使用分布式缓存有两点需要注意，这是今天折腾了一天的体会。
1）利用DistributedCache类添加缓存文件的语句要紧紧跟在Configuration实例之后
1 Configuration conf=new Configuration(); 2 DistributedCache.addCacheFile(new URI(cachePath),conf);//添加分布式缓存 3 FileSystem fs=FileSystem.get(URI.create(cachePath),conf); 4 System.out.println(fs.getUri().toString()); 5 fs.delete(new Path(outUri),true); 6 conf.set("rightMatrixNum","5"); 7 conf.set("u","5"); 8 Job job=new Job(conf,"MultiMatrix"); 9 //DistributedCache.addCacheFile(new URI(cachePath),conf);//添加分布式缓存
原先添加在第9行，运行一直报“空引用”的错，将 DistributedCache.addCacheFile(new URI(cachePath),conf);添加到第2行，紧跟在conf之后，就OK了(不过要满足以下第2点)
2）第2点就是使用分布式缓存的自定义mapper/reducer类必须定义为内部类。
3)当满足以上两点之后，程序中就可以正常的使用分布式缓存了，不过运行又会遇到一个问题，“FileNotFoundException....”后面就是缓存文件的路径，程序找不到缓存的文件，MR程序中读取文件时，默认FileSystem是hdfs,也就是从集群上读取，但是这些缓存文件恰恰是放在数据节点本地文件系统中的，所以程序中当然会“找不到文件”，解决方法很简单，在使用DistributedCache.getLocalCacheFiles()得到路径的基础上，在其前面追加字符串”file://“就可以了，如下所示(第6行)(http://hugh-wangp.iteye.com/blog/1468989)：
1 public void setup(Context context) throws IOException { 2 Configuration conf=context.getConfiguration(); 3 System.out.println("map setup() start!"); 4 //URI[] cacheFiles=DistributedCache.getCacheFiles(context.getConfiguration()); 5 Path[] cacheFiles=DistributedCache.getLocalCacheFiles(conf); 6 String localCacheFile="file://"+cacheFiles[0].toString(); 7 System.out.println("local path is:"+cacheFiles[0].toString()); 8 // URI[] cacheFiles=DistributedCache.getCacheFiles(context.getConfiguration()); 9 FileSystem fs =FileSystem.get(URI.create(localCacheFile), conf); 10 SequenceFile.Reader reader=null; 11 reader=new SequenceFile.Reader(fs,new Path(localCacheFile),conf); 12 IntWritable key= (IntWritable)ReflectionUtils.newInstance(reader.getKeyClass(),conf); 13 DoubleArrayWritable value= (DoubleArrayWritable)ReflectionUtils.newInstance(reader.getValueClass(),conf); 14 int valueLength=Array.getLength(value.toArray()); 15 while (reader.next(key,value)){ 16 obValue=value.toArray(); 17 leftMatrix[key.get()]=new double[valueLength]; 18 for (int i=0;i<valueLength;++i){ 19 leftMatrix[key.get()][i]=Double.parseDouble(Array.get(obValue, i).toString()); 20 } 21 22 } 23 }
将“data/F100”作为缓存文件，然后在setup()函数中读取这个缓存文件，将其值存入double[][]leftMatrix二维数组中，也就是存储了左矩阵，这就涉及到一个问题，leftMatrix是MyMapper类的成员变量，应该是在其构造函数中完成初始化，这就需要得到矩阵的行数和列数，而获得行数和列数的方法目前我知道的就只有通过conf.setInt(“leftColumn”,100),conf.getInt("leftColumn",100)得到，但是前提是获得context，然后context.getConfiguration().getInt(...),也就是说MyMapper的构造函数需要以Context context作为参数，那就写一个嘛，然后运行，不成功。说明在MR框架在创建MyMapper对象的时候，使用的应该是一个无参的构造函数。所以最终只能是在声明成员变量leftMatrix的时候直接初始化。然后乘“data/input/b100”中的b100矩阵，以下就是完整的代码：
1 /** 2 * Created with IntelliJ IDEA. 3 * User: hadoop 4 * Date: 16-3-6 5 * Time: 下午12:47 6 * To change this template use File | Settings | File Templates. 7 */ 8 import org.apache.hadoop.conf.Configuration; 9 import org.apache.hadoop.fs.FileSystem; 10 import java.io.IOException; 11 import java.lang.reflect.Array; 12 import java.net.URI; 13 14 import org.apache.hadoop.fs.Path; 15 import org.apache.hadoop.io.