Passing different parameters to each mapper

Question

I have a job that uses multiple mappers and one reducer. The mappers are almost identical, except they differ in the value of a String that they use to produce the result.

Currently I have several classes, one for each value of the String I mentioned — it feels like there should be a better way, that doesn't require so much code duplication. Is there a way to pass these String values as parameters to the mappers?

My job looks like this:

Input File A  ---->  Mapper A using
                       String "Foo"  ----+
                                         |--->  Reducer
                     Mapper B using  ----+
Input File B  ---->    String "Bar" 

I want to turn it into something like this:

Input File A  ---->  GenericMapper parameterized
                               with String "Foo" ----+
                                                     |--->  Reducer
                     GenericMapper parameterized ----+ 
Input File B  ---->            with String "Bar"

Edit: Here are two simplified mapper class that I currently have. They accurately represent my actual situation.

class MapperA extends Mapper<Text, Text, Text, Text> {
    public void map(Text key, Text value, Context context) {
        context.write(key, new Text(value.toString() + "Foo"));
    }
}

class MapperB extends Mapper<Text, Text, Text, Text> {
    public void map(Text key, Text value, Context context) {
        context.write(key, new Text(value.toString() + "Bar"));
    }
}

Edit: What string each mapper should use depends only on which file the data comes from. There is no way to differentiate between the files, except through the file name.

Answer 1

Assuming you use file input formats, you can get you current input file name in the mapper like this:

if (context.getInputSplit() instanceof FileSplit) {
    FileSplit fileSplit = (FileSplit) context.getInputSplit();
    Path inputPath = fileSplit.getPath();
    String fileId = ... //parse inputPath into a file id
    ...
}

You can parse inputPath however you want, eg use file name only or partition id only, etc to generate a unique id identifying the input file. For example:

/some/path/A -> A
/some/path/B -> B

Configure your properties for each possible file "id" in your driver:

conf.set("my.property.A", "foo");
conf.set("my.property.B", "bar"); 

In the mapper compute file "id" as stated above and get the value:

conf.get("my.property." + fileId);

Answer 2

Maybe you would use if sentence inside your mapper for choose between strings. What depends the use of one string or another?

Or maybe use Abstract Mapper class.

Answer 3

Maybe something like this ?

abstract class AbstractMapper extends Mapper<Text, Text, Text, Text> {
    protected String text;
    public void map(Text key, Text value, Context context) {
        context.write(key, new Text(value.toString() + text));
    }
}
class MapperImpl1 extends AbstractMapper{
    @Override
    public void map(Text key, Text value, Context context) {
        text = "foo";
        super.map();
    }
}
class MapperImpl2 extends AbstractMapper{
        @Override
        public void map(Text key, Text value, Context context) {
            text = "bar";
            super.map();
        }
    }