19.2]# mkdir input
[root@localhost hadoop-0.19.2]# cp CHANGES.txt LICENSE.txt NOTICE.txt README.txt input/
然后，将本地目录input上传到HDFS文件系统上，执行如下命令：
[root@localhost hadoop-0.19.2]# bin/hadoop fs -put input/ input
（4）启动wordcount任务
执行如下命令行：
[root@localhost hadoop-0.19.2]# bin/hadoop jar hadoop-0.19.2-examples.jar wordcount input output
元数据目录为input，输出数据目录为output。
任务执行信息如下所示：
10/08/01 19:06:15 INFO mapred.FileInputFormat: Total input paths to process : 4
10/08/01 19:06:15 INFO mapred.JobClient: Running job: job_201008011904_0002
10/08/01 19:06:16 INFO mapred.JobClient: map 0% reduce 0%
10/08/01 19:06:22 INFO mapred.JobClient: map 20% reduce 0%
10/08/01 19:06:24 INFO mapred.JobClient: map 40% reduce 0%
10/08/01 19:06:25 INFO mapred.JobClient: map 60% reduce 0%
10/08/01 19:06:27 INFO mapred.JobClient: map 80% reduce 0%
10/08/01 19:06:28 INFO mapred.JobClient: map 100% reduce 0%
10/08/01 19:06:38 INFO mapred.JobClient: map 100% reduce 26%
10/08/01 19:06:40 INFO mapred.JobClient: map 100% reduce 100%
10/08/01 19:06:41 INFO mapred.JobClient: Job complete: job_201008011904_0002
10/08/01 19:06:41 INFO mapred.JobClient: Counters: 16
10/08/01 19:06:41 INFO mapred.JobClient: File Systems
10/08/01 19:06:41 INFO mapred.JobClient: HDFS bytes read=301489
10/08/01 19:06:41 INFO mapred.JobClient: HDFS bytes written=113098
10/08/01 19:06:41 INFO mapred.JobClient: Local bytes read=174004
10/08/01 19:06:41 INFO mapred.JobClient: Local bytes written=348172
10/08/01 19:06:41 INFO mapred.JobClient: Job Counters
10/08/01 19:06:41 INFO mapred.JobClient: Launched reduce tasks=1
10/08/01 19:06:41 INFO mapred.JobClient: Launched map tasks=5
10/08/01 19:06:41 INFO mapred.JobClient: Data-local map tasks=5
10/08/01 19:06:41 INFO mapred.JobClient: Map-Reduce Framework
10/08/01 19:06:41 INFO mapred.JobClient: Reduce input groups=8997
10/08/01 19:06:41 INFO mapred.JobClient: Combine output records=10860
10/08/01 19:06:41 INFO mapred.JobClient: Map input records=7363
10/08/01 19:06:41 INFO mapred.JobClient: Reduce output records=8997
10/08/01 19:06:41 INFO mapred.JobClient: Map output bytes=434077
10/08/01 19:06:41 INFO mapred.JobClient: Map input bytes=299871
10/08/01 19:06:41 INFO mapred.JobClient: Combine input records=39193
10/08/01 19:06:41 INFO mapred.JobClient: Map output records=39193
10/08/01 19:06:41 INFO mapred.JobClient: Reduce input records=10860
（5）查看任务执行结果
可以通过如下命令行：
bin/hadoop fs -cat output/*
执行结果，截取部分显示如下所示：
vijayarenu 20
violations. 1
virtual 3
vis-a-vis 1
visible 1
visit 1
volume 1
volume, 1
volumes 2
volumes. 1
w.r.t 2
wait 9
waiting 6
waiting. 1
waits 3
want 1
warning 7
warning, 1
warnings 12
warnings. 3
warranties 1
warranty 1
warranty, 1
（6）终止Hadoop相关后台进程
执行如下命令行：
[root@localhost hadoop-0.19.2]# bin/stop-all.sh
执行信息如下所示：
stopping jobtracker
localhost: stopping tasktracker
stopping namenode
localhost: stopping datanode
localhost: stopping secondarynamenode
已经将上面列出的5个进程jobtracker、tasktracker、namenode、datanode、secondarynamenode
异常分析
在进行上述实践过程中，可能会遇到某种异常情况，大致分析如下：
1、Call to localhost/127.0.0.1:9000 failed on local exception异常
（1）异常描述
可能你会在执行如下命令行的时候出现：
[root@localhost hadoop-0.19.2]# bin/hadoop jar hadoop-0.19.0-examples.jar wordcount input output
出错异常信息如下所示：
10/08/01 19:50:55 INFO ipc.Client: Retrying connect to server: localhost/127.0.0.1:9000. Already tried 0 time(s).
