java Hadoop 中的 NoClassDefFoundError 错误
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NoClassDefFoundError Error in Hadoop
提问by SLearner
So after going through many posts on SO and revising some java basics, I still get this error
因此，在阅读了许多关于 SO 的帖子并修改了一些 Java 基础知识之后，我仍然收到此错误
Exception in thread "main" java.lang.NoClassDefFoundError: org/apache/hadoop/io/Writable
at java.lang.Class.getDeclaredMethods0(Native Method)
at java.lang.Class.privateGetDeclaredMethods(Class.java:2451)
at java.lang.Class.getMethod0(Class.java:2694)
at java.lang.Class.getMethod(Class.java:1622)
at sun.launcher.LauncherHelper.getMainMethod(LauncherHelper.java:494)
at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:486)
I know the difference between java.lang.NoClassDefFoundErrorand ClassNotFoundExceptionin Java, and have looked through what usually causes it. In a nutshell, it is because some class is unavailable to the program during Run Time but is available during Compile time. Hence I get no compile time errors.
我知道Javajava.lang.NoClassDefFoundError和ClassNotFoundExceptionJava 中的区别，并且查看了通常导致它的原因。简而言之，这是因为某些类在运行时对程序不可用，但在编译时可用。因此我没有得到编译时错误。
I have added two classpaths, one to commons-logging-1.1.3.jarand the other to the hadoop-core.*jar.
I am pretty confident that the classpaths are correct.
我已经添加了两个类路径，一个commons-logging-1.1.3.jar和其他的hadoop-core.*jar。我非常有信心类路径是正确的。
Here are the imports in my Program
这是我的程序中的导入
import java.io.*;
import org.apache.hadoop.io.SequenceFile;
import org.apache.hadoop.io.SequenceFile.*;
import org.apache.hadoop.io.SequenceFile.Writer;
import org.apache.hadoop.io.*;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.io.Writable;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.conf.*;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.*;
采纳答案by Tejas Patil
You need to create a jar out of the java code like given here:
您需要使用此处给出的 java 代码创建一个 jar ：
$ mkdir my_classes
$ javac -classpath $HADOOP_HOME/hadoop-$HADOOP_VERSION-core.jar -d my_classes <name of the main class>
$ jar -cvf <name of the jar> -C my_classes .
Run the jar this way:
以这种方式运行 jar：
$HADOOP_HOME/bin/hadoop jar <name of the jar> <name of the main class> <arguments to the program>
See the documentation pageof hadoop for the description of jar command.
有关jar 命令的说明，请参阅hadoop的文档页面。
