hadoop在yarn执行MapReduce任务时，找不到主类导致的报错信息：
Last 4096 bytes of prelaunch.err :
Last 4096 bytes of stderr :
错误: 找不到或无法加载主类 org.apache.hadoop.mapreduce.v2.app.MRAppMaster

解决办法

在命令行输入：

hadoop classpath

输出的值添加到yarn-site.xml文件对应的属性 yarn.application.classpath下面

[admin@master hadoop]$ hadoop classpath
/opt/module/hadoop/etc/hadoop:/opt/module/hadoop/share/hadoop/common/lib/*:/opt/module/hadoop/share/hadoop/common/*:/opt/module/hadoop/share/hadoop/hdfs:/opt/module/hadoop/share/hadoop/hdfs/lib/*:/opt/module/hadoop/share/hadoop/hdfs/*:/opt/module/hadoop/share/hadoop/mapreduce/lib/*:/opt/module/hadoop/share/hadoop/mapreduce/*:/opt/module/hadoop/share/hadoop/yarn:/opt/module/hadoop/share/hadoop/yarn/lib/*:/opt/module/hadoop/share/hadoop/yarn/*

比如：

vi yarn-site.xml

比如：

<property>
        <name>yarn.application.classpath</name>
        <value>/opt/module/hadoop/etc/hadoop:/opt/module/hadoop/share/hadoop/common/lib/*:/opt/module/hadoop/share/hadoop/common/*:/opt/module/hadoop/share/hadoop/hdfs:/opt/module/hadoop/share/hadoop/hdfs/lib/*:/opt/module/hadoop/share/hadoop/hdfs/*:/opt/module/hadoop/share/hadoop/mapreduce/lib/*:/opt/module/hadoop/share/hadoop/mapreduce/*:/opt/module/hadoop/share/hadoop/yarn:/opt/module/hadoop/share/hadoop/yarn/lib/*:/opt/module/hadoop/share/hadoop/yarn/*</value>
</property>

重启yarn，重新跑MapReduce任务。

注意事项：

1.注意yarn.application.classpath要加入自己的classpath
2.注意文件目录
cd /export/servers/hadoop-2.7.4/etc/hadoop/yarn-site.xml