10/08/01 19:50:56 INFO ipc.Client: Retrying connect to server: localhost/127.0.0.1:9000. Already tried 1 time(s).
10/08/01 19:50:57 INFO ipc.Client: Retrying connect to server: localhost/127.0.0.1:9000. Already tried 2 time(s).
10/08/01 19:50:58 INFO ipc.Client: Retrying connect to server: localhost/127.0.0.1:9000. Already tried 3 time(s).
10/08/01 19:50:59 INFO ipc.Client: Retrying connect to server: localhost/127.0.0.1:9000. Already tried 4 time(s).
10/08/01 19:51:00 INFO ipc.Client: Retrying connect to server: localhost/127.0.0.1:9000. Already tried 5 time(s).
10/08/01 19:51:01 INFO ipc.Client: Retrying connect to server: localhost/127.0.0.1:9000. Already tried 6 time(s).
10/08/01 19:51:02 INFO ipc.Client: Retrying connect to server: localhost/127.0.0.1:9000. Already tried 7 time(s).
10/08/01 19:51:03 INFO ipc.Client: Retrying connect to server: localhost/127.0.0.1:9000. Already tried 8 time(s).
10/08/01 19:51:04 INFO ipc.Client: Retrying connect to server: localhost/127.0.0.1:9000. Already tried 9 time(s).
java.lang.RuntimeException: java.io.IOException: Call to localhost/127.0.0.1:9000 failed on local exception: Connection refused
at org.apache.hadoop.mapred.JobConf.getWorkingDirectory(JobConf.java:323)
at org.apache.hadoop.mapred.FileInputFormat.setInputPaths(FileInputFormat.java:295)
at org.apache.hadoop.mapred.FileInputFormat.setInputPaths(FileInputFormat.java:268)
at org.apache.hadoop.examples.WordCount.run(WordCount.java:146)
at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:65)
at org.apache.hadoop.examples.WordCount.main(WordCount.java:155)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.apache.hadoop.util.ProgramDriver$ProgramDescription.invoke(ProgramDriver.java:68)
at org.apache.hadoop.util.ProgramDriver.driver(ProgramDriver.java:141)
at org.apache.hadoop.examples.ExampleDriver.main(ExampleDriver.java:61)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.apache.hadoop.util.RunJar.main(RunJar.java:165)
at org.apache.hadoop.mapred.JobShell.run(JobShell.java:54)
at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:65)
at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:79)
at org.apache.hadoop.mapred.JobShell.main(JobShell.java:68)
Caused by: java.io.IOException: Call to localhost/127.0.0.1:9000 failed on local exception: Connection refused
at org.apache.hadoop.ipc.Client.call(Client.java:699)
at org.apache.hadoop.ipc.RPC$Invoker.invoke(RPC.java:216)
at $Proxy0.getProtocolVersion(Unknown Source)
at org.apache.hadoop.ipc.RPC.getProxy(RPC.java:319)
at org.apache.hadoop.hdfs.DFSClient.createRPCNamenode(DFSClient.java:104)
at org.apache.hadoop.hdfs.DFSClient.<init>(DFSClient.java:177)
at org.apache.hadoop.hdfs.DistributedFileSystem.initialize(DistributedFileSystem.java:74)
at org.apache.hadoop.fs.FileSystem.createFileSystem(FileSystem.java:1367)
at org.apache.hadoop.fs.FileSystem.access$200(FileSystem.java:56)
at org.apache.hadoop.fs.FileSystem$Cache.get(FileSystem.java:1379)
at org.apache.hadoop.fs.FileSystem.get(FileSystem.java:215)
at org.apache.hadoop.fs.FileSystem.get(FileSystem.java:120)
at org.apache.hadoop.mapred.JobConf.getWorkingDirectory(JobConf.java:319)
... 21 more
Caused by: java.net.ConnectException: Connection refused
at sun.nio.ch.SocketChannelImpl.checkConnect(Native Method)
at sun.nio.ch.SocketChannelImpl.finishConnect(SocketChannelImpl.java:574)
at sun.nio.ch.SocketAdaptor.connect(SocketAdaptor.java:100)
at org.apache.hadoop.ipc.Client$Connection.setupIOstreams(Client.java:299)
at org.apache.hadoop.ipc.Client$Connection.access$1700(Client.java:176)
at org.apache.hadoop.ipc.Client.getConnection(Client.java:772)
at org.apache.hadoop.ipc.Client.call(Client.java:685)
... 33 more
（2）异常分析
从上述异常信息分析，这句是关键：
Retrying connect to server: localhost/127.0.0.1:9000.
