概述
序列化（Serialization）是指把结构化对象转化为字节流。
反序列化（Deserialization）是序列化的逆过程。把字节流转为结构化对象。
当要在进程间传递对象或持久化对象的时候，就需要序列化对象成字节流，反之当要将接收到或从磁盘读取的字节流转换为对象，就要进行反序列化。
Java 的序列化（Serializable）是一个重量级序列化框架，一个对象被序列化后，会附带很多额外的信息（各种校验信息，header，继承体系…），不便于在网络中高效传输；所以，hadoop 自己开发了一套序列化机制（ Writable），精简，高效。不用像 java 对象类一样传输多层的父子关系，需要哪个属性就传输哪个属性值，大大的减少网络传输的开销。
Writable是Hadoop的序列化格式，hadoop定义了这样一个Writable接口。一个类要支持可序列化只需实现这个接口即可。
public interface Writable { void write(DataOutput out) throws IOException; void readFields(DataInput in) throws IOException; }
如需要将自定义的 bean 放在 key 中传输，则还需要实现 comparable 接口，因为 mapreduce 框中的 shuffle 过程一定会对 key 进行排序,此时，自定义的bean 实现的接口应该是：WritableComparable
代码示例
1 ． 需求
统计每一个用户（手机号）所耗费的总上行流量、下行流量，总流量结果的基础之上再加一个需求：将统计结果按照总流量倒序排序。
准备数据
1363157985066 13726230503 00-FD-07-A4-72-B8:CMCC 120.196.100.82 i02.c.aliimg.com 24 27 2481 24681 200 1363157985066 13726230503 00-FD-07-A4-72-B8:CMCC 120.196.100.82 i02.c.aliimg.com 24 27 2481 24681 200 1363157985066 13726230503 00-FD-07-A4-72-B8:CMCC 120.196.100.82 i02.c.aliimg.com 24 27 2481 24681 200 1363157985066 13726230503 00-FD-07-A4-72-B8:CMCC 120.196.100.82 i02.c.aliimg.com 24 27 2481 24681 200 1363157985066 13726230503 00-FD-07-A4-72-B8:CMCC 120.196.100.82 i02.c.aliimg.com 24 27 2481 24681 200 1363157985066 13726230503 00-FD-07-A4-72-B8:CMCC 120.196.100.82 i02.c.aliimg.com 24 27 2481 24681 200 1363157995052 13826544101 5C-0E-8B-C7-F1-E0:CMCC 120.197.40.4 4 0 264 0 200 1363157991076 13926435656 20-10-7A-28-CC-0A:CMCC 120.196.100.99 2 4 132 1512 200 1363154400022 13926251106 5C-0E-8B-8B-B1-50:CMCC 120.197.40.4 4 0 240 0 200 1363157993044 18211575961 94-71-AC-CD-E6-18:CMCC-EASY 120.196.100.99 iface.qiyi.com 视频网站 15 12 1527 2106 200 1363157995074 84138413 5C-0E-8B-8C-E8-20:7DaysInn 120.197.40.4 122.72.52.12 20 16 4116 1432 200 1363157993055 13560439658 C4-17-FE-BA-DE-D9:CMCC 120.196.100.99 18 15 1116 954 200 1363157995033 15920133257 5C-0E-8B-C7-BA-20:CMCC 120.197.40.4 sug.so.360.cn 信息安全 20 20 3156 2936 200 1363157983019 13719199419 68-A1-B7-03-07-B1:CMCC-EASY 120.196.100.82 4 0 240 0 200 1363157984041 13660577991 5C-0E-8B-92-5C-20:CMCC-EASY 120.197.40.4 s19.cnzz.com 站点统计 24 9 6960 690 200 1363157973098 15013685858 5C-0E-8B-C7-F7-90:CMCC 120.197.40.4 rank.ie.sogou.com 搜索引擎 28 27 3659 3538 200 1363157986029 15989002119 E8-99-C4-4E-93-E0:CMCC-EASY 120.196.100.99 www.umeng.com 站点统计 3 3 1938 180 200 1363157992093 13560439658 C4-17-FE-BA-DE-D9:CMCC 120.196.100.99 15 9 918 4938 200 1363157992093 13560439658 C4-17-FE-BA-DE-D9:CMCC 120.196.100.99 15 9 918 4938 200 1363157992093 13560439658 C4-17-FE-BA-DE-D9:CMCC 120.196.100.99 15 9 918 4938 200 1363157992093 13560439658 C4-17-FE-BA-DE-D9:CMCC 120.196.100.99 15 9 918 