hadoop运行mapreduce报错Error: Could not find or load main class org.apache.hadoop.mapreduce.v2.app.MRApp

异常问题：

今天运行word count单词统计示例，hadoop运行mapreduce报错Error: Could not find or load main class org.apache.hadoop.mapreduce.v2.app.MRAppMaster

详细错误信息如下：

1. Error: Could not find or load main class org.apache.hadoop.mapreduce.v2.app.MRAppMaster
3. Please check whether your etc/hadoop/mapred-site.xml contains the below configuration:
4. <property>
5. <name>yarn.app.mapreduce.am.env</name>
6. <value>HADOOP_MAPRED_HOME=${full path of your hadoop distribution directory}</value>
7. </property>
8. <property>
9. <name>mapreduce.map.env</name>
10. <value>HADOOP_MAPRED_HOME=${full path of your hadoop distribution directory}</value>
11. </property>
12. <property>
13. <name>mapreduce.reduce.env</name>
14. <value>HADOOP_MAPRED_HOME=${full path of your hadoop distribution directory}</value>
15. </property>

解决办法：

先手hadoop classpath命令得到classpath:
hadoop classpath
如下：

1. [root@hd1 mapreduce]# hadoop classpath
2. /home/hadoop-3.1.4/etc/hadoop:/home/hadoop-3.1.4/share/hadoop/common/lib/*:/home/hadoop-3.1.4/share/hadoop/common/*:/home/hadoop-3.1.4/share/hadoop/hdfs:/home/hadoop-3.1.4/share/hadoop/hdfs/lib/*:/home/hadoop-3.1.4/share/hadoop/hdfs/*:/home/hadoop-3.1.4/share/hadoop/mapreduce/lib/*:/home/hadoop-3.1.4/share/hadoop/mapreduce/*:/home/hadoop-3.1.4/share/hadoop/yarn:/home/hadoop-3.1.4/share/hadoop/yarn/lib/*:/home/hadoop-3.1.4/share/hadoop/yarn/*

再修改mapred-site.xml配置，增加如下：

1. <property>
2. <name>yarn.app.mapreduce.am.env</name> <value>HADOOP_MAPRED_HOME=/home/hadoop-3.1.4/etc/hadoop:/home/hadoop-3.1.4/share/hadoop/common/lib/*:/home/hadoop-3.1.4/share/hadoop/common/*:/home/hadoop-3.1.4/share/hadoop/hdfs:/home/hadoop-3.1.4/share/hadoop/hdfs/lib/*:/home/hadoop-3.1.4/share/hadoop/hdfs/*:/home/hadoop-3.1.4/share/hadoop/mapreduce/lib/*:/home/hadoop-3.1.4/share/hadoop/mapreduce/*:/home/hadoop-3.1.4/share/hadoop/yarn:/home/hadoop-3.1.4/share/hadoop/yarn/lib/*:/home/hadoop-3.1.4/share/hadoop/yarn/*</value>
3. </property>
4. <property>
5. <name>mapreduce.map.env</name>
6. <value>HADOOP_MAPRED_HOME=/home/hadoop-3.1.4/etc/hadoop:/home/hadoop-3.1.4/share/hadoop/common/lib/*:/home/hadoop-3.1.4/share/hadoop/common/*:/home/hadoop-3.1.4/share/hadoop/hdfs:/home/hadoop-3.1.4/share/hadoop/hdfs/lib/*:/home/hadoop-3.1.4/share/hadoop/hdfs/*:/home/hadoop-3.1.4/share/hadoop/mapreduce/lib/*:/home/hadoop-3.1.4/share/hadoop/mapreduce/*:/home/hadoop-3.1.4/share/hadoop/yarn:/home/hadoop-3.1.4/share/hadoop/yarn/lib/*:/home/hadoop-3.1.4/share/hadoop/yarn/*</value>
7. </property>
8. <property>
9. <name>mapreduce.reduce.env</name>
10. <value>HADOOP_MAPRED_HOME=/home/hadoop-3.1.4/etc/hadoop:/home/hadoop-3.1.4/share/hadoop/common/lib/*:/home/hadoop-3.1.4/share/hadoop/common/*:/home/hadoop-3.1.4/share/hadoop/hdfs:/home/hadoop-3.1.4/share/hadoop/hdfs/lib/*:/home/hadoop-3.1.4/share/hadoop/hdfs/*:/home/hadoop-3.1.4/share/hadoop/mapreduce/lib/*:/home/hadoop-3.1.4/share/hadoop/mapreduce/*:/home/hadoop-3.1.4/share/hadoop/yarn:/home/hadoop-3.1.4/share/hadoop/yarn/lib/*:/home/hadoop-3.1.4/share/hadoop/yarn/*</value>
11. </property>

注意：上面的配置的value都是上面命令得到的值，复制到value中即可