hadoop2.2编程: 重写comparactor

要点：

1. 类型比较在hadoop的mapreduce中非常重要，主要用来比较keys;

2. hadoop中的RawComparator<T>接口继承自java的comparator, 主要用来比较序列化的objects;

3. hadoop中的WritableComparator class更全面，提供了两种主要的比较方法，一种是直接比较object，另一种是较serialized representations;

　　　　举例来说 比较object: compare(new IntWritable(21), new IntWritable(998)); 比较serialized representations： compare(serialize(new 　　　　　　IntWritable(21)), serialize(new IntWritable(998))),

提示：继承关系

1.org.apache.hadoop.io
Interface RawComparator<T>
//description
public interface RawComparator<T>
extends Comparator<T>
//method
int compare(byte[] b1, int s1, int l1, byte[] b2, int s2, int l2)
2.org.apache.hadoop.io
Interface WritableComparable<T>
//description
public interface WritableComparable<T>
extends Writable, Comparable<T>
//method
Methods inherited from interface org.apache.hadoop.io.Writable
readFields, write
3.java.lang.Object
     |__ org.apache.hadoop.io.WritableComparator
//description
public class WritableComparator
extends Object
implements RawComparator
//methods
int compare(byte[] b1, int s1, int l1, byte[] b2, int s2, int l2)
int compare(Object a, Object b)
int compare(WritableComparable a, WritableComparable b)
static int compareBytes(byte[] b1, int s1, int l1, byte[] b2, int s2, int l2)
4.java.util
Interface Comparator<T>
//description
public interface Comparator<T>
//methods
int compare(T o1, T o2)
boolean equals(Object obj)

代码：

 import java.lang.Byte;
 import java.io.DataOutputStream;
 import java.io.ByteArrayOutputStream;
 
 import org.apache.hadoop.io.IntWritable;
 import org.apache.hadoop.io.WritableComparator;
 import org.apache.hadoop.io.RawComparator;
 
 public class MyIntWritableComparactor {
 
   public static byte[] serialize(IntWritable writable) throws Exception {
     ByteArrayOutputStream out = new ByteArrayOutputStream();
     DataOutputStream dataOut = new DataOutputStream(out);
     writable.write(dataOut);
     dataOut.close();
     return out.toByteArray();
   }
 
   @SuppressWarnings("unchecked")
   public static void main(String[] args) throws Exception {
     RawComparator<IntWritable> comparator = WritableComparator.get(IntWritable.class);
     IntWritable w1 = new IntWritable(13);
     IntWritable w2 = new IntWritable(12);
     System.out.println("w1: " + w1 + " w2: " + w2);
     System.out.println("w1 compare w2 : " + comparator.compare(w1,w2));
 
     byte[] b1 = serialize(w1);
     byte[] b2 = serialize(w2);
     System.out.println("b1.length: " + b1.length);
     System.out.println("b2.length: " + b2.length);
     System.out.println("b1.length compare b2.length: " + comparator.compare(b1, 0, b1.length, b2, 0, b2.length));
 
   }
 }

编译，运行：

//注意我用的是hadoop2.2
$ source $YARN_HOME/libexec/hadoop-config.sh
$ mkdir myclass
$ javac -d myclass MyIntWritableCompare.java
$ jar -cvf  mycompare.jar -C myclass ./
$ export HADOOP_CLASSPATH=$CLASSPATH:mycompare.jar
$ yarn MyIntWritableCompare

输出：

$ yarn jar text.jar Text
w1:  w2:
w1 compare w2 :
b1.length:
b2.length:
b1.length compare b2.length: