运行MR作业出现的问题之三（非插件形式）

  最近一直在研究MR作业的问题，在《Hadoop开发者第四期》中运行实例时遇到了如下问题，这个问题还比较常见，乍一看写的源码才发现了问题的所在，首先将错误log贴出：

2012-04-13 19:38:07,940 WARN  mapred.JobClient - No job jar file set.  User classes may not be found. See JobConf(Class) or JobConf#setJar(String).
2012-04-13 19:38:07,956 INFO  input.FileInputFormat - Total input paths to process : 1
2012-04-13 19:38:08,661 INFO  mapred.JobClient - Running job: job_201204121502_0021
2012-04-13 19:38:09,664 INFO  mapred.JobClient -  map 0% reduce 0%
2012-04-13 19:38:15,683 INFO  mapred.JobClient - Task Id : attempt_201204121502_0021_m_000000_0, Status : FAILED
java.lang.RuntimeException: java.lang.ClassNotFoundException: com.loogn.cache.JoinWithDistribute$JoinMapper
    at org.apache.hadoop.conf.Configuration.getClass(Configuration.java:996)
    at org.apache.hadoop.mapreduce.JobContext.getMapperClass(JobContext.java:212)
    at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:602)
    at org.apache.hadoop.mapred.MapTask.run(MapTask.java:323)
    at org.apache.hadoop.mapred.Child$4.run(Child.java:270)
    at java.security.AccessController.doPrivileged(Native Method)
    at javax.security.auth.Subject.doAs(Subject.java:396)
    at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1157)
    at org.apache.hadoop.mapred.Child.main(Child.java:264)
Caused by: java.lang.ClassNotFoundException: com.loogn.cache.JoinWithDistribute$JoinMapper
    at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
    at java.lang.ClassLoader.loadClass(Clas
2012-04-13 19:38:20,792 INFO  mapred.JobClient - Task Id : attempt_201204121502_0021_m_000000_1, Status : FAILED
java.lang.RuntimeException: java.lang.ClassNotFoundException: com.loogn.cache.JoinWithDistribute$JoinMapper
    at org.apache.hadoop.conf.Configuration.getClass(Configuration.java:996)
    at org.apache.hadoop.mapreduce.JobContext.getMapperClass(JobContext.java:212)
    at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:602)
    at org.apache.hadoop.mapred.MapTask.run(MapTask.java:323)
    at org.apache.hadoop.mapred.Child$4.run(Child.java:270)
    at java.security.AccessController.doPrivileged(Native Method)
    at javax.security.auth.Subject.doAs(Subject.java:396)
    at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1157)
    at org.apache.hadoop.mapred.Child.main(Child.java:264)
Caused by: java.lang.ClassNotFoundException: com.loogn.cache.JoinWithDistribute$JoinMapper
    at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
    at java.lang.ClassLoader.loadClass(Clas
2012-04-13 19:38:25,873 INFO  mapred.JobClient - Task Id : attempt_201204121502_0021_m_000000_2, Status : FAILED
java.lang.RuntimeException: java.lang.ClassNotFoundException: com.loogn.cache.JoinWithDistribute$JoinMapper
    at org.apache.hadoop.conf.Configuration.getClass(Configuration.java:996)
    at org.apache.hadoop.mapreduce.JobContext.getMapperClass(JobContext.java:212)
    at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:602)
    at org.apache.hadoop.mapred.MapTask.run(MapTask.java:323)
    at org.apache.hadoop.mapred.Child$4.run(Child.java:270)
    at java.security.AccessController.doPrivileged(Native Method)
    at javax.security.auth.Subject.doAs(Subject.java:396)
    at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1157)
    at org.apache.hadoop.mapred.Child.main(Child.java:264)
Caused by: java.lang.ClassNotFoundException: com.loogn.cache.JoinWithDistribute$JoinMapper
    at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
    at java.lang.ClassLoader.loadClass(Clas

  错误为： No job jar file set. User classes may not be found. See JobConf(Class) or JobConf#setJar(String).这是因为在写MR作业配置时出了问题，将

conf.set("mapred.jar", jarFile.toString());

中的第一个参数人为地输入错了，在开始时写入是conf.set("mared",jarFile.toString());导致在运行作业时，无法上传jar到集群中，从而分布式系统提示找不到Map类，从而导致整个作业无法完成。