*; 16 import org.apache.hadoop.mapreduce.InputSplit; 17 import org.apache.hadoop.mapreduce.Job; 18 import org.apache.hadoop.mapreduce.lib.input.FileInputFormat; 19 import org.apache.hadoop.mapreduce.lib.input.FileSplit; 20 import org.apache.hadoop.mapreduce.lib.input.SequenceFileInputFormat; 21 import org.apache.hadoop.mapreduce.lib.output.SequenceFileOutputFormat; 22 import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat; 23 import org.apache.hadoop.mapreduce.Reducer; 24 import org.apache.hadoop.mapreduce.Mapper; 25 import org.apache.hadoop.filecache.DistributedCache; 26 import org.apache.hadoop.util.ReflectionUtils; 27 28 public class MutiDoubleInputMatrixProduct { 29 public static void main(String[]args) throws IOException, ClassNotFoundException, InterruptedException { 30 String uri="data/input/b100"; 31 String outUri="sOutput"; 32 String cachePath="data/F100"; 33 Configuration conf=new Configuration(); 34 DistributedCache.addCacheFile(URI.create(cachePath),conf);//添加分布式缓存 35 FileSystem fs=FileSystem.get(URI.create(uri),conf); 36 fs.delete(new Path(outUri),true); 37 conf.setInt("leftMatrixColumnNum",100); 38 conf.setFloat("u",0.5f); 39 Job job=new Job(conf,"MultiMatrix"); 40 //DistributedCache.addCacheFile(URI.create(cachePath),conf);//添加分布式缓存 41 //DistributedCache.addLocalFiles(conf,cachePath); 42 43 job.setJarByClass(MutiDoubleInputMatrixProduct.class); 44 job.setInputFormatClass(SequenceFileInputFormat.class); 45 job.setOutputFormatClass(SequenceFileOutputFormat.class); 46 job.setMapperClass(MyMapper.class); 47 job.setReducerClass(MyReducer.class); 48 job.setMapOutputKeyClass(IntWritable.class); 49 job.setMapOutputValueClass(DoubleArrayWritable.class); 50 job.setOutputKeyClass(IntWritable.class); 51 job.setOutputValueClass(DoubleArrayWritable.class); 52 FileInputFormat.setInputPaths(job, new Path(uri)); 53 FileOutputFormat.setOutputPath(job,new Path(outUri)); 54 System.exit(job.waitForCompletion(true)?0:1); 55 } 56 public static class MyMapper extends Mapper<IntWritable,DoubleArrayWritable,IntWritable,DoubleArrayWritable>{ 57 public DoubleArrayWritable map_value=new DoubleArrayWritable(); 58 public double[][] leftMatrix=new double[100][100];/******************************************/ 59 public Object obValue=null; 60 public DoubleWritable[] arraySum=null; 61 public double sum=0; 62 public void setup(Context context) throws IOException { 63 Configuration conf=context.getConfiguration(); 64 System.out.println("map setup() start!"); 65 //URI[] cacheFiles=DistributedCache.getCacheFiles(context.getConfiguration()); 66 Path[] cacheFiles=DistributedCache.getLocalCacheFiles(conf); 67 String localCacheFile="file://"+cacheFiles[0].toString(); 68 //URI[] cacheFiles=DistributedCache.getCacheFiles(conf); 69 //DistributedCache. 70 System.out.println("local path is:"+cacheFiles[0].toString()); 71 // URI[] cacheFiles=DistributedCache.getCacheFiles(context.getConfiguration()); 72 FileSystem fs =FileSystem.get(URI.create(localCacheFile), conf); 73 SequenceFile.Reader reader=null; 74 reader=new SequenceFile.Reader(fs,new Path(localCacheFile),conf); 75 IntWritable key= (IntWritable)ReflectionUtils.newInstance(reader.getKeyClass(),conf); 76 DoubleArrayWritable value= (DoubleArrayWritable)ReflectionUtils.newInstance(reader.getValueClass(),conf); 77 int valueLength=0; 78 while (reader.next(key,value)){ 79 obValue=value.toArray(); 80 if(key.get()<1){ 81 valueLength=Array.getLength(obValue); 82 } 83 //leftMatrix=new double[valueLength][Integer.parseInt(context.getConfiguration().get("leftMatrixColumnNum"))]; 84 for (int i=0;i<valueLength;++i){ 85 leftMatrix[key.get()][i]=Double.parseDouble(Array.get(obValue, i).toString()); 86 } 87 88 } 89 } 90 public void map(IntWritable key,DoubleArrayWritable value,Context context) throws IOException, InterruptedException { 91 obValue=value.toArray(); 92 InputSplit inputSplit=context.getInputSplit(); 93 String fileName=((FileSplit)inputSplit).getPath().getName(); 94 if (fileName.startsWith("FB")) { 95 context.write(key,value); 96 } 97 else{ 98 arraySum=new DoubleWritable[leftMatrix.length]; 99 for (int i=0;i<leftMatrix.length;++i){ 100 sum=0; 101 for (int j=0;j<leftMatrix[0].length;++j){ 102 sum+= leftMatrix[i][j]*Double.parseDouble(Array.get(obValue,j).toString())*(double)(context.getConfiguration().getFloat("u",1f)); 103 } 104 arraySum[i]=new DoubleWritable(sum); 105 //arraySum[i].set(sum); 106 } 107 map_value.set(arraySum); 108 context.write(key,map_value); 109 } 110 } 111 } 112 public static class MyReducer extends Reducer<IntWritable,DoubleArrayWritable,IntWritable,DoubleArrayWritable>{ 113 public DoubleWritable[] sum=null; 114 public Object obValue=null; 115 public DoubleArrayWritable valueArrayWritable=null; 116 117 public void setup(Context context){ 118 int leftMatrixColumnNum=context.getConfiguration().getInt("leftMatrixColumnNum",100); 119 sum=new DoubleWritable[leftMatrixColumnNum]; 120 for (int i=0;i<leftMatrixColumnNum;++i){ 121 sum[i]=new DoubleWritable(0.0); 122 } 123 } 124 125 public void reduce(IntWritable key,Iterable<DoubleArrayWritable>value,Context context) throws IOException, InterruptedException { 126 int valueLength=0; 127 for(DoubleArrayWritable doubleValue:value){ 128 obValue=doubleValue.toArray(); 129 valueLength=Array.getLength(obValue); 130 for (int i=0;i<valueLength;++i){ 131 sum[i]=new DoubleWritable(Double.parseDouble(Array.get(obValue,i).toString())+sum[i].get()); 132 } 133 } 134 valueArrayWritable=new DoubleArrayWritable(); 135 valueArrayWritable.set(sum); 136 context.write(key,valueArrayWritable); 137 for (int i=0;i<sum.length;++i){ 138 sum[i].set(0.0); 139 } 140 141 } 142 } 143 } 144 class DoubleArrayWritable extends ArrayWritable { 145 public DoubleArrayWritable(){ 146 super(DoubleWritable.class); 147 } 148 /* 149 public String toString(){ 150 StringBuilder sb=new StringBuilder(); 151 for (Writable val:get()){ 152 DoubleWritable doubleWritable=(DoubleWritable)val; 153 sb.append(doubleWritable.get()); 154 sb.append(","); 155 } 156 sb.deleteCharAt(sb.length()-1); 157 return sb.toString(); 158 } 159 */ 160 }
如果在声明成员变量leftMatrix的时候没有初始化，而是采用83行的方式初始化，那么在setup()函数中完成了leftMatrix的初始化，一切正常，但是当程序执行到map()函数中的时候，leftMatrix中的元素又变成了全0，最后的结果自然就全是0，原因是什么？？原因是少了第87行代码(http://www.cnblogs.com/lz3018/p/5252814.html)。最新版本如下：
