1. wordcount源程序结构为： 

2. 将wordcount的源程序打包乘wordcount.jar过程如下：

3. hadoop运行打包好的jar文件，执行下面命令，出现异常：

[hadoop01@hadoop01 export]$ hadoop jar wordcount.jar wordcount /user/hadoop/input/file* /user/hadoop/output
Exception in thread "main" java.lang.ClassNotFoundException: wordcountat java.net.URLClassLoader.findClass(URLClassLoader.java:382)at java.lang.ClassLoader.loadClass(ClassLoader.java:418)at java.lang.ClassLoader.loadClass(ClassLoader.java:351)at java.lang.Class.forName0(Native Method)at java.lang.Class.forName(Class.java:348)at org.apache.hadoop.util.RunJar.run(RunJar.java:219)at org.apache.hadoop.util.RunJar.main(RunJar.java:141)

 4. 解决办法

出现问题的原因是受到了官方示例wordcount的影响：

[hadoop01@hadoop01 hadoop-2.7.7]$ hadoop jar share/hadoop/mapreduce/hadoop-mapreduce-examples-2.7.7.jar wordcount /user/hadoop/input/file* /user/hadoop/output

应该把wordcount改为wordcount.jar中主类的绝对路径com.skp.wordcount.WordMain，命令如下：

[hadoop01@hadoop01 export]$ hadoop jar wordcount.jar com.skp.wordcount.WordMain /user/hadoop/input/file* /user/hadoop/output

运行结果如下：

 参考文章：

Exception in thread "main" java.lang.ClassNotFoundException: