我试图运行一个运行在底层数据库之上的mapreduce程序。当我安装hadoop的发行版时，它可以在hadoop下载中找到。这些程序在这个发行版中运行良好。但是当我编译我自己的hadoop发行版并试图运行相同的程序时，我得到了下面的错误。我遵循了将mysql连接器jar放在hadoop/lib目录中，并将一个放在分布式缓存中的过程。虽然这些过程适用于hadoop下载下可用的发行版，但它们不适用于我创建的发行版。谁能告诉我哪里出了问题?我尝试了所有其他方法，如更新classpath和HADOOP_CLASSPATH变量，但没有工作。
hduser@ramanujan:~$ hadoop jar SimpleConn.jar
13/04/15 13:50:16 WARN util.NativeCodeLoader: Unable to load native-hadoop library for your platform... using builtin-java classes where applicable
13/04/15 13:50:17 INFO service.AbstractService: Service:org.apache.hadoop.yarn.client.YarnClientImpl is inited.
13/04/15 13:50:17 INFO service.AbstractService: Service:org.apache.hadoop.yarn.client.YarnClientImpl is started.
13/04/15 13:50:17 INFO mapreduce.JobSubmitter: Cleaning up the staging area /tmp/hadoop-yarn/staging/hduser/.staging/job_1366013851608_0001
Exception in thread "main" java.lang.RuntimeException: java.lang.RuntimeException: java.lang.ClassNotFoundException: com.mysql.jdbc.Driver
at org.apache.hadoop.mapreduce.lib.db.DBInputFormat.setConf(DBInputFormat.java:169)
at org.apache.hadoop.util.ReflectionUtils.setConf(ReflectionUtils.java:70)
at org.apache.hadoop.util.ReflectionUtils.newInstance(ReflectionUtils.java:130)
at org.apache.hadoop.mapreduce.JobSubmitter.writeNewSplits(JobSubmitter.java:470)
at org.apache.hadoop.mapreduce.JobSubmitter.writeSplits(JobSubmitter.java:490)
at org.apache.hadoop.mapreduce.JobSubmitter.submitJobInternal(JobSubmitter.java:387)
at org.apache.hadoop.mapreduce.Job$11.run(Job.java:1218)
at org.apache.hadoop.mapreduce.Job$11.run(Job.java:1215)
at java.security.AccessController.doPrivileged(Native Method)
at javax.security.auth.Subject.doAs(Subject.java:415)
at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1489)
at org.apache.hadoop.mapreduce.Job.submit(Job.java:1215)
at org.apache.hadoop.mapreduce.Job.waitForCompletion(Job.java:1236)
at DBCountPageView.run(DBCountPageView.java:227)
at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:70)
at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:84)
at DBCountPageView.main(DBCountPageView.java:236)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:601)
at org.apache.hadoop.util.RunJar.main(RunJar.java:212)
Caused by: java.lang.RuntimeException: java.lang.ClassNotFoundException: com.mysql.jdbc.Driver
at org.apache.hadoop.mapreduce.lib.db.DBInputFormat.getConnection(DBInputFormat.java:195)
at org.apache.hadoop.mapreduce.lib.db.DBInputFormat.setConf(DBInputFormat.java:163)
... 21 more
Caused by: java.lang.ClassNotFoundException: com.mysql.jdbc.Driver
at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
at java.lang.ClassLoader.loadClass(ClassLoader.java:423)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
at java.lang.ClassLoader.loadClass(ClassLoader.java:356)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:188)
at org.apache.hadoop.mapreduce.lib.db.DBConfiguration.getConnection(DBConfiguration.java:148)
at org.apache.hadoop.mapreduce.lib.db.DBInputFormat.getConnection(DBInputFormat.java:189)
... 22 more
确保在提交作业时向HADOOP_CLASSPATH和-libjars添加任何依赖项，如下面的示例所示:
使用以下命令从当前目录和lib目录中添加所有jar依赖项:
export HADOOP_CLASSPATH=$HADOOP_CLASSPATH:`echo *.jar`:`echo lib/*.jar | sed 's/ /:/g'`
请记住，当通过hadoop jar启动作业时，您还需要通过使用-libjars将任何依赖项的jar传递给它。我喜欢使用:
hadoop jar <jar> <class> -libjars `echo ./lib/*.jar | sed 's/ /,/g'` [args...]
注意: sed命令需要一个不同的分隔符;HADOOP_CLASSPATH用:分隔，-libjars用,分隔。