java - Hadoop 作业返回异常 "classNotFound"

Question

我正在尝试在 hadoop 上启动一个字数统计图 reduce。当我要求 hadoop 完成这项工作时，它返回以下内容:

Exception in thread "main" java.lang.ClassNotFoundException: sdz.hadoop.wordcount.WordCountDriver
    at java.base/java.net.URLClassLoader.findClass(URLClassLoader.java:471)
    at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:588)
    at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:521)
    at java.base/java.lang.Class.forName0(Native Method)
    at java.base/java.lang.Class.forName(Class.java:398)
    at org.apache.hadoop.util.RunJar.run(RunJar.java:232)
    at org.apache.hadoop.util.RunJar.main(RunJar.java:153)

这是输出:

 hadoop jar wordcount.jar sdz.hadoop.wordcount.WordCountDriver ../source.txt ../results

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jar 文件如下所示:

$jar tf wordcount.jar 
META-INF/
META-INF/MANIFEST.MF
WordCountReducer.class
WordCountMapper.class
WordCountDriver.class

---

已使用以下命令/输出生成 jar 文件:

$jar -cvf wordcount.jar .
added manifest
adding: WordCountReducer.class(in = 1720) (out= 723)(deflated 57%)
adding: WordCountMapper.class(in = 2347) (out= 936)(deflated 60%)
adding: WordCountDriver.class(in = 2278) (out= 1118)(deflated 50%)

---

这些类是从以下命令生成的:

javac -classpath $HADOOP_CLASSPATH WordCount*.java

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Java 文件包含(我知道无用的导入):

• WordCountReducer.java

• WordCountMapper.java

• WordCountDriver.java

---

关于这个的其他问题说要添加“job.setJarByClass(WordCountDriver.class);”。但我已经有了。那么我的错误是什么？

Answer 1

作业将失败，因为它会尝试(但失败)在 jar 中的以下位置找到您的类:

sdz/hadoop/wordcount/WordCountDriver.class

最快的解决方案是简单地从运行命令中删除包名称(以及类本身中的任何包声明):

hadoop jar wordcount.jar WordCountDriver ../source.txt ../results

一个更稳定的解决方案是使用 Maven 或 Gradle 等构建工具正确构建您的 jar。