1.Create a new java project, then copy examples folder from /home/hadoop/hadoop-1.0.4/src;
Create a new folder named src, then Paste to the project to this folder.
Error: Could not find or load main class
right-click src folder, --> build Path --> Use as source Folder
2.Copy hadoop-1.0.4-eclipse-plugin.jar to eclipse/plugin . Then restart eclipse.
3.Set the hadoop install directory and configure the hadoop location.
4.Attched the hadoop source code for the project, then you can check hadoop source code freely.
5.Java heap space Error
java.lang.OutOfMemoryError: Java heap space at org.apache.hadoop.mapred.MapTask$MapOutputBuffer.<init>(MapTask.java:949) at org.apache.hadoop.mapred.MapTask$NewOutputCollector.<init>(MapTask.java:674) at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:756) at org.apache.hadoop.mapred.MapTask.run(MapTask.java:370) at org.apache.hadoop.mapred.LocalJobRunner$Job.run(LocalJobRunner.java:212) int maxMemUsage = sortmb << 20; int recordCapacity = (int)(maxMemUsage * recper); recordCapacity -= recordCapacity % RECSIZE; kvbuffer = new byte[maxMemUsage - recordCapacity];
so we should configure the value of io.sort.mb to avoid this.
我运行的机器环境配置比较低，three nodes, all 512M memory .
我没有在core-site.xml中设置这个参数的值，为了这次job,我直接设置在job的driver code中,
conf.set("io.sort.mb","10");
6.sample test data for WordCount:
10 9 8 7 6 5 4 3 2 1 line1 line3 line2 line5 Line4 运行结果文件是： 1 1 10 1 2 1 3 1 4 1 5 1 6 1 7 1 8 1 9 1 line1 2 line2 2 line3 2 line4 2 line5 2 line6 1
还有一个文件是_Success.表明job执行成功。
可以看到执行后的文件是排过序的。是根据key 值的类型进行排序的，我们wordcount示例中，key值是string类型。
7.在Wordcount示例中，没有专门处理如果输出目录已经存在的情况，为了方便测试，我们添加如下的代码来处理目录.
Path outPath = new Path(args[1]); FileSystem dfs = FileSystem.get(outPath.toUri(), conf); if (dfs.exists(outPath)) { dfs.delete(outPath, true); }
8.why the wordcount demo 's mapper and reduce class are both static?
（为什么WordCount示例中的mapper和reducer都设计成static的，难道非要这样吗？）
Let me remove the static key word for mapper class, then run the job, you will get exception as follow:
java.lang.RuntimeException: java.lang.NoSuchMethodException: org.apache.hadoop.examples.WordCount$TokenizerMapper.<init>() at org.apache.hadoop.util.ReflectionUtils.newInstance(ReflectionUtils.java:115) at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:719)at org.apache.hadoop.mapred.MapTask.run(MapTask.java:370) at org.apache.hadoop.mapred.LocalJobRunner$Job.run(LocalJobRunner.java:212) Caused by: java.lang.NoSuchMethodException: org.apache.hadoop.examples.WordCount$TokenizerMapper.<init>() at java.lang.Class.getConstructor0(Class.java:2730)
在这个时候，mapper类变成了wordcount类的内部类，反射辅助类无法准备地找到它的构造函数，无法实例化。
解决方案，把mapper类从内部类转成非内部类，从wordcount类中拿出来，放到外面去或另起一个文件，这样
执行依然可以。
我们可以看到，我们的示例，尽可能地简单，都放在一个类里面了，使用static就可以保证可以正确运行，如果我们的mapper和reducer不是特别复杂，这样的设计也无可厚非。如果复杂的话，最好单拎出来放一个类。
9.默认我们在eclipse里面直接调试运行或直接运行的时候，我们并非是执行在hadoop cluster上面的，而是进程中模拟执行的，这样方便我们进行调试，我们可以看到console中会有输出类似LocalJobRunner的字样，而不是JobTracker去执行。
这就是为什么，即使我们设置reducetask number大于1的时候，我们仍会在输出的目录里面看到一个part-0000之类的输出，是因为localjobrunner只支持一个.