1 /** 2 * Created with IntelliJ IDEA. 3 * User: hadoop 4 * Date: 16-3-6 5 * Time: 下午12:47 6 * To change this template use File | Settings | File Templates. 7 */ 8 import org.apache.hadoop.conf.Configuration; 9 import org.apache.hadoop.fs.FileSystem; 10 import java.io.IOException; 11 import java.lang.reflect.Array; 12 import java.net.URI; 13 14 import org.apache.hadoop.fs.Path; 15 import org.apache.hadoop.io.*; 16 import org.apache.hadoop.mapreduce.InputSplit; 17 import org.apache.hadoop.mapreduce.Job; 18 import org.apache.hadoop.mapreduce.lib.input.FileInputFormat; 19 import org.apache.hadoop.mapreduce.lib.input.FileSplit; 20 import org.apache.hadoop.mapreduce.lib.input.SequenceFileInputFormat; 21 import org.apache.hadoop.mapreduce.lib.output.SequenceFileOutputFormat; 22 import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat; 23 import org.apache.hadoop.mapreduce.Reducer; 24 import org.apache.hadoop.mapreduce.Mapper; 25 import org.apache.hadoop.filecache.DistributedCache; 26 import org.apache.hadoop.util.ReflectionUtils; 27 28 public class MutiDoubleInputMatrixProduct { 29 public static void main(String[]args) throws IOException, ClassNotFoundException, InterruptedException { 30 String uri="data/input/b100"; 31 String outUri="sOutput"; 32 String cachePath="data/F100"; 33 Configuration conf=new Configuration(); 34 DistributedCache.addCacheFile(URI.create(cachePath),conf);//添加分布式缓存 35 FileSystem fs=FileSystem.get(URI.create(uri),conf); 36 fs.delete(new Path(outUri),true); 37 conf.setInt("leftMatrixColumnNum",100); 38 conf.setInt("leftMatrixRowNum",100); 39 conf.setFloat("u",0.5f); 40 Job job=new Job(conf,"MultiMatrix"); 41 //DistributedCache.addCacheFile(URI.create(cachePath),conf);//添加分布式缓存 42 //DistributedCache.addLocalFiles(conf,cachePath); 43 44 job.setJarByClass(MutiDoubleInputMatrixProduct.class); 45 job.setInputFormatClass(SequenceFileInputFormat.class); 46 job.setOutputFormatClass(SequenceFileOutputFormat.class); 47 job.setMapperClass(MyMapper.class); 48 job.setReducerClass(MyReducer.class); 49 job.setMapOutputKeyClass(IntWritable.class); 50 job.setMapOutputValueClass(DoubleArrayWritable.class); 51 job.setOutputKeyClass(IntWritable.class); 52 job.setOutputValueClass(DoubleArrayWritable.class); 53 FileInputFormat.setInputPaths(job, new Path(uri)); 54 FileOutputFormat.setOutputPath(job,new Path(outUri)); 55 System.exit(job.waitForCompletion(true)?0:1); 56 } 57 public static class MyMapper extends Mapper<IntWritable,DoubleArrayWritable,IntWritable,DoubleArrayWritable>{ 58 public DoubleArrayWritable map_value=new DoubleArrayWritable(); 59 public double[][] leftMatrix=null;/******************************************/ 60 public Object obValue=null; 61 public DoubleWritable[] arraySum=null; 62 public double sum=0; 63 public void setup(Context context) throws IOException { 64 Configuration conf=context.getConfiguration(); 65 leftMatrix=new double[conf.getInt("leftMatrixRowNum",10)][conf.getInt("leftMatrixColumnNum",10)]; 66 System.out.println("map setup() start!"); 67 //URI[] cacheFiles=DistributedCache.getCacheFiles(context.getConfiguration()); 68 Path[] cacheFiles=DistributedCache.getLocalCacheFiles(conf); 69 String localCacheFile="file://"+cacheFiles[0].toString(); 70 //URI[] cacheFiles=DistributedCache.getCacheFiles(conf); 71 //DistributedCache. 