问题
执行wordcount命令时，hadoop找不到或无法加载主类，org.apache.hadoop.mapreduce.v2.app.MRAppMaster 
- 输入命令 hadoop classpath
2.将输出的内容直接复制到yarn-site.xml文件中：
yarn.application.classpath
/usr/local/hadoop3.1.2/etc/hadoop:/usr/local/hadoop3.1.2/share/hadoop/common/lib/*:/usr/local/hadoop3.1.2/share/hadoop/common/*:/usr/local/hadoop3.1.2/share/hadoop/hdfs:/usr/local/hadoop3.1.2/share/hadoop/hdfs/lib/*:/usr/local/hadoop3.1.2/share/hadoop/hdfs/*:/usr/local/hadoop3.1.2/share/hadoop/mapreduce/lib/*:/usr/local/hadoop3.1.2/share/hadoop/mapreduce/*:/usr/local/hadoop3.1.2/share/hadoop/yarn:/usr/local/hadoop3.1.2/share/hadoop/yarn/lib/*:/usr/local/hadoop3.1.2/share/hadoop/yarn/*
