1 public class STjoinEx { 2     private static final String TIMES = "TIMES"; 3  4     public static void main(String[] args) throws Exception { 5         Configuration configuration = new Configuration(); 6         configuration.setInt(TIMES, 1); 7         String[] remainingArgs = new GenericOptionsParser(configuration, args).getRemainingArgs(); 8         if (remainingArgs.length != 2) { 9             System.err.println("STjoinEx <input> <output>");10             System.exit(2);11         }12 13         Job job = new Job(configuration, STjoinEx.class.getSimpleName());14         job.setJarByClass(STjoinEx.class);15         job.setMapperClass(Map.class);16         job.setReducerClass(Reduce.class);17         job.setInputFormatClass(KeyValueTextInputFormat.class);18         job.setOutputFormatClass(TextOutputFormat.class);19         job.setOutputKeyClass(Text.class);20         job.setOutputValueClass(Text.class);21 22         FileInputFormat.setInputPaths(job, new Path(remainingArgs[0]));23         FileOutputFormat.setOutputPath(job, new Path(remainingArgs[1]));24 25         System.exit(job.waitForCompletion(true) ? 0 : 1);26 27     }28 29     public static class Map extends Mapper<Text, Text, Text, Text> {30         final static Text LEFT_TABLE = new Text();31         final static Text RIGHT_TABLE = new Text();32 33         @Override34         protected void map(Text key, Text value, Context context) throws IOException, InterruptedException {35             // left table36             LEFT_TABLE.set("1 " + value);37             context.write(key, LEFT_TABLE);38             // right table39             RIGHT_TABLE.set("2 " + key);40             context.write(value, RIGHT_TABLE);41         }42     }43 44     public static class Reduce extends Reducer<Text, Text, Text, Text> {45         private static final int INDENT = 2;46         private static final Text GRAND_PARENT = new Text();47         private static final Text GRAND_CHILD = new Text();48 49         @Override50         protected void reduce(Text key, Iterable<Text> values, Context context) throws IOException, InterruptedException {51             // output header52             int times = context.getConfiguration().getInt(TIMES, 1);53             if (times == 1) {54                 context.write(new Text("grandChild"), new Text("grandParent"));55                 context.getConfiguration().setInt(TIMES, ++times);56             }57 58             // prepare matrix59             int headChar = 0;60             String[] grandChild = new String[10];61             String[] grandParent = new String[10];62             int grandChildNum = 0;63             int grandParentNum = 0;64 65             for (Text value : values) {66                 headChar = value.charAt(0);67                 if (headChar == ‘1‘) {68                     grandParent[grandParentNum] = value.toString().substring(2);69                     grandParentNum++;70                 } else {71                     grandChild[grandChildNum] = value.toString().substring(2);72                     grandChildNum++;73                 }74             }75 76             // multiply77             if (grandChildNum != 0 && grandChildNum != 0) {78                 for (int i = 0; i < grandChildNum; i++) {79                     GRAND_CHILD.set(grandChild[i]);80                     for (int j = 0; j < grandParentNum; j++) {81                         GRAND_PARENT.set(grandParent[j]);82                         context.write(GRAND_CHILD, GRAND_PARENT);83                     }84                 }85             }86         }87     }88 }

 1 14/10/07 11:12:51 INFO mapred.JobClient:  map 0% reduce 0% 2 14/10/07 11:12:54 INFO mapred.JobClient:  map 100% reduce 0% 3 14/10/07 11:13:01 INFO mapred.JobClient:  map 100% reduce 33% 4 14/10/07 11:13:04 INFO mapred.JobClient: Task Id : attempt_201410021756_0048_r_000000_0, Status : FAILED 5 java.lang.NullPointerException 6     at org.apache.hadoop.io.Text.encode(Text.java:388) 7     at org.apache.hadoop.io.Text.set(Text.java:178) 8     at main.ch5.STjoinEx$Reduce.reduce(STjoinEx.java:96) 9     at main.ch5.STjoinEx$Reduce.reduce(STjoinEx.java:61)10     at org.apache.hadoop.mapreduce.Reducer.run(Reducer.java:177)11     at org.apache.hadoop.mapred.ReduceTask.runNewReducer(ReduceTask.java:649)12     at org.apache.hadoop.mapred.ReduceTask.run(ReduceTask.java:418)13     at org.apache.hadoop.mapred.Child$4.run(Child.java:255)14     at java.security.AccessController.doPrivileged(Native Method)15     at javax.security.auth.Subject.doAs(Subject.java:396)16     at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1190)17     at org.apache.hadoop.mapred.Child.main(Child.java:249)

 1 grandChild    grandParent 2 Jone    Alice 3 Jone    Jesse 4 Tom    Alice 5 Tom    Jesse 6 Tom    Mary 7 Tom    Ben 8 Jone    Mary 9 Jone    Ben10 Philip    Alice11 Philip    Jesse12 Mark    Alice13 Mark    Jesse