4938 200 1363157992093 13560439658 C4-17-FE-BA-DE-D9:CMCC 120.196.100.99 15 9 918 4938 200 1363157992093 13560439658 C4-17-FE-BA-DE-D9:CMCC 120.196.100.99 15 9 918 4938 200 1363157992093 13560439658 C4-17-FE-BA-DE-D9:CMCC 120.196.100.99 15 9 918 4938 200 1363157992093 13560439658 C4-17-FE-BA-DE-D9:CMCC 120.196.100.99 15 9 918 4938 200 1363157986041 13480253104 5C-0E-8B-C7-FC-80:CMCC-EASY 120.197.40.4 3 3 180 180 200 1363157986041 13480253104 5C-0E-8B-C7-FC-80:CMCC-EASY 120.197.40.4 3 3 180 180 200 1363157986041 13480253104 5C-0E-8B-C7-FC-80:CMCC-EASY 120.197.40.4 3 3 180 180 200 1363157986041 13480253104 5C-0E-8B-C7-FC-80:CMCC-EASY 120.197.40.4 3 3 180 180 200 1363157986041 13480253104 5C-0E-8B-C7-FC-80:CMCC-EASY 120.197.40.4 3 3 180 180 200 1363157986041 13480253104 5C-0E-8B-C7-FC-80:CMCC-EASY 120.197.40.4 3 3 180 180 200 1363157986041 13480253104 5C-0E-8B-C7-FC-80:CMCC-EASY 120.197.40.4 3 3 180 180 200 1363157986041 13480253104 5C-0E-8B-C7-FC-80:CMCC-EASY 120.197.40.4 3 3 180 180 200 1363157986041 13480253104 5C-0E-8B-C7-FC-80:CMCC-EASY 120.197.40.4 3 3 180 180 200 1363157986041 13480253104 5C-0E-8B-C7-FC-80:CMCC-EASY 120.197.40.4 3 3 180 180 200 1363157984040 13602846565 5C-0E-8B-8B-B6-00:CMCC 120.197.40.4 2052.flash2-http.qq.com 综合门户 15 12 1938 2910 200 1363157995093 13922314466 00-FD-07-A2-EC-BA:CMCC 120.196.100.82 img.qfc.cn 12 12 3008 3720 200 1363157982040 13502468823 5C-0A-5B-6A-0B-D4:CMCC-EASY 120.196.100.99 y0.ifengimg.com 综合门户 57 102 7335 110349 200 1363157986072 18320173382 84-25-DB-4F-10-1A:CMCC-EASY 120.196.100.99 input.shouji.sogou.com 搜索引擎 21 18 9531 2412 200 1363157990043 13925057413 00-1F-64-E1-E6-9A:CMCC 120.196.100.55 t3.baidu.com 搜索引擎 69 63 11058 48243 200 1363157988072 13760778710 00-FD-07-A4-7B-08:CMCC 120.196.100.82 2 2 120 120 200 1363157985066 13726238888 00-FD-07-A4-72-B8:CMCC 120.196.100.82 i02.c.aliimg.com 24 27 2481 24681 200 1363157993055 13560436666 C4-17-FE-BA-DE-D9:CMCC 120.196.100.99 18 15 1116 954 200
2 ． 分析
实现自定义的bean 来封装流量信息，并将bean 作为 map 输出的 key 来传输
MR 程序在处理数据的过程中会对数据排序(map 输出的 kv 对传输到 reduce之前，会排序)，排序的依据是 map 输出的 key。所以，我们如果要实现自己需要的排序规则，则可以考虑将排序因素放到 key 中，让 key 实现接口：WritableComparable，然后重写 key 的 compareTo 方法。
3 ． 未排序的实现
自定义JavaBean
public class FlowBean implements WritableComparable<FlowBean>{ private long upFlow; private long downFlow; private long sumFlow; public FlowBean() { } public FlowBean(long upFlow, long downFlow, long sumFlow) { this.upFlow = upFlow; this.downFlow = downFlow; this.sumFlow = sumFlow; } public FlowBean(long upFlow, long downFlow) { this.upFlow = upFlow; this.downFlow = downFlow; this.sumFlow = upFlow + downFlow; } public void set(long upFlow, long downFlow) { this.upFlow = upFlow; this.downFlow = downFlow; this.sumFlow = upFlow + downFlow; } public long getUpFlow() { return upFlow; } public void setUpFlow(long upFlow) { this.upFlow = upFlow; } public long getDownFlow() { return downFlow; } public void setDownFlow(long downFlow) { this.downFlow = downFlow; } public long getSumFlow() { return sumFlow; } public void setSumFlow(long sumFlow) { this.sumFlow = sumFlow; } @Override public String toString() { return upFlow + " " + downFlow + " " + sumFlow; } /** * 序列化方法 * @param out * @throws IOException */ @Override public void write(DataOutput out) throws IOException { out.writeLong(upFlow); out.writeLong(downFlow); out.writeLong(sumFlow); } /** * 反序列化方法 * 先序列化的先反序列化 * @param in * @throws IOException */ @Override public void readFields(DataInput in) throws IOException { this.upFlow = in.readLong(); this.downFlow = in.readLong(); this.sumFlow = in.readLong(); } /** * 指定对象排序的方法 * 如果指定的数与参数相等返回 0。 * 如果指定的数小于参数返回 -1。 * 如果指定的数大于参数返回 1。 */ @Override public int compareTo(FlowBean o) { return this.getSumFlow() > o.getSumFlow() ? -1 : 1 ;//按照指定的总流量的倒序排序 // return this.getSumFlow() > o.getSumFlow() ? 1 : -1 ;//按照指定的总流量的正序排序 } }
Mapper方法
public class FlowSumMapper extends Mapper<LongWritable,Text,Text,FlowBean> { Text k = new Text(); FlowBean v = new FlowBean(); @Override protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException { String line = value.toString(); String[] fields = line.split(" "); String phoNum = fields[1];//提前目标文件中的手机号 long upFlow = Long.parseLong(fields[fields.length-3]);//提取目标文件中的上行流量 long downFlow = Long.parseLong(fields[fields.length-2]);//提取目标文件中的下行流量 k.set(phoNum); v.set(upFlow,downFlow); context.write(k,v); } }
Reducer方法
public class FlowSumReducer extends Reducer<Text,FlowBean,Text,FlowBean> { FlowBean v = new FlowBean(); @Override protected void reduce(Text key, Iterable<FlowBean> values, Context context) throws IOException, InterruptedException { long sumUpFlow = 0; long sumDownFlowd = 0; for (FlowBean value : values) { sumUpFlow += value.getUpFlow();//获取每条记录的上行流量并计算总和 sumDownFlowd += value.getDownFlow();//获取每条记录的下行流量并计算总和 } v.set(sumUpFlow ,sumDownFlowd); context.write(key,v); } }
主方法
public class FlowSumRunner { public static void main(String[] args) throws Exception{ Configuration conf = new Configuration(); //指定mr程序使用本地模式模拟一套环境执行mr程序，一般用于本地代码测试 conf.set("mapreduce.framework.name","local"); //通过job方法获得mr程序运行的实例 Job job = Job.getInstance(conf); //指定本次mr程序的运行主类 job.setJarByClass(FlowSumRunner.class); //指定本次mr程序使用的mapper reduce job.setMapperClass(FlowSumMapper.class); job.setReducerClass(FlowSumReducer.class); //指定本次mr程序map输出的数据类型 job.setMapOutputKeyClass(Text.class); job.setMapOutputValueClass(FlowBean.class); //指定本次mr程序reduce输出的数据类型，也就是说最终的输出类型 job.setOutputKeyClass(Text.class); job.setOutputValueClass(FlowBean.class); //指定本次mr程序待处理数据目录 输出结果存放目录 FileInputFormat.addInputPath(job,new Path("D:\flowsum\input")); FileOutputFormat.setOutputPath(job,new Path("D:\flowsum\output")); //提交本次mr程序 boolean b = job.waitForCompletion(true); System.exit(b ? 