为了方便我们直接在这里写完代码，就模拟在集群上执行，是很有必要的，有时候是因为你写的代码不在集群上执行就
不能及时地发现错误（分布式应用程序写的时候还是需要注意很多事项的）。
因为提交到集群其实需要做的一件事就是打包你的代码为jar文件，然后提交到集群中去，所以这里需要做这些事情。
我使用spork兄的EJob类来完成这件事，如果你熟悉可以自己写，可以参照http://www.cnblogs.com/spork/archive/2010/04/21/1717592.html.
参照文章，然后在驱动代码中进行部分调整即可。
10.
如果我想把单词中第一个字母小于N的放在第一个reduce task中完成，其他的放在第二个reduce task中输出，该怎么做呢？
写自己的partitioner类，默认的partitioner类是HashPartitioner类，我们简单实现自己的，然后设置一下就可以了。
11.附上修改后的完整的WordCount类源码：
package org.apache.hadoop.examples;
import java.io.File;
import java.io.IOException;
import java.util.StringTokenizer;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapred.JobConf;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.Mapper;
import org.apache.hadoop.mapreduce.Partitioner;
import org.apache.hadoop.mapreduce.Reducer;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
import org.apache.hadoop.util.GenericOptionsParser;
class TokenizerMapper
extends Mapper<Object, Text, Text, IntWritable>{
private final IntWritable one = new IntWritable(1);
private Text word = new Text();
@SuppressWarnings("unused")
public void map(Object key, Text value, Context context
) throws IOException, InterruptedException {
if(false){
StringTokenizer itr = new StringTokenizer(value.toString());
while (itr.hasMoreTokens()) {
word.set(itr.nextToken());
context.write(word, one);
}
}else
{
String s = value.toString();
String[] words = s.split("\s+");
for (int i = 0; i < words.length; i++) {
words[i] = words[i].replaceAll("[^\w]", "");
// System.out.println(words[i]);
word.set(words[i].toUpperCase());
if(words[i].length()>0)
context.write(word,one);
}
}
}
}
public class WordCount {
public static class MyPartitioner<K, V> extends Partitioner<K, V> {
public int getPartition(K key, V value,
int numReduceTasks) {
if(key.toString().toUpperCase().toCharArray()[0]<'N') return 0;
else return 1;
}
}
public static class IntSumReducer
extends Reducer<Text,IntWritable,Text,IntWritable> {
private IntWritable result = new IntWritable();
public void reduce(Text key, Iterable<IntWritable> values,
Context context
) throws IOException, InterruptedException {
int sum = 0;
for (IntWritable val : values) {
sum += val.get();
}
result.set(sum);
context.write(key, result);
}
}
public static void main(String[] args) throws Exception {
args= "hdfs://namenode:9000/user/hadoop/englishwords hdfs://namenode:9000/user/hadoop/out".split(" ");
File jarFile = EJob.createTempJar("bin");
EJob.addClasspath("/home/hadoop/hadoop-1.0.4/conf");
//conf.set("mapred.job.tracker","namenode:9001");
ClassLoader classLoader = EJob.getClassLoader();
Thread.currentThread().setContextClassLoader(classLoader);
Configuration conf = new Configuration();
String[] otherArgs = new GenericOptionsParser(conf, args).getRemainingArgs();
if (otherArgs.length != 2) {
System.err.println("Usage: wordcount <in> <out>");
System.exit(2);
}
//drop output directory if exists
Path outPath = new Path(args[1]);
FileSystem dfs = FileSystem.get(outPath.toUri(), conf);
if (dfs.exists(outPath)) {
dfs.delete(outPath, true);
}
conf.set("io.sort.mb","10");
Job job = new Job(conf, "word count");
((JobConf) job.getConfiguration()).setJar(jarFile.toString());
job.setNumReduceTasks(2);//use to reducer process to process work
job.setPartitionerClass(MyPartitioner.class);
job.setJarByClass(WordCount.class);
job.setMapperClass(TokenizerMapper.class);
job.setCombinerClass(IntSumReducer.class);
job.setReducerClass(IntSumReducer.class);
job.setOutputKeyClass(Text.class);
job.setOutputValueClass(IntWritable.class);
FileInputFormat.addInputPath(job, new Path(otherArgs[0]));
FileOutputFormat.setOutputPath(job, new Path(otherArgs[1]));
System.exit(job.waitForCompletion(true) ? 0 : 1);
}
}