JAVA高级面试进阶训练营视频教程

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一．简介

   要在Windows下的 Eclipse上调试Hadoop2代码，所以我们在windows下的Eclipse配置hadoop-eclipse-plugin- 2.6.0.jar插件，并在运行Hadoop的WordCount代码时出现了一系列的问题，搞了好几天终于能运行起代码。接下来我们来看看问题并怎么解决，提供给跟我同样遇到的问题作为参考。

Hadoop2的WordCount.java统计代码如下：

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.output.FileOutputFormat;

public class WordCount {

  public static class TokenizerMapper
       extends Mapper<Object, Text, Text, IntWritable>{

    private final static IntWritable one = new IntWritable(1);
    private Text word = new Text();

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

  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 {
    Configuration conf = new Configuration();
　　 conf.set("mapred.job.tracker", "hadoopmaster:9001");//在windows下面必须设置
    conf.set("fs.default.name", "hdfs://hadoopmaster:9000");
    Job job = Job.getInstance(conf, "word count");
    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(args[0]));
    FileOutputFormat.setOutputPath(job, new Path(args[1]));
    System.exit(job.waitForCompletion(true) ? 0 : 1);
  }
}

问题一.An internal error occurred during: "Map/Reducelocation status updater".java.lang.NullPointerException

把hadoop-eclipse-plugin-2.6.0.jar放到Eclipse的plugins目录下，我们的Eclipse目录是 F:\tool\eclipse-jee-juno-SR2\eclipse-jee-juno-SR2\plugins，重启一下Eclipse，然 后，打开Window-->Preferens，可以看到Hadoop Map/Reduc选项，然后点击出现了An internal error occurredduring: "Map/Reduce location status updater".java.lang.NullPointerException，如图所示：

解决：

我们发现刚配置部署的Hadoop2还没创建输入和输出目录，先在hdfs上建个文件夹 。

#bin/hdfs dfs -mkdir –p /user/root/input

#bin/hdfs dfs -mkdir -p  /user/root/output

 我们在Eclipse的DFS Locations目录下看到我们这两个目录，如图所示：

问题二.Exception in thread "main" java.lang.NullPointerException atjava.lang.ProcessBuilder.start(Unknown Source)

运行Hadoop2的WordCount.java代码时出现了这样错误：

log4j:WARNPlease initialize the log4j system properly.
log4j:WARN Seehttp://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.
Exception in thread "main" java.lang.NullPointerException
       atjava.lang.ProcessBuilder.start(Unknown Source)
       atorg.apache.hadoop.util.Shell.runCommand(Shell.java:482)
       atorg.apache.hadoop.util.Shell.run(Shell.java:455)
       atorg.apache.hadoop.util.Shell$ShellCommandExecutor.execute(Shell.java:715)
       atorg.apache.hadoop.util.Shell.execCommand(Shell.java:808)
       atorg.apache.hadoop.util.Shell.execCommand(Shell.java:791)
       at

分析：下载Hadoop2以上版本时，在Hadoop2的bin目录下没有winutils.exe

解决：

1.下载https://codeload.github.com/srccodes/hadoop-common-2.2.0-bin/zip /master下载hadoop-common-2.2.0-bin-master.zip，然后解压后，把hadoop-common-2.2.0- bin-master下的bin全部复制放到我们下载的Hadoop2的binHadoop2/bin目录下。如图所示：

2.Eclipse-》window-》Preferences 下的Hadoop Map/Peduce 把下载放在我们的磁盘的Hadoop目录引进来，如图所示：

3.Hadoop2配置变量环境HADOOP_HOME 和path，如图所示：

问题三.Exception in thread "main"java.lang.UnsatisfiedLinkError:org.apache.hadoop.io.nativeio.NativeIO$Windows.access0(Ljava/lang/String;I)Z

当我们解决了问题三时，在运行WordCount.java代码时，出现这样的问题:

