Could not find or load main class org.apache.hadoop.mapreduce.v2.app.MRAppMaster

新部署 Hadoop 3.x 提交 MapReduce 时会出现以下报错：

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>

而网上很多的解决方案是：在mapred-site.xml配置中加入配置信息之后，分发到各个节点，再重启，但依然会有以上问题。

    <property>
      <name>yarn.app.mapreduce.am.env</name>
      <value>HADOOP_MAPRED_HOME=${HADOOP_HOME}</value>
    </property>
    <property>
      <name>mapreduce.map.env</name>
      <value>HADOOP_MAPRED_HOME=${HADOOP_HOME}</value>
    </property>
    <property>
      <name>mapreduce.reduce.env</name>
      <value>HADOOP_MAPRED_HOME=${HADOOP_HOME}</value>
    </property>

而报错中最核心的是 Could not find or load main class org.apache.hadoop.mapreduce.v2.app.MRAppMaster

在命令行执行 hadoop classpath 获取 Hadoop classpath
然后在 yarn-site.xml，添加信息：

  <property>
    <name>yarn.application.classpath</name>
    <value>复制的Hadoop classpath信息</value>
  </property>