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Hadoop中WritableComparable 和 comparator
1.WritableComparable
查看HadoopAPI,如图所示：
WritableComparable继承自Writable和java.lang.Comparable接口，是一个Writable也是一个Comparable，也就是说，既可以序列化，也可以比较！
再看看它的实现类，发现BooleanWritable, BytesWritable, ByteWritable, DoubleWritable, FloatWritable, IntWritable, LongWritable, MD5Hash, NullWritable, Record, RecordTypeInfo, Text, VIntWritable, VLongWritable都实现了WritableComparable类！
WritableComparable的实现类之间相互来比较，在Map/Reduce中，任何用作键来使用的类都应该实现WritableComparable接口！
Example:
1 package cn.roboson.writable; 2 3 import java.io.DataInput; 4 import java.io.DataOutput; 5 import java.io.IOException; 6 7 import org.apache.hadoop.io.WritableComparable; 8 9 /**10 * 1.自定义一个类，继承WritableComparable11 * 2.发现有三个未实现的方法，两个是Writable接口的（序列化），一个是Comparable接口的（用来比较）12 * 3.自定义比较，这里以counter来作为比较13 * @author roboson14 *15 */16 public class MyWritableComparable implements WritableComparable<MyWritableComparable>{17 18 private int counter;19 private long timestamp;20 public MyWritableComparable() {21 // TODO Auto-generated constructor stub22 }23 24 public MyWritableComparable(int counter,long timestamp) {25 // TODO Auto-generated constructor stub26 this.counter = counter;27 this.timestamp = timestamp;28 }29 30 @Override31 public void readFields(DataInput in) throws IOException {32 // TODO Auto-generated method stub33 34 //将输入流中的字节流数据转化为结构化数据35 counter = in.readInt();36 timestamp = in.readLong();37 }38 39 @Override40 public void write(DataOutput out) throws IOException {41 // TODO Auto-generated method stub42 43 //讲结构化数据写入输出流44 out.writeInt(counter);45 out.writeLong(timestamp);46 }47 48 @Override49 public int compareTo(MyWritableComparable other) {50 // TODO Auto-generated method stub51 int thisValue = http://www.mamicode.com/this.counter;52 int otherValue =http://www.mamicode.com/ other.counter;53 return (thisValue < otherValue ? -1 : (thisValue =http://www.mamicode.com/= otherValue ? 0 : 1));54 }55 56 public int getCounter() {57 return counter;58 }59 60 public void setCounter(int counter) {61 this.counter = counter;62 }63 64 public long getTimestamp() {65 return timestamp;66 }67 68 public void setTimestamp(long timestamp) {69 this.timestamp = timestamp;70 }71 72 73 public static void main(String[] args) {74 MyWritableComparable comparable = new MyWritableComparable(3,4);75 MyWritableComparable otherComparable = new MyWritableComparable(4, 5);76 int value =http://www.mamicode.com/ comparable.compareTo(otherComparable);77 if(value=http://www.mamicode.com/=-1){78 System.out.println("comparable<otherComparable");79 }else if(value=http://www.mamicode.com/=0){80 System.out.println("comparable=otherComparable");81 }else{82 System.out.println("comparable>otherComparable");83 }84 }85 }
运行结果：
2.RawComparator
对于MapReduce来说，因为中间有个基于键的排序阶段，所以类型的比较是非常重要的。Hadoop中提供了原生的比较接口RawComparator，该接口继承子Java Comparator接口。RawComparator接口允许其实现直接比较数据流中的记录，无需先把数据流饭序列化为对象，这样便避免了新建对象的额外开销。
1 package org.apache.hadoop.io; 2 3 import java.util.Comparator; 4 5 public interface RawComparator<T> extends Comparator<T>{ 6 7 //自己的方法 8 public int compare(byte[] b1, int s1, int l1, byte[] b2,int s2, int l2); 9 10 //继承自Comparator的方法11 @Override12 public int compare(T o1, T o2);13 14 @Override15 public boolean equals(Object obj);16 }
查看HadoopAPI：
该类并非被多数的衍生类所实现，其具体的子类为WritableComparator，多数情况下是作为实现Writable接口的类的内置类，提供序列化字节的比较。如下图说所示：BooleanWritable, BytesWritable, ByteWritable, org.apache.hadoop.io.serializer.DeserializerComparator, DoubleWritable, FloatWritable, IntWritable, JavaSerializationComparator, LongWritable, LongWritable, MD5Hash, NullWritable， RecordComparator, Text, UTF8,都实现了RawComparator，作为其内部类。
而WritableComparator则是其的具体子类。
3.WritableComparator
