轉：http://blog.csdn.net/cnmilan/article/details/7986750

Could not find the main class: org.apache.hadoop.hdfs.server.namenode.NameNode.解決方法

hadoop-0.21.0 配置好1.core-site.xml 2.hdfs-site.xml 3.mapred-site.xml 格式化namenode 報錯
Exception in thread "main" java.lang.NoClassDefFoundError: org/apache/hadoop/hdfs/server/namenode/NameNode
Caused by: java.lang.ClassNotFoundException: org.apache.hadoop.hdfs.server.namenode.NameNode
        at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:248)
Could not find the main class: org.apache.hadoop.hdfs.server.namenode.NameNode. Program will exit.

解決辦法：修改 conf/hadoop-env.sh，增加
                  export HADOOP_COMMON_HOME=/usr/hadoop/hadoop （hadoop的安裝目錄)