当我运行MapReduce作业时,我会收到以下错误。
我的工作类别如下:
package mutualfriends;
import org.apache.hadoop.conf.Configured;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
import org.apache.hadoop.util.Tool;
import org.apache.hadoop.util.ToolRunner;
public class MutualSuggest extends Configured implements Tool {
@Override
public int run(String[] args) throws Exception {
// TODO Auto-generated method stub
if(args.length !=2)
{
System.err.println("Usage: MutualSuggest <input path> <outputpath>");
System.exit(-1);
}
Job job = new Job();
job.setJarByClass(MutualSuggest.class);
job.setJobName("Mutual Friends");
FileInputFormat.addInputPath(job, new Path(args[0]));
FileOutputFormat.setOutputPath(job,new Path(args[1]));
job.setMapperClass(MutualSuggestMapper.class);
job.setCombinerClass(MutualSuggestReducer.class);
job.setReducerClass(MutualSuggestReducer.class);
job.setOutputKeyClass(Text.class);
job.setOutputValueClass(Text.class);
System.exit(job.waitForCompletion(true) ? 0:1);
boolean success = job.waitForCompletion(true);
return success ? 0 : 1;
}
public static void main(String[] args) throws Exception
{
MutualSuggest driver = new MutualSuggest();
int exitCode = ToolRunner.run(driver, args);
System.exit(exitCode);
}
}
下面提到了我的Mapper类:
package mutualfriends;
import java.io.IOException;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;
@SuppressWarnings({ "unchecked","rawtypes"})
public class MutualSuggestMapper extends Mapper<Object, Text, Text, Text>{
public String sorted(String name) {
char[] chars = name.toCharArray();
Arrays.sort(chars);
String sorted = new String(chars);
return sorted;
}
@Override
public void map(Object key, Text value, Context context)
throws IOException, InterruptedException {
String line = value.toString();
String[] spl = line.split("=");
String user=spl[0];
String[] friends = spl[1].split(",");
Map m = new HashMap();
for (int i=0;i<friends.length;i++)
{
m.put(sorted(user+friends[i]), sorted(spl[1].replace(",","")));
}
Set x=m.keySet();
Iterator ite=x.iterator();
while (ite.hasNext())
{
Object z=ite.next();
context.write(new Text((String) z),new Text((String) m.get(z)));
}
}
}
我的减速器类如下所述:
package mutualfriends;
import java.io.IOException;
import java.util.HashMap;
import java.util.Map;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Reducer;
public class MutualSuggestReducer extends Reducer<Text, Text, Text, Text>{
@SuppressWarnings({ "rawtypes", "unchecked" })
@Override
public void reduce(Text key, Iterable<Text> values,Context context)
throws IOException, InterruptedException {
Map hm=new HashMap();
int z=1;
for (Text val:values)
{
hm.put(z, new Text(val));
z+=1;
}
String s=new String();
String t=new String();
s= hm.get(1).toString();
t= hm.get(2).toString();
//System.out.println(s+" "+t);
String x = s.replaceAll("[^" + t + "]", "");
System.out.println(key+" "+new Text(x));
context.write(new Text(key),new Text(x));
}
}
在打印时,我得到了正确的输出,如:
AB CD
AC BD
AD BC
BC ADE
BD ACE
BE CD
CD ABE
CE BD
DE BC
但是在编写输出时,如:
context.write(key,new Text(x));
我得到以下错误:
15/07/03 16:13:10 WARN mapred.LocalJobRunner: job_local1502108935_0001
java.lang.NullPointerException
at mutualfriends.MutualSuggestReducer.reduce(MutualSuggestReducer.java:26)
at mutualfriends.MutualSuggestReducer.reduce(MutualSuggestReducer.java:1)
at org.apache.hadoop.mapreduce.Reducer.run(Reducer.java:177)
at org.apache.hadoop.mapred.ReduceTask.runNewReducer(ReduceTask.java:649)
at org.apache.hadoop.mapred.ReduceTask.run(ReduceTask.java:418)
at org.apache.hadoop.mapred.LocalJobRunner$Job.run(LocalJobRunner.java:398)
15/07/03 16:13:11 INFO mapred.JobClient: map 100% reduce 0%
如何解决这个问题?
提前谢谢。
您正在对hm.get(1) and hm.get(2)
进行索引,而不检查映射中是否存在这些关键字。在从HashMap
获取值之前进行检查非常基本的错误
我通过一个简单的修改解决了这个问题。。
通过删除MutualSuggest
类中的job.setCombinerClass(MutualSuggestReducer.class);
语句。
出现此错误的原因是,通过调用job.setCombinerClass(MutualSuggestReducer.class);
,它执行了一次Reducer函数,通过再次调用job.setReducerClass(MutualSuggestReducer.class);
,程序尝试再次执行Reducer功能。所以我删除了job.setCombinerClass(MutualSuggestReducer.class);
。程序运行良好。