hadoop3.1.1：找不到或没法加载主类 org.apache.hadoop.mapreduce.v2.app.MRAppMaster

yarn执行MapReduce任务时，找不到主类致使的

解决：

一、在命令行输入：hadoop classpath

[hadoop@localhost ~]$ hadoop classpath /data/hadoop/hadoop//etc/hadoop:/data/hadoop/hadoop//share/hadoop/common/lib/*:/data/hadoop/hadoop//share/hadoop/common/*:/data/hadoop/hadoop//share/hadoop/hdfs:/data/hadoop/hadoop//share/hadoop/hdfs/lib/*:/data/hadoop/hadoop//share/hadoop/hdfs/*:/data/hadoop/hadoop//share/hadoop/mapreduce/lib/*:/data/hadoop/hadoop//share/hadoop/mapreduce/*:/data/hadoop/hadoop//share/hadoop/yarn:/data/hadoop/hadoop//share/hadoop/yarn/lib/*:/data/hadoop/hadoop//share/hadoop/yarn/*

二、把上述输出的值添加到yarn-site.xml文件对应的属性 <name>yarn.application.classpath</name>下面，eg：

<property>
         <name>yarn.application.classpath</name>
         <value>/data/hadoop/hadoop//etc/hadoop:/data/hadoop/hadoop//share/hadooo
p/common/lib/*:/data/hadoop/hadoop//share/hadoop/common/*:/data/hadoop/hadoop//ss hare/hadoop/hdfs:/data/hadoop/hadoop//share/hadoop/hdfs/lib/*:/data/hadoop/hadooo p//share/hadoop/hdfs/*:/data/hadoop/hadoop//share/hadoop/mapreduce/lib/*:/data/hh adoop/hadoop//share/hadoop/mapreduce/*:/data/hadoop/hadoop//share/hadoop/yarn:/dd ata/hadoop/hadoop//share/hadoop/yarn/lib/*:/data/hadoop/hadoop//share/hadoop/yarr n/*</value> </property>

三、重启yarn，从新跑MapReduce任务。