一、错误: 找不到或无法加载主类 org.apache.hadoop.mapreduce.v2.app.MRAppMaster

错误信息:
Container exited with a non-zero exit code 1. Error file: prelaunch.err.
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

解决方法:

在主机中运行:hadoop classpath

记下返回的结果

vi $HADOOP_HOME/etc/hadoop/yarn-site.xml

添加一个配置

<property>
        <name>yarn.application.classpath</name>
        <value>hadoop classpath返回信息</value>
</property>

二、Hadoop格式化后start.all.sh运行出错

1、master，slave都需要修改start-dfs.sh，stop-dfs.sh，start-yarn.sh，stop-yarn.sh四个文件
2、如果你的Hadoop是另外启用其它用户来启动，记得将root改为对应用户

HDFS格式化后启动dfs出现以下错误：

[root@hadoop101 hadoop-3.2.0]# sbin/start-all.sh
Starting namenodes on [master]
ERROR: Attempting to operate on hdfs namenode as root
ERROR: but there is no HDFS_NAMENODE_USER defined. Aborting operation.
Starting datanodes
ERROR: Attempting to operate on hdfs datanode as root
ERROR: but there is no HDFS_DATANODE_USER defined. Aborting operation.
Starting secondary namenodes [slave1]
ERROR: Attempting to operate on hdfs secondarynamenode as root
ERROR: but there is no HDFS_SECONDARYNAMENODE_USER defined. Aborting operation.

在/hadoop/sbin路径下：
将start-dfs.sh，stop-dfs.sh两个文件顶部添加以下参数

#!/usr/bin/env bash
HDFS_DATANODE_USER=root 
HDFS_DATANODE_SECURE_USER=hdfs 
HDFS_NAMENODE_USER=root 
HDFS_SECONDARYNAMENODE_USER=root

还有，start-yarn.sh，stop-yarn.sh顶部也需添加以下：

#!/usr/bin/env bash
YARN_RESOURCEMANAGER_USER=root
HADOOP_SECURE_DN_USER=yarn
YARN_NODEMANAGER_USER=root

修改后重启 ./start-all.sh，成功！

[root@master sbin]# ./start-dfs.sh
WARNING: HADOOP_SECURE_DN_USER has been replaced by HDFS_DATANODE_SECURE_USER. Using value of HADOOP_SECURE_DN_USER.
Starting namenodes on [master]
上一次登录：日 6月  3 03:01:37 CST 2018从 slave1pts/2 上
master: Warning: Permanently added 'master,192.168.43.161' (ECDSA) to the list of known hosts.
Starting datanodes
上一次登录：日 6月  3 04:09:05 CST 2018pts/1 上
Starting secondary namenodes [slave1]
上一次登录：日 6月  3 04:09:08 CST 2018pts/1 上