0 : 1);//程序执行成功，退出状态码为0，退出程序，否则为1 } }
3 ． 排序的实现
使用上面的输出作为该需求的输入
Mapper方法
public class FlowSumSortMapper extends Mapper<LongWritable,Text,FlowBean,Text> { FlowBean k = new FlowBean(); Text v = new Text(); @Override protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException { String line = value.toString(); String[] fileds = line.split(" "); String phoNum = fileds[0]; long sumUpFlow = Long.parseLong(fileds[1]); long sumDownFlow = Long.parseLong(fileds[2]); v.set(phoNum); k.set(sumUpFlow,sumDownFlow); context.write(k,v); } }
Reducer方法
public class FlowSumSortReducer extends Reducer<FlowBean,Text,Text,FlowBean> { @Override protected void reduce(FlowBean key, Iterable<Text> values, Context context) throws IOException, InterruptedException { Text phoNum = values.iterator().next();//iterator中只有一个值 context.write(phoNum,key); } }
主方法
1 //得出上题结果的基础之上再加一个需求：将统计结果按照总流量倒序排序 2 public class FlowSumSortDriver { 3 public static void main(String[] args) throws Exception{ 4 Configuration conf = new Configuration(); 5 //指定mr程序使用本地模式模拟一套环境执行mr程序，一般用于本地代码测试 6 conf.set("mapreduce.framework.name","local"); 7 8 //通过job方法获得mr程序运行的实例 9 Job job = Job.getInstance(conf); 10 11 //指定本次mr程序的运行主类 12 job.setJarByClass(FlowSumSortDriver.class); 13 //指定本次mr程序使用的mapper reduce 14 job.setMapperClass(FlowSumSortMapper.class); 15 job.setReducerClass(FlowSumSortReducer.class); 16 //指定本次mr程序map输出的数据类型 17 job.setMapOutputKeyClass(FlowBean.class); 18 job.setMapOutputValueClass(Text.class); 19 //指定本次mr程序reduce输出的数据类型，也就是说最终的输出类型 20 job.setOutputKeyClass(Text.class); 21 job.setOutputValueClass(FlowBean.class); 22 //指定本次mr程序待处理数据目录 输出结果存放目录 23 FileInputFormat.addInputPath(job,new Path("D:\flowsum\output")); 24 FileOutputFormat.setOutputPath(job,new Path("D:\flowsum\outsortput")); 25 26 //提交本次mr程序 27 boolean b = job.waitForCompletion(true); 28 System.exit(b ? 0 : 1);//程序执行成功，退出状态码为0，退出程序，否则为1 29 } 30 }
Mapreduce的分区—Partitioner
1 ． 需求
将流量汇总统计结果按照手机归属地不同省份输出到不同文件中。
2 ． 分析
Mapreduce 中会将 map 输出的 kv 对，按照相同 key 分组，然后分发给不同的 reducetask。
默认的分发规则为：根据 key 的 hashcode%reducetask 数来分发
所以：如果要按照我们自己的需求进行分组，则需要改写数据分发（分组）组件 Partitioner，自定义一个 CustomPartitioner 继承抽象类：Partitioner，然后在job 对象中，设置自定义 partitioner： job.setPartitionerClass(CustomPartitioner.class）
3 ． 实现
自定义partitioner类
