Hadoop: How to using two mapper to do different thing
In my work, I run a situation that I want to use A mapper reading a file with to fields (questionId, questionTags) and outpute format likes key: questionId value: questionTags, while B mapper reading a dir which contains many files named by questionId with questionContent as its file content and output format likes key: questionId/fileName value: questionContent. Then a reducer do some string operations.The framework above is
A mapper
> reducer
B mapper
The problem can't be solved by ChainMapper.
I found that the two mapper's output format is the same. So, the other way is to adopt one mapper to read questions dir and tags file.
two problems;
a.
QuestionTagsWritable e1 = null, e2 = null;
for (QuestionTagsWritable e : values) {
System.out.println("xx = " + e.toString());
if (e.isTags) {
e1 = e;
} else {
e2 = e;
}
}
solution: e1 = new QuestionTagsWritable(true,tmp.content); //pass value not address
b.
FileSplit fileSplit = (FileSplit) context.getInputSplit();
java.lang.ClassCastException:org.apache.hadoop.mapreduce.lib.input.TaggedInputSplit cannot be cast toorg.apache.hadoop.mapreduce.lib.input.FileSplit
solution:
InputSplit split = context.getInputSplit();
Class<? extends InputSplit> splitClass = split.getClass();
FileSplit fileSplit = null;
if (splitClass.equals(FileSplit.class)) {
fileSplit = (FileSplit) split;
} else if (splitClass.getName().equals(
"org.apache.hadoop.mapreduce.lib.input.TaggedInputSplit")) {
// begin reflection hackery...
try {
Method getInputSplitMethod = splitClass
.getDeclaredMethod("getInputSplit");
getInputSplitMethod.setAccessible(true);
fileSplit = (FileSplit) getInputSplitMethod.invoke(split);
} catch (Exception e) {
// wrap and re-throw error
throw new IOException(e);
}
// end reflection hackery
}
see:http://stackoverflow.com/questions/11130145/hadoop-multipleinputs-fails-with-classcastexception
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