在《Hadoop权威指南》中，说到这儿，很模糊，只说WritableComparator是对继承自WritableComparable类的RawCompartor类的一个通用实现。让人看着很迷惑，这句话什么意思呢？
首先、在第二个小标题RawComparator中，我门都知道WritableComparator实现了RawComparator这个接口，也就是说，WritableComparator是RawComparator的实现。
其次、是对继承自WritableComparable类的RawComparator的一个通用实现。那么继承自WritableComparable类的RawComparator都有哪些呢？也就是说那些类，继承自WritableComparator，并且实现了RawComparator？在第二个小标题RawComparator中有也都说明清楚了，上面的红色部分!同理，实现了WritableComparable类的在第一个小标题WritableComparable中也有说明，红色部分字体！也就谁说WritableComparator是对BooleanWritable.Comparator, BytesWritable.Comparator, ByteWritable.Comparator, DoubleWritable.Comparator, FloatWritable.Comparator, IntWritable.Comparator, LongWritable.Comparator, MD5Hash.Comparator, NullWritable.Comparator, RecordComparator, Text.Comparator, UTF8.Comparator这些类的一个通用实现！这句话就引出了WritableComparator的两个功能：第一，它提供了对原始compare()方法的一个默认实现。该方法能够饭序列化将流中进行比较的对象，并调用对象的compara()方法。第二，它充当的是RawComparator实例的工厂（已注册Writable的实现）。例如，为了获得IntWratable的comparator，我们直接如下调用：
RawComparator<IntWritable> comparator = WritableComparator.get(IntWratable.class);
再来看看WritableComparator这个类是如何定义的，如下图所示：
WritableComparator类类似于一个注册表，里面记录了所有Comparator类的集合。Comparators成员用一张Hash表记录Key=Class，value=http://www.mamicode.com/WritableComprator的注册信息.这就是它能够充当RawComparator实例工厂的原因!因为它本省的实现中有意个HashMap集合，HashMap<Class,WritableComparator>根据对应的Class，就能返回一个响应的WritableComparator!
Example:
1 package cn.roboson.writable; 2 3 import java.io.ByteArrayInputStream; 4 import java.io.ByteArrayOutputStream; 5 import java.io.DataInputStream; 6 import java.io.DataOutputStream; 7 import java.io.IOException; 8 9 import org.apache.hadoop.io.IntWritable;10 import org.apache.hadoop.io.RawComparator;11 import org.apache.hadoop.io.Writable;12 import org.apache.hadoop.io.WritableComparator;13 14 /**15 * 1.通过WritableComparator获得IntWritable类的RawComparator实例16 * 2.通过两种方式来比较17 * @author roboson18 *19 */20 21 public class ComparableFinish {22 23 public static void main(String[] args) throws IOException {24 25 //创建两个IntWritable来比较26 IntWritable writable1 = new IntWritable(163);27 IntWritable writable2 = new IntWritable(165);28 29 //获得IntWritable的RawComparator实例30 RawComparator<IntWritable> intRawComparator = WritableComparator.get(IntWritable.class);31 32 //直接比较对象33 int value1 =intRawComparator.compare(writable1, writable2);34 35 if(value1==-1){36 System.out.println("writable1<writable2");37 }else if(value1==0){38 System.out.println("writable1=writable2");39 }else{40 System.out.println("writable1>writable2");41 }42 43 //序列化两个对象，获得其字节流44 byte[] byte1 = serizlize(writable1);45 byte[] byte2 = serizlize(writable2);46 47 //直接通过字符流比较大小48 int value2 = intRawComparator.compare(byte1, 0, 4, byte2, 0, 4);49 if(value2==-1){50 System.out.println("writable1<writable2");51 }else if(value2==0){52 System.out.println("writable1=writable2");53 }else{54 System.out.println("writable1>writable2");55 }56 }57 58 public static byte[] serizlize(Writable writable) throws IOException{59 60 //创建一个输出字节流对象61 ByteArrayOutputStream out = new ByteArrayOutputStream();62 DataOutputStream dataout = new DataOutputStream(out);63 64 //将结构化数据的对象writable写入到输出字节流。65 writable.write(dataout);66 return out.toByteArray();67 }68 69 public static byte[] deserizlize(Writable writable,byte[] bytes) throws IOException{70 71 //创建一个输入字节流对象，将字节数组中的数据，写入到输入流中72 ByteArrayInputStream in = new ByteArrayInputStream(bytes);73 DataInputStream datain = new DataInputStream(in);74 75 //将输入流中的字节流数据反序列化76 writable.readFields(datain);77 return bytes;78 79 }80 }
运行结果：
关于序列化方面的知识，可以参考我的博客《Hadoop序列化》地址如下：
http://www.cnblogs.com/robert-blue/p/4157768.html
参考博文：
http://blog.csdn.net/keda8997110/article/details/8518255
http://www.360doc.com/content/12/0827/09/9318309_232551844.shtml
Hadoop中WritableComparable 和 comparator