Hadoop 1.2.1 - mapreduce 编译期间的“java.lang.NoClassDefFoundError: org/apache/hadoop/mapred/JobConf”

Question

我正在尝试学习如何使用 Hadoop 1.2.1。

我已经创建了我的第一个集群，但无法在 java 中编译 mapreduce 示例（Eclipse 2021-03）

这是我的源代码：

import java.io.*;
import java.util.*;    

import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.*;
import org.apache.hadoop.mapred.*;
import org.apache.hadoop.mapred.Reducer;

public class WordCount {

    public static class WordCountMapper extends MapReduceBase
            implements Mapper<LongWritable, Text, Text, IntWritable> {

            private Text word = new Text();

            public void map(LongWritable key, Text value,
                            OutputCollector<Text, IntWritable> output, Reporter reporter)
                            throws IOException {
                    String line = value.toString();
                    StringTokenizer tokenizer = new StringTokenizer(line);
                    while (tokenizer.hasMoreTokens()) {
                            word.set(tokenizer.nextToken());
                            output.collect(word, new IntWritable(1));
                    }
            }
    }

    public static class WordCountReducer extends MapReduceBase
                    implements Reducer<Text, IntWritable, Text, IntWritable> {

            public void reduce(Text key, Iterator<IntWritable> values,
                            OutputCollector<Text, IntWritable> output, Reporter reporter)
                            throws IOException {

                    int total = 0;
                    while (values.hasNext()) {
                            total += values.next().get();
                    }
                    output.collect(key,  new IntWritable(total));
            }
    }

    public static void main(String[] args) throws Exception {
            JobConf conf = new JobConf(WordCount.class);
            conf.setJobName("Word Count");

            conf.setOutputKeyClass(Text.class);
            conf.setOutputValueClass(IntWritable.class);

            conf.setMapperClass(WordCountMapper.class);
            conf.setReducerClass(WordCountReducer.class);

            conf.setInputFormat(TextInputFormat.class);
            conf.setOutputFormat(TextOutputFormat.class);

            FileInputFormat.addInputPath(conf, new Path(args[0]));
            FileOutputFormat.setOutputPath(conf, new Path(args[1]));

            JobClient.runJob(conf);
    }
}

我不是此代码的作者，只是在网络某处找到示例。

While trying to run application, it shows me this error:

 Exception in thread "main" java.lang.NoClassDefFoundError: org/apache/hadoop/mapred/JobConf
        at WordCount.main(WordCount.java:45)
Caused by: java.lang.ClassNotFoundException: org.apache.hadoop.mapred.JobConf
        at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:606)
        at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:168)
        at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:522)
        ... 1 more

找不到 JobConf 类的原因是什么？ 在引用的库中，我有 hadoop-core-1.2.1.jar 和 hadoop 目录中的其他库。

谁能告诉我我做错了什么？ :)

最好的问候！

Answer 1

主要是为作业设置配置。 但你还没有进口。所以尝试导入以下内容：

import org.apache.hadoop.mapred.JobConf;