是说在尝试10次连接到“server”时都无法成功，这就说明到server的通信链路是不通的。我们已经在hadoop-site.xml中配置了namenode结点的值，如下所示：
<property>
<name>fs.default.name</name>
<value>hdfs://localhost:9000</value>
</property>
所以，很可能namenode进程根本就没有启动，更不必谈要执行任务了。
上述异常，过程是：
格式化了HDFS，但是没有执行bin/start-all.sh，直接启动wordcount任务，就出现上述异常。
所以，应该执行bin/start-all.sh以后再启动wordcount任务。
2、Input path does not exist异常
（1）异常描述
当你在当前hadoop目录下面创建一个input目录，并cp某些文件到里面，开始执行：
[root@localhost hadoop-0.19.2]# bin/hadoop namenode -format
[root@localhost hadoop-0.19.2]# bin/start-all.sh
这时候，你认为input已经存在，应该可以执行wordcount任务了：
[root@localhost hadoop-0.19.2]# bin/hadoop jar hadoop-0.19.0-examples.jar wordcount input output
结果抛出一堆异常，信息如下：
org.apache.hadoop.mapred.InvalidInputException: Input path does not exist: hdfs://localhost:9000/user/root/input
at org.apache.hadoop.mapred.FileInputFormat.listStatus(FileInputFormat.java:179)
at org.apache.hadoop.mapred.FileInputFormat.getSplits(FileInputFormat.java:190)
at org.apache.hadoop.mapred.JobClient.submitJob(JobClient.java:782)
at org.apache.hadoop.mapred.JobClient.runJob(JobClient.java:1127)
at org.apache.hadoop.examples.WordCount.run(WordCount.java:149)
at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:65)
at org.apache.hadoop.examples.WordCount.main(WordCount.java:155)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.apache.hadoop.util.ProgramDriver$ProgramDescription.invoke(ProgramDriver.java:68)
at org.apache.hadoop.util.ProgramDriver.driver(ProgramDriver.java:141)
at org.apache.hadoop.examples.ExampleDriver.main(ExampleDriver.java:61)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.apache.hadoop.util.RunJar.main(RunJar.java:165)
at org.apache.hadoop.mapred.JobShell.run(JobShell.java:54)
at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:65)
at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:79)
at org.apache.hadoop.mapred.JobShell.main(JobShell.java:68)
上述异常，我模拟的过程是：
[root@localhost hadoop-0.19.2]# bin/hadoop fs -rmr input
Deleted hdfs://localhost:9000/user/root/input
[root@localhost hadoop-0.19.2]# bin/hadoop fs -rmr output
Deleted hdfs://localhost:9000/user/root/output
（2）异常分析
本地的input目录并没有上传到HDFS上，所出现org.apache.hadoop.mapred.InvalidInputException: Input path does not exist: hdfs://localhost:9000/user/root/input
只需要执行上传的命令即可：
[root@localhost hadoop-0.19.2]# bin/hadoop fs -put input/ input