使用MR求解多个矩阵的乘积之后

首先介绍涉及到的知识点，如下：

1）value的类型是IntArrayWritable，将整型数组值取出的方法有两种。

a.其一，就是使用value的toArray()方法,返回值是一个Object obValue,然后使用Array.get(obValue,index)，就可以获得数组中的元素（Object类型），其中index是数组索引，然后再利用Integer.parseInt(Array.get(obValue,index).toString())将其转化为int型即可。

b.第二种方法是使用value的get()方法获得数组，不过是Writable[],但是不能直接使用IntWritable[] intArr=(IntWritable[])value.get()，不能完成Writable[]到IntWritable[]的转化。正确的方式是：

for(Writable val:value.get()){
IntWritable intVal=(IntWritable)val;

}

2)利用InputSplit获得分片的文件名，因为待求解的问题如下所示：

X=(B+u(s+b+d+f))*F

矩阵s、b、d、f与F相乘之前，要扩大u倍，而B矩阵则不需要，通过检测文件名来判断是否先扩大u倍然后再与F相乘。获得文件名的代码如下：

InputSplit inputSplit=context.getInputSplit();
String fileName=((FileSplit)inputSplit).getPath().getName();

如果是要获得文件的上级目录名，可以使用以下代码(http://blog.csdn.net/shallowgrave/article/details/7757914)：

String dirName = ((FileSplit) inputSplit).getPath().getParent().getName();

矩阵文件如下所示：




map()中实现矩阵乘法的代码如下：

 obValue=value.toArray();
           InputSplit inputSplit=context.getInputSplit();
           String fileName=((FileSplit)inputSplit).getPath().getName();

           if (!(fileName.startsWith("10"))) {
            for (int i=0;i<rightMatrix[0].length;++i){
                sum=0;
                for (int j=0;j<rightMatrix.length;++j){
                      sum+= Integer.parseInt(Array.get(obValue,j).toString())*2*rightMatrix[j][i];
                }
                arraySum[i]=new IntWritable(sum);
            }
             map_value.set(arraySum);
           }
            else{
               for (int i=0;i<rightMatrix[0].length;++i){
                   sum=0;
                   for (int j=0;j<rightMatrix.length;++j){
                       sum+= Integer.parseInt(Array.get(obValue,j).toString())*rightMatrix[j][i];
                   }
                   arraySum[i]=new IntWritable(sum);
               }
               map_value.set(arraySum);
           }
           context.write(key,map_value);
       }

3)第三个知识点就是IntArrayWritable中toString()方法的定义中StringBuilder的使用。

   public String toString(){
         StringBuilder sb=new StringBuilder();
         for (Writable val:get()){
             IntWritable intWritable=(IntWritable)val;//使用1)中的第二种方法
             sb.append(intWritable.get());//在sb后追加一个字符
             sb.append(",");//再追加一个','字符
         }
         sb.deleteCharAt(sb.length()-1);//删除最后一个字符，也就是最后一个‘,’
         return sb.toString();//将sb转化为字符串返回

完整代码如下：

 /**
  * Created with IntelliJ IDEA.
  * User: hadoop
  * Date: 16-3-5
  * Time: 上午9:24
  * To change this template use File | Settings | File Templates.
  */
 import org.apache.hadoop.conf.Configuration;
 import org.apache.hadoop.fs.FileSystem;
 import java.io.IOException;
 import java.lang.reflect.Array;
 import java.net.URI;
 import java.util.Arrays;

 import org.apache.hadoop.fs.Path;
 import org.apache.hadoop.io.IntWritable;
 import org.apache.hadoop.io.ArrayWritable;
 import org.apache.hadoop.io.Text;
 import org.apache.hadoop.io.Writable;
 import org.apache.hadoop.mapreduce.InputSplit;
 import org.apache.hadoop.mapreduce.Job;
 import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
 import org.apache.hadoop.mapreduce.lib.input.FileSplit;
 import org.apache.hadoop.mapreduce.lib.input.SequenceFileInputFormat;
 import org.apache.hadoop.mapreduce.lib.output.TextOutputFormat;
 import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
 import org.apache.hadoop.mapreduce.Reducer;
 import org.apache.hadoop.mapreduce.Mapper;