72 System.out.println("local path is:"+cacheFiles[0].toString()); 73 // URI[] cacheFiles=DistributedCache.getCacheFiles(context.getConfiguration()); 74 FileSystem fs =FileSystem.get(URI.create(localCacheFile), conf); 75 SequenceFile.Reader reader=null; 76 reader=new SequenceFile.Reader(fs,new Path(localCacheFile),conf); 77 IntWritable key= (IntWritable)ReflectionUtils.newInstance(reader.getKeyClass(),conf); 78 DoubleArrayWritable value= (DoubleArrayWritable)ReflectionUtils.newInstance(reader.getValueClass(),conf); 79 int valueLength=0; 80 int rowIndex=0; 81 while (reader.next(key,value)){ 82 obValue=value.toArray(); 83 rowIndex=key.get(); 84 if(rowIndex<1){ 85 valueLength=Array.getLength(obValue); 86 } 87 leftMatrix[rowIndex]=new double[conf.getInt("leftMatrixColumnNum",10)]; 88 //this.leftMatrix=new double[valueLength][Integer.parseInt(context.getConfiguration().get("leftMatrixColumnNum"))]; 89 for (int i=0;i<valueLength;++i){ 90 this.leftMatrix[rowIndex][i]=Double.parseDouble(Array.get(obValue, i).toString()); 91 } 92 93 } 94 } 95 public void map(IntWritable key,DoubleArrayWritable value,Context context) throws IOException, InterruptedException { 96 obValue=value.toArray(); 97 InputSplit inputSplit=context.getInputSplit(); 98 String fileName=((FileSplit)inputSplit).getPath().getName(); 99 if (fileName.startsWith("FB")) { 100 context.write(key,value); 101 } 102 else{ 103 arraySum=new DoubleWritable[this.leftMatrix.length]; 104 for (int i=0;i<this.leftMatrix.length;++i){ 105 sum=0; 106 for (int j=0;j<this.leftMatrix[0].length;++j){ 107 sum+= this.leftMatrix[i][j]*Double.parseDouble(Array.get(obValue,j).toString())*(double)(context.getConfiguration().getFloat("u",1f)); 108 } 109 arraySum[i]=new DoubleWritable(sum); 110 //arraySum[i].set(sum); 111 } 112 map_value.set(arraySum); 113 context.write(key,map_value); 114 } 115 } 116 } 117 public static class MyReducer extends Reducer<IntWritable,DoubleArrayWritable,IntWritable,DoubleArrayWritable>{ 118 public DoubleWritable[] sum=null; 119 public Object obValue=null; 120 public DoubleArrayWritable valueArrayWritable=null; 121 122 public void setup(Context context){ 123 int leftMatrixColumnNum=context.getConfiguration().getInt("leftMatrixColumnNum",100); 124 sum=new DoubleWritable[leftMatrixColumnNum]; 125 for (int i=0;i<leftMatrixColumnNum;++i){ 126 sum[i]=new DoubleWritable(0.0); 127 } 128 } 129 130 public void reduce(IntWritable key,Iterable<DoubleArrayWritable>value,Context context) throws IOException, InterruptedException { 131 int valueLength=0; 132 for(DoubleArrayWritable doubleValue:value){ 133 obValue=doubleValue.toArray(); 134 valueLength=Array.getLength(obValue); 135 for (int i=0;i<valueLength;++i){ 136 sum[i]=new DoubleWritable(Double.parseDouble(Array.get(obValue,i).toString())+sum[i].get()); 137 } 138 } 139 valueArrayWritable=new DoubleArrayWritable(); 140 valueArrayWritable.set(sum); 141 context.write(key,valueArrayWritable); 142 for (int i=0;i<sum.length;++i){ 143 sum[i].set(0.0); 144 } 145 146 } 147 } 148 } 149 class DoubleArrayWritable extends ArrayWritable { 150 public DoubleArrayWritable(){ 151 super(DoubleWritable.class); 152 } 153 /* 154 public String toString(){ 155 StringBuilder sb=new StringBuilder(); 156 for (Writable val:get()){ 157 DoubleWritable doubleWritable=(DoubleWritable)val; 158 sb.append(doubleWritable.get()); 159 sb.append(","); 160 } 161 sb.deleteCharAt(sb.length()-1); 162 return sb.toString(); 163 } 164 */ 165 }
以上就是单机模式下(IDEA)中使用分布式缓存进行矩阵乘法的实例，路径必须是本地路径就可以了(33~37)，就是我创建的工程中的路径。