情景：mapreduce任务出现如下错误
    错误：
    Error: Could not find or load main class org.apache.hadoop.mapreduce.v2.app.MRAppMaster
    全过程：

    1. [hadoop@hadoop102 ~]$ hadoop jar wc.jar com.muzhi.mapreduce.wordcount1.WordCountDriver /input /output
    2. 2021-08-21 10:53:30,543 INFO client.RMProxy: Connecting to ResourceManager at hadoop103/192.168.220.103:8032
    3. 2021-08-21 10:53:31,430 WARN mapreduce.JobResourceUploader: Hadoop command-line option parsing not performed. Implement the Tool interface and execute your application with ToolRunner to remedy this.
    4. 2021-08-21 10:53:31,461 INFO mapreduce.JobResourceUploader: Disabling Erasure Coding for path: /tmp/hadoop-yarn/staging/hadoop/.staging/job_1629442479906_0002
    5. 2021-08-21 10:53:31,614 INFO sasl.SaslDataTransferClient: SASL encryption trust check: localHostTrusted = false, remoteHostTrusted = false
    6. 2021-08-21 10:53:31,823 INFO input.FileInputFormat: Total input files to process : 1
    7. 2021-08-21 10:53:31,878 INFO sasl.SaslDataTransferClient: SASL encryption trust check: localHostTrusted = false, remoteHostTrusted = false
    8. 2021-08-21 10:53:31,927 INFO sasl.SaslDataTransferClient: SASL encryption trust check: localHostTrusted = false, remoteHostTrusted = false
    9. 2021-08-21 10:53:31,961 INFO mapreduce.JobSubmitter: number of splits:1
    10. 2021-08-21 10:53:32,201 INFO sasl.SaslDataTransferClient: SASL encryption trust check: localHostTrusted = false, remoteHostTrusted = false
    11. 2021-08-21 10:53:32,272 INFO mapreduce.JobSubmitter: Submitting tokens for job: job_1629442479906_0002
    12. 2021-08-21 10:53:32,272 INFO mapreduce.JobSubmitter: Executing with tokens: []
    13. 2021-08-21 10:53:32,620 INFO conf.Configuration: resource-types.xml not found
    14. 2021-08-21 10:53:32,620 INFO resource.ResourceUtils: Unable to find 'resource-types.xml'.
    15. 2021-08-21 10:53:32,712 INFO impl.YarnClientImpl: Submitted application application_1629442479906_0002
    16. 2021-08-21 10:53:32,774 INFO mapreduce.Job: The url to track the job: http://hadoop103:8088/proxy/application_1629442479906_0002/
    17. 2021-08-21 10:53:32,777 INFO mapreduce.Job: Running job: job_1629442479906_0002
    18. 2021-08-21 10:53:36,844 INFO mapreduce.Job: Job job_1629442479906_0002 running in uber mode : false
    19. 2021-08-21 10:53:36,847 INFO mapreduce.Job:  map 0% reduce 0%
    20. 2021-08-21 10:53:36,878 INFO mapreduce.Job: Job job_1629442479906_0002 failed with state FAILED due to: Application application_1629442479906_0002 failed 2 times due to AM Container for appattempt_1629442479906_0002_000002 exited with  exitCode: 1
    21. Failing this attempt.Diagnostics: [2021-08-21 10:53:35.878]Exception from container-launch.
    22. Container id: container_1629442479906_0002_02_000001
    23. Exit code: 1
    25. [2021-08-21 10:53:35.923]Container exited with a non-zero exit code 1. Error file: prelaunch.err.
    26. Last 4096 bytes of prelaunch.err :
    27. Last 4096 bytes of stderr :
    28. Error: Could not find or load main class org.apache.hadoop.mapreduce.v2.app.MRAppMaster
    30. Please check whether your etc/hadoop/mapred-site.xml contains the below configuration:
    31. <property>
    32.   <name>yarn.app.mapreduce.am.env</name>
    33.   <value>HADOOP_MAPRED_HOME=${full path of your hadoop distribution directory}</value>
    34. </property>
    35. <property>
    36.   <name>mapreduce.map.env</name>
    37.   <value>HADOOP_MAPRED_HOME=${full path of your hadoop distribution directory}</value>
    38. </property>
    39. <property>
    40.   <name>mapreduce.reduce.env</name>
    41.   <value>HADOOP_MAPRED_HOME=${full path of your hadoop distribution directory}</value>
    42. </property>
    44. [2021-08-21 10:53:35.924]Container exited with a non-zero exit code 1. Error file: prelaunch.err.
    45. Last 4096 bytes of prelaunch.err :
    46. Last 4096 bytes of stderr :
    47. Error: Could not find or load main class org.apache.hadoop.mapreduce.v2.app.MRAppMaster
    49. Please check whether your etc/hadoop/mapred-site.xml contains the below configuration:
    50. <property>
    51.   <name>yarn.app.mapreduce.am.env</name>
    52.   <value>HADOOP_MAPRED_HOME=${full path of your hadoop distribution directory}</value>
    53. </property>
    54. <property>
    55.   <name>mapreduce.map.env</name>
    56.   <value>HADOOP_MAPRED_HOME=${full path of your hadoop distribution directory}</value>
    57. </property>
    58. <property>
    59.   <name>mapreduce.reduce.env</name>
    60.   <value>HADOOP_MAPRED_HOME=${full path of your hadoop distribution directory}</value>
    61. </property>
    63. For more detailed output, check the application tracking page: http://hadoop103:8088/cluster/app/application_1629442479906_0002 Then click on links to logs of each attempt.
    64. . Failing the application.
    65. 2021-08-21 10:53:36,929 INFO mapreduce.Job: Counters: 0
    66. [hadoop@hadoop102 ~]$