log4j:WARN No appenders could be found forlogger (org.apache.hadoop.metrics2.lib.MutableMetricsFactory).
log4j:WARN Please initialize the log4jsystem properly.
log4j:WARN Seehttp://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.
Exception in thread "main"java.lang.UnsatisfiedLinkError:org.apache.hadoop.io.nativeio.NativeIO$Windows.access0(Ljava/lang/String;I)Z
       atorg.apache.hadoop.io.nativeio.NativeIO$Windows.access0(Native Method)
       atorg.apache.hadoop.io.nativeio.NativeIO$Windows.access(NativeIO.java:557)
       atorg.apache.hadoop.fs.FileUtil.canRead(FileUtil.java:977)
       atorg.apache.hadoop.util.DiskChecker.checkAccessByFileMethods(DiskChecker.java:187)
       atorg.apache.hadoop.util.DiskChecker.checkDirAccess(DiskChecker.java:174)
       atorg.apache.hadoop.util.DiskChecker.checkDir(DiskChecker.java:108)
       atorg.apache.hadoop.fs.LocalDirAllocator$AllocatorPerContext.confChanged(LocalDirAllocator.java:285)
       atorg.apache.hadoop.fs.LocalDirAllocator$AllocatorPerContext.getLocalPathForWrite(LocalDirAllocator.java:344)
       atorg.apache.hadoop.fs.LocalDirAllocator.getLocalPathForWrite(LocalDirAllocator.java:150)
       atorg.apache.hadoop.fs.LocalDirAllocator.getLocalPathForWrite(LocalDirAllocator.java:131)
       atorg.apache.hadoop.fs.LocalDirAllocator.getLocalPathForWrite(LocalDirAllocator.java:115)
       atorg.apache.hadoop.mapred.LocalDistributedCacheManager.setup(LocalDistributedCacheManager.

分析：C:\Windows\System32下缺少hadoop.dll,把这个文件拷贝到C:\Windows\System32下面即可。

解决：hadoop-common-2.2.0-bin-master下的bin的hadoop.dll放到C:\Windows\System32下，然后重启电脑，也许还没那么简单，还是出现这样的问题。

我们在继续分析：我们在出现错误的的atorg.apache.hadoop.io.nativeio.NativeIO$Windows.access(NativeIO.java:557)我们来看这个类NativeIO的557行，如图所示：

Windows的唯一方法用于检查当前进程的请求，在给定的路径的访问权限，所以我们先给以能进行访问，我们自己先修改源代码，return true 时允许访问。我们下载对应hadoop源代码，hadoop-2.6.0-src.tar.gz解压，hadoop-2.6.0-src\hadoop- common-project\hadoop-common\src\main\java\org\apache\hadoop\io\nativeio 下NativeIO.java 复制到对应的Eclipse的project，然后修改557行为return true如图所示：

问题四：org.apache.hadoop.security.AccessControlException: Permissiondenied: user=zhengcy, access=WRITE,inode="/user/root/output":root:supergroup:drwxr-xr-x

我们在执行运行WordCount.java代码时，出现这样的问题:

2014-12-18 16:03:24,092  WARN (org.apache.hadoop.mapred.LocalJobRunner:560) - job_local374172562_0001
org.apache.hadoop.security.AccessControlException: Permission denied: user=zhengcy, access=WRITE, inode="/user/root/output":root:supergroup:drwxr-xr-x
    at org.apache.hadoop.hdfs.server.namenode.FSPermissionChecker.checkFsPermission(FSPermissionChecker.java:271)
    at org.apache.hadoop.hdfs.server.namenode.FSPermissionChecker.check(FSPermissionChecker.java:257)
    at org.apache.hadoop.hdfs.server.namenode.FSPermissionChecker.check(FSPermissionChecker.java:238)
    at org.apache.hadoop.hdfs.server.namenode.FSPermissionChecker.checkPermission(FSPermissionChecker.java:179)
    at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.checkPermission(FSNamesystem.java:6512)
    at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.checkPermission(FSNamesystem.java:6494)
    at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.checkAncestorAccess(FSNamesystem.java:6446)
    at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.mkdirsInternal(FSNamesystem.java:4248)
    at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.mkdirsInt(FSNamesystem.java:4218)
    at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.mkdirs(FSNamesystem.java:4191)
    at org.apache.hadoop.hdfs.server.namenode.NameNodeRpcServer.mkdirs(NameNodeRpcServer.java:8

分析：我们没权限访问output目录。

解决：我们 在设置hdfs配置的目录是在hdfs-site.xml配置hdfs文件存放的地方,我们在这个etc/hadoop下的hdfs-site.xml添加

<property> 
     <name>dfs.permissions</name> 
     <value>false</value> 
  </property>

设置没有权限，不过我们在正式的 服务器上不能这样设置。

问题五：File/usr/root/input/file01._COPYING_ could only be replicated to 0 nodes instead ofminRepLication (=1) There are 0 datanode(s) running and no node(s) are excludedin this operation

如图所示：

分析：  我们在第一次执行#hadoop namenode –format 完然后在执行#sbin/start-all.sh 再执行#jps，能看到Datanode，在执行#hadoop namenode –format然后执行#jps这时看不到Datanode ,如图所示：

然后我们想把文本放到输入目录执行bin/hdfs dfs -put/usr/local/hadoop/hadoop-2.6.0/test/* /user/root/input  把/test/*文件上传到hdfs的/user/root/input中,出现这样的问题。

 解决：是我们执行太多次了hadoopnamenode –format，创建了多个，去我们对应的hdfs目录删除hdfs-site.xml配置的保存datanode和namenode目录即可解决。