 public class MutiInputMatrixProduct {
     public static class MyMapper extends Mapper<IntWritable,IntArrayWritable,IntWritable,IntArrayWritable>{
         public IntArrayWritable map_value=new IntArrayWritable();
         public static  int[][] rightMatrix=new int[][]{{10,10,10,10,10},{10,10,10,10,10},{10,10,10,10,10},
                 {10,10,10,10,10}};
         public Object obValue=null;
         public IntWritable[] arraySum=new IntWritable[rightMatrix[0].length];
         public int sum=0;
        public void map(IntWritable key,IntArrayWritable value,Context context) throws IOException, InterruptedException {
            obValue=value.toArray();
            InputSplit inputSplit=context.getInputSplit();
            String fileName=((FileSplit)inputSplit).getPath().getName();

            if (!(fileName.equals("10IntArray"))) {
             for (int i=0;i<rightMatrix[0].length;++i){
                 sum=0;
                 for (int j=0;j<rightMatrix.length;++j){
                       sum+= Integer.parseInt(Array.get(obValue,j).toString())*2*rightMatrix[j][i];
                 }
                 arraySum[i]=new IntWritable(sum);
             }
              map_value.set(arraySum);
            }
             else{
                for (int i=0;i<rightMatrix[0].length;++i){
                    sum=0;
                    for (int j=0;j<rightMatrix.length;++j){
                        sum+= Integer.parseInt(Array.get(obValue,j).toString())*rightMatrix[j][i];
                    }
                    arraySum[i]=new IntWritable(sum);
                }
                map_value.set(arraySum);
            }
            context.write(key,map_value);
        }
     }
     public static class MyReducer extends Reducer<IntWritable,IntArrayWritable,IntWritable,Text>{
         public int[] sum=null;
         public Object obValue=null;
         public Text resultText=null;

         public void setup(Context context){
             sum=new int[Integer.parseInt(context.getConfiguration().get("leftMatrixNum"))];
         }

         public void reduce(IntWritable key,Iterable<IntArrayWritable>value,Context context) throws IOException, InterruptedException {
           for(IntArrayWritable intValue:value){
               obValue=intValue.toArray();
               for (int i=0;i<Array.getLength(obValue);++i){
                   sum[i]+=Integer.parseInt(Array.get(obValue,i).toString());
               }

           }
          resultText=new Text(Arrays.toString(sum));
             for (int i=0;i<sum.length;++i){
                 sum[i]=0;
             }
           context.write(key,resultText);
         }

     }

     public static void main(String[]args) throws IOException, ClassNotFoundException, InterruptedException {
         String uri="/home/hadoop/2016Test/Input";
         String outUri="/home/hadoop/2016Test/Output";
         Configuration conf=new Configuration();
         FileSystem fs=FileSystem.get(URI.create(uri),conf);
         fs.delete(new Path(outUri),true);
         conf.set("leftMatrixNum","5");
         Job job=new Job(conf,"MultiMatrix");
         job.setJarByClass(MutiInputMatrixProduct.class);
         job.setInputFormatClass(SequenceFileInputFormat.class);
         job.setOutputFormatClass(TextOutputFormat.class);
         job.setMapperClass(MyMapper.class);
         job.setReducerClass(MyReducer.class);
         job.setMapOutputKeyClass(IntWritable.class);
         job.setMapOutputValueClass(IntArrayWritable.class);
         job.setOutputKeyClass(IntWritable.class);
         job.setOutputValueClass(Text.class);
         FileInputFormat.setInputPaths(job, new Path(uri));
         FileOutputFormat.setOutputPath(job,new Path(outUri));
         System.exit(job.waitForCompletion(true)?0:1);
     }
 }
 class IntArrayWritable extends ArrayWritable {
     public IntArrayWritable(){
         super(IntWritable.class);
     }
     public String toString(){
         StringBuilder sb=new StringBuilder();
         for (Writable val:get()){
             IntWritable intWritable=(IntWritable)val;
             sb.append(intWritable.get());
             sb.append(",");
         }
         sb.deleteCharAt(sb.length()-1);
         return sb.toString();
     }
 }