#### 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：
```xml
Error: Could not find or load main class org.apache.hadoop.mapreduce.v2.app.MRAppMaster

Please check whether your etc/hadoop/mapred-site.xml contains the below configuration:
<property>
  <name>yarn.app.mapreduce.am.env</name>
  <value>HADOOP_MAPRED_HOME=${full path of your hadoop distribution directory}</value>
</property>
<property>
  <name>mapreduce.map.env</name>
  <value>HADOOP_MAPRED_HOME=${full path of your hadoop distribution directory}</value>
</property>
<property>
  <name>mapreduce.reduce.env</name>
  <value>HADOOP_MAPRED_HOME=${full path of your hadoop distribution directory}</value>
</property>
```

#### 解决办法：

先手hadoop classpath命令得到classpath:
hadoop classpath
如下：
```shell
[root@hd1 mapreduce]# hadoop classpath
/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：
```xml
<property>
  <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>
</property>
<property>
  <name>mapreduce.map.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>
</property>
<property>
  <name>mapreduce.reduce.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>
</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

如要知道我是怎么安装hadoop的，可以查看我这篇文章
[《安装Hadoop伪分布式模式教程，版本为Hadoop3.1.4》](http://www.classinstance.cn/detail/154.html "《安装Hadoop伪分布式模式教程，版本为Hadoop3.1.4》")

ClassInstance

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

相关文章

推荐文章

热门文章

标签列表