Combine small files to Sequence file

Combine small files to sequence file or avro files are a good method to feed hadoop.

Small files in hadoop will take more namenode memory resource.

SequenceFileInputFormat 是一种Key value 格式的文件格式。

Key和Value的类型可以自己实现其序列化和反序列化内容。

SequenceFile示例内容：

其默认的key,value之间的分隔符 是\001,这个与hive文件的存储格式是匹配的，这样也方便直接把这种文件加载到hive里面。

以下的代码仅供参考作用，真实的项目中使用的时候，可以做适当的调整，以更高地节约资源和满足项目的需要。

示例代码如下：

package myexamples;

import java.io.File;
import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

import org.apache.commons.io.FileUtils;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.SequenceFile;
import org.apache.hadoop.io.Text;

public class localf2seqfile {
/*
 * Local folder has a lot of txt file
 * we need handle it in map reduce
 * so we want to load these files to one sequence file
 * key: source file name
 * value: file content
 * */
	static void write2Seqfile(FileSystem fs,Path hdfspath,HashMap<Text,Text> hm)  
	{
	    SequenceFile.Writer writer=null;  
	      
	    try {  
	      writer=SequenceFile.createWriter(fs, fs.getConf(), hdfspath, Text.class, Text.class);  
	      
	      for(Map.Entry<Text,Text> entry:hm.entrySet())    
	        writer.append(entry.getKey(),entry.getValue());  
	      
	    } catch (IOException e) {  
	    	e.printStackTrace();  
	    }finally{  
	    	try{writer.close();}catch(IOException ioe){}
	    }  
	}
	static HashMap<Text,Text> collectFiles(String localpath) throws IOException
	{
		HashMap<Text,Text> hm = new HashMap<Text,Text>();
		File f = new File(localpath);
		if(!f.isDirectory()) return hm;
		for(File file:f.listFiles())
			hm.put(new Text(file.getName()), new Text(FileUtils.readFileToString(file)));
		
		return hm;
	}
	static void readSeqFile(FileSystem fs, Path hdfspath) throws IOException
	{
		 SequenceFile.Reader reader = new SequenceFile.Reader(fs,hdfspath,fs.getConf());  
		         Text key = new Text();  
		         Text value = new Text();  
		         while (reader.next(key, value)) {  
		        	 System.out.print(key +" ");  
		        	 System.out.println(value);  
		         }  
		         reader.close();
		
	}
	public static void main(String[] args) throws IOException {
		args = "/home/hadoop/test/sub".split(" ");
		
		Configuration conf = new Configuration();
		conf.set("fs.default.name", "hdfs://namenode:9000");
		
		FileSystem fs = FileSystem.get(conf);
		System.out.println(fs.getUri()); 
		Path file = new Path("/user/hadoop/seqfiles/seqdemo.seq");
		if (fs.exists(file)) fs.delete(file,false);
		HashMap<Text,Text> hm = collectFiles(args[0]);
		write2Seqfile(fs,file,hm);
		readSeqFile(fs,file);
		
		fs.close();
	}
}