Hadoop 基础的 MapReduce 程序中的 java.lang.NoClassDefFoundError

Question

我正在尝试 Hadoop 的基本 MapReduce 程序，其教程在 http://java.dzone.com/articles/hadoop-basics-creating 上

类的完整代码是(代码在上面的网址上)

import java.io.IOException;
import java.util.StringTokenizer;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.Mapper;
import org.apache.hadoop.mapreduce.Reducer;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.input.KeyValueTextInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
import org.apache.hadoop.util.GenericOptionsParser;

public class Dictionary {
public static class WordMapper extends Mapper<Text, Text, Text, Text> {
    private Text word = new Text();

    public void map(Text key, Text value, Context context) throws IOException, InterruptedException {
        StringTokenizer itr = new StringTokenizer(value.toString(), ",");
        while (itr.hasMoreTokens()) {
            word.set(itr.nextToken());
            context.write(key, word);
        }
    }
}

public static class AllTranslationsReducer extends Reducer<Text, Text, Text, Text> {
    private Text result = new Text();

    public void reduce(Text key, Iterable<Text> values, Context context) throws IOException, InterruptedException {
        String translations = "";
        for (Text val : values) {
            translations += "|" + val.toString();
        }
        result.set(translations);
        context.write(key, result);
    }
}

public static void main(String[] args) throws Exception {
    System.out.println("welcome to Java 1");
    Configuration conf = new Configuration();
    System.out.println("welcome to Java 2");
    Job job = new Job(conf, "dictionary");
    job.setJarByClass(Dictionary.class);
    job.setMapperClass(WordMapper.class);
    job.setReducerClass(AllTranslationsReducer.class);
    job.setOutputKeyClass(Text.class);
    job.setOutputValueClass(Text.class);
    job.setInputFormatClass(KeyValueTextInputFormat.class);
    FileInputFormat.addInputPath(job, new Path("/tmp/hadoop-cscarioni/dfs/name/file"));
    FileOutputFormat.setOutputPath(job, new Path("output"));
    System.exit(job.waitForCompletion(true) ? 0 : 1);
}
}

但是在eclipse中运行之后；我收到错误，

welcome to Java 1
Exception in thread "main" java.lang.NoClassDefFoundError: org/apache/commons/logging/LogFactory
at org.apache.hadoop.conf.Configuration.<clinit>(Configuration.java:73)
at Dictionary.main(Dictionary.java:43)
Caused by: java.lang.ClassNotFoundException: org.apache.commons.logging.LogFactory
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
... 2 more

Answer 1

请注意异常是 NoClassDefFoundError 而不是 ClassNotFoundException。

注意:当类在运行时不可见但在编译时可见时，将抛出 NoClassDefFoundError。这可能发生在 JAR 文件的分发或生成中，其中没有包含所有必需的类文件。

修复:请检查您的构建时间和运行时类路径中的差异。

NoClassDefFoundError 和 ClassNotFoundException 是不同的。一个是错误，另一个是异常。

NoClassDefFoundError:由于 JVM 在查找它期望找到的类时遇到问题。由于找不到类文件，在编译时运行的程序无法运行。

ClassNotFoundException:此异常表示在类路径中未找到该类，即我们正在尝试加载类定义，但类路径中不存在包含该类的类/jar。