    解决办法：
    1.在 mapred-site.xml 中如下内容

    1. <property>
    2.         <name>mapreduce.application.classpath</name>
    3.         <value>$HADOOP_MAPRED_HOME/share/hadoop/mapreduce/*:$HADOOP_MAPRED_HOME/share/hadoop/mapreduce/lib/*</value>
    4.     </property>
    5.     <property>
    6.       <name>yarn.app.mapreduce.am.env</name>
    7.       <value>HADOOP_MAPRED_HOME=${HADOOP_HOME}</value>
    8.     </property>
    9.     <property>
    10.       <name>mapreduce.map.env</name>
    11.       <value>HADOOP_MAPRED_HOME=${HADOOP_HOME}</value>
    12.     </property>
    13.     <property>
    14.       <name>mapreduce.reduce.env</name>
    15.       <value>HADOOP_MAPRED_HOME=${HADOOP_HOME}</value>
    16.     </property>

    2.还需在 mapred-site.xml 继续添加 yarn.application.classpath
    查看：

    1. [hadoop@hadoop102 hadoop-3.1.3]$ hadoop classpath
    2. /opt/module/hadoop-3.1.3/etc/hadoop:/opt/module/hadoop-3.1.3/share/hadoop/common/lib/*:/opt/module/hadoop-3.1.3/share/hadoop/common/*:/opt/module/hadoop-3.1.3/share/hadoop/hdfs:/opt/module/hadoop-3.1.3/share/hadoop/hdfs/lib/*:/opt/module/hadoop-3.1.3/share/hadoop/hdfs/*:/opt/module/hadoop-3.1.3/share/hadoop/mapreduce/lib/*:/opt/module/hadoop-3.1.3/share/hadoop/mapreduce/*:/opt/module/hadoop-3.1.3/share/hadoop/yarn:/opt/module/hadoop-3.1.3/share/hadoop/yarn/lib/*:/opt/module/hadoop-3.1.3/share/hadoop/yarn/*

    把“：”改成“，”
    结果如下

    1. <property>
    2.     <name>yarn.application.classpath</name>
    3.     <value>/opt/module/hadoop-3.1.3/etc/hadoop,
    4.     /opt/module/hadoop-3.1.3/share/hadoop/common/lib/*,
    5.     /opt/module/hadoop-3.1.3/share/hadoop/common/*,
    6.     /opt/module/hadoop-3.1.3/share/hadoop/hdfs,
    7.     /opt/module/hadoop-3.1.3/share/hadoop/hdfs/lib/*,
    8.     /opt/module/hadoop-3.1.3/share/hadoop/hdfs/*,
    9.     /opt/module/hadoop-3.1.3/share/hadoop/mapreduce/lib/*,
    10.     /opt/module/hadoop-3.1.3/share/hadoop/mapreduce/*,
    11.     /opt/module/hadoop-3.1.3/share/hadoop/yarn,
    12.     /opt/module/hadoop-3.1.3/share/hadoop/yarn/lib/*,
    13.     /opt/module/hadoop-3.1.3/share/hadoop/yarn/*
    14.     </value>
    15.   </property>

    总共需要添加的内容

    1. <!--用于集群跑mr任务-->
    2.     <property>
    3.         <name>mapreduce.application.classpath</name>
    4.         <value>$HADOOP_MAPRED_HOME/share/hadoop/mapreduce/*:$HADOOP_MAPRED_HOME/share/hadoop/mapreduce/lib/*</value>
    5.     </property>
    6.     <property>
    7.       <name>yarn.app.mapreduce.am.env</name>
    8.       <value>HADOOP_MAPRED_HOME=${HADOOP_HOME}</value>
    9.     </property>
    10.     <property>
    11.       <name>mapreduce.map.env</name>
    12.       <value>HADOOP_MAPRED_HOME=${HADOOP_HOME}</value>
    13.     </property>
    14.     <property>
    15.       <name>mapreduce.reduce.env</name>
    16.       <value>HADOOP_MAPRED_HOME=${HADOOP_HOME}</value>
    17.     </property>
    18. <property>
    19.     <name>yarn.application.classpath</name>
    20.     <value>/opt/module/hadoop-3.1.3/etc/hadoop,
    21.     /opt/module/hadoop-3.1.3/share/hadoop/common/lib/*,
    22.     /opt/module/hadoop-3.1.3/share/hadoop/common/*,
    23.     /opt/module/hadoop-3.1.3/share/hadoop/hdfs,
    24.     /opt/module/hadoop-3.1.3/share/hadoop/hdfs/lib/*,
    25.     /opt/module/hadoop-3.1.3/share/hadoop/hdfs/*,
    26.     /opt/module/hadoop-3.1.3/share/hadoop/mapreduce/lib/*,
    27.     /opt/module/hadoop-3.1.3/share/hadoop/mapreduce/*,
    28.     /opt/module/hadoop-3.1.3/share/hadoop/yarn,
    29.     /opt/module/hadoop-3.1.3/share/hadoop/yarn/lib/*,
    30.     /opt/module/hadoop-3.1.3/share/hadoop/yarn/*
    31.     </value>
    32.   </property>