public class ProvincePartitioner extends Partitioner<Text, FlowBean> { public static HashMap<String, Integer> provinceMap = new HashMap<String, Integer>(); static{ provinceMap.put("134", 0); provinceMap.put("135", 1); provinceMap.put("136", 2); provinceMap.put("137", 3); provinceMap.put("138", 4); } @Override public int getPartition(Text key, FlowBean value, int numPartitions) { Integer code = provinceMap.get(key.toString().substring(0, 3)); if (code != null) { return code; } return 5; } }
Mapper、Reducer及主方法
1 public class FlowSumProvince { 2 public static class FlowSumProvinceMapper extends Mapper<LongWritable, Text, Text, FlowBean>{ 3 Text k = new Text(); 4 FlowBean v = new FlowBean(); 5 6 @Override 7 protected void map(LongWritable key, Text value,Context context) 8 throws IOException, InterruptedException { 9 //拿取一行文本转为String 10 String line = value.toString(); 11 //按照分隔符 进行分割 12 String[] fileds = line.split(" "); 13 //获取用户手机号 14 String phoneNum = fileds[1]; 15 16 long upFlow = Long.parseLong(fileds[fileds.length-3]); 17 long downFlow = Long.parseLong(fileds[fileds.length-2]); 18 19 k.set(phoneNum); 20 v.set(upFlow, downFlow); 21 context.write(k,v); 22 } 23 } 24 25 public static class FlowSumProvinceReducer extends Reducer<Text, FlowBean, Text, FlowBean>{ 26 FlowBean v = new FlowBean(); 27 @Override 28 protected void reduce(Text key, Iterable<FlowBean> flowBeans,Context context) throws IOException, InterruptedException { 29 long upFlowCount = 0; 30 long downFlowCount = 0; 31 32 for (FlowBean flowBean : flowBeans) { 33 upFlowCount += flowBean.getUpFlow(); 34 downFlowCount += flowBean.getDownFlow(); 35 } 36 v.set(upFlowCount, downFlowCount); 37 context.write(key, v); 38 } 39 40 public static void main(String[] args) throws Exception{ 41 Configuration conf = new Configuration(); 42 Job job = Job.getInstance(conf); 43 44 //指定我这个 job 所在的 jar包位置 45 job.setJarByClass(FlowSumProvince.class); 46 //指定我们使用的Mapper是那个类 reducer是哪个类 47 job.setMapperClass(FlowSumProvinceMapper.class); 48 job.setReducerClass(FlowSumProvinceReducer.class); 49 // 设置我们的业务逻辑 Mapper 类的输出 key 和 value 的数据类型 50 job.setMapOutputKeyClass(Text.class); 51 job.setMapOutputValueClass(FlowBean.class); 52 // 设置我们的业务逻辑 Reducer 类的输出 key 和 value 的数据类型 53 job.setOutputKeyClass(Text.class); 54 job.setOutputValueClass(FlowBean.class); 55 56 //这里设置运行reduceTask的个数 57 job.setNumReduceTasks(6); 58 59 //这里指定使用我们自定义的分区组件 60 job.setPartitionerClass(ProvincePartitioner.class); 61 62 FileInputFormat.setInputPaths(job, new Path("D:\flowsum\input")); 63 // 指定处理完成之后的结果所保存的位置 64 FileOutputFormat.setOutputPath(job, new Path("D:\flowsum\outputProvince")); 65 boolean res = job.waitForCompletion(true); 66 System.exit(res ? 0 : 1); 67 } 68 } 69 }