hadoop运行报错： java.lang.ClassNotFoundException解决方法

在创建自定义的Mapper时候，编译正确，但上传到集群执行时出现错误： 11/12/11 22:53:16 INFO mapred.JobClient: Task Id : attempt_201111301626_0015_m_000000_0, Status : FAILED java.lang.RuntimeException: java.lang.ClassNotFoundException: actiondemo.MyJob$MapClass at org.apache.hadoop.conf.Configuration.getClass(Configuration.java:866) at org.apache.hadoop.mapreduce.JobContext.getMapperClass(JobContext.java:199) at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:719) at org.apache.hadoop.mapred.MapTask.run(MapTask.java:370) at org.apache.hadoop.mapred.Child$4.run(Child.java:255) at java.security.AccessController.doPrivileged(Native Method) at javax.security.auth.Subject.doAs(Subject.java:396) at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1059) at org.apache.hadoop.mapred.Child.main(Child.java:249) Caused by: java.lang.ClassNotFoundException: actiondemo.MyJob$MapClass at java.net.URLClassLoader$1.run(URLClassLoader.java:202) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(URLClassLoader.java:190) at java.lang.ClassLoader.loadClass(ClassLoader.java:306) at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301) at java.lang.ClassLoader.loadClass(ClassLoader.java:247) at java.lang.Class.forName0(Native Method) at java.lang.Class.forName(Class.java:247) at org.apache.hadoop.conf.Configuration.getClassByName(Configuration.java:819) at org.apache.hadoop.conf.Configuration.getClass(Configuration.java:864) ... 8 more

经过验证，发现问题原因及解决办法如下：

因为使用的是0.20以上的Hadoop版本，在调用jar中的自定义mapper时，需要设置setJarByClass方法，设置方法如下： job.setJarByClass(MyJob.class); 其实，在输出日志中也有提示信息： 11/12/11 22:53:03 WARN mapred.JobClient: No job jar file set. User classes may not be found. See JobConf(Class) or JobConf#setJar(String). job.setJarByClass(MyJob.class);

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