HDFS中文件的压缩与解压

　　文件的压缩有两大好处：1、可以减少存储文件所需要的磁盘空间；2、可以加速数据在网络和磁盘上的传输。尤其是在处理大数据时，这两大好处是相当重要的。

　　下面是一个使用gzip工具压缩文件的例子。将文件/user/hadoop/aa.txt进行压缩，压缩后为/user/hadoop/text.gz

 1 package com.hdfs;
 2
 3 import java.io.IOException;
 4 import java.io.InputStream;
 5 import java.io.OutputStream;
 6 import java.net.URI;
 7
 8 import org.apache.hadoop.conf.Configuration;
 9 import org.apache.hadoop.fs.FSDataInputStream;
10 import org.apache.hadoop.fs.FSDataOutputStream;
11 import org.apache.hadoop.fs.FileSystem;
12 import org.apache.hadoop.fs.Path;
13 import org.apache.hadoop.io.IOUtils;
14 import org.apache.hadoop.io.compress.CompressionCodec;
15 import org.apache.hadoop.io.compress.CompressionCodecFactory;
16 import org.apache.hadoop.io.compress.CompressionInputStream;
17 import org.apache.hadoop.io.compress.CompressionOutputStream;
18 import org.apache.hadoop.util.ReflectionUtils;
19
20 public class CodecTest {
21     //压缩文件
22     public static void compress(String codecClassName) throws Exception{
23         Class<?> codecClass = Class.forName(codecClassName);
24         Configuration conf = new Configuration();
25         FileSystem fs = FileSystem.get(conf);
26         CompressionCodec codec = (CompressionCodec)ReflectionUtils.newInstance(codecClass, conf);
27         //指定压缩文件路径
28         FSDataOutputStream outputStream = fs.create(new Path(“/user/hadoop/text.gz”));
29         //指定要被压缩的文件路径
30         FSDataInputStream in = fs.open(new Path(“/user/hadoop/aa.txt”));
31         //创建压缩输出流
32         CompressionOutputStream out = codec.createOutputStream(outputStream);
33         IOUtils.copyBytes(in, out, conf);
34         IOUtils.closeStream(in);
35         IOUtils.closeStream(out);
36     }
37
38     //解压缩
39     public static void uncompress(String fileName) throws Exception{
40         Class<?> codecClass = Class.forName(“org.apache.hadoop.io.compress.GzipCodec”);
41         Configuration conf = new Configuration();
42         FileSystem fs = FileSystem.get(conf);
43         CompressionCodec codec = (CompressionCodec)ReflectionUtils.newInstance(codecClass, conf);
44         FSDataInputStream inputStream = fs.open(new Path(“/user/hadoop/text.gz”));
45          //把text文件里到数据解压，然后输出到控制台
46         InputStream in = codec.createInputStream(inputStream);
47         IOUtils.copyBytes(in, System.out, conf);
48         IOUtils.closeStream(in);
49     }
50
51     //使用文件扩展名来推断二来的codec来对文件进行解压缩
52     public static void uncompress1(String uri) throws IOException{
53         Configuration conf = new Configuration();
54         FileSystem fs = FileSystem.get(URI.create(uri), conf);
55
56         Path inputPath = new Path(uri);
57         CompressionCodecFactory factory = new CompressionCodecFactory(conf);
58         CompressionCodec codec = factory.getCodec(inputPath);
59         if(codec == null){
60             System.out.println(“no codec found for “ + uri);
61             System.exit(1);
62         }
63         String outputUri = CompressionCodecFactory.removeSuffix(uri, codec.getDefaultExtension());
64         InputStream in = null;
65         OutputStream out = null;
66         try {
67             in = codec.createInputStream(fs.open(inputPath));
68             out = fs.create(new Path(outputUri));
69             IOUtils.copyBytes(in, out, conf);
70         } finally{
71             IOUtils.closeStream(out);
72             IOUtils.closeStream(in);
73         }
74     }
75
76     public static void main(String[] args) throws Exception {
77         //compress(“org.apache.hadoop.io.compress.GzipCodec”);
78         //uncompress(“text”);
79         uncompress1(“hdfs://master:9000/user/hadoop/text.gz”);
80     }
81
82 }

　　首先执行77行进行压缩，压缩后执行第78行进行解压缩，这里解压到标准输出，所以执行78行会再控制台看到文件/user/hadoop
/aa.txt的内容。如果执行79行的话会将文件解压到/user/hadoop/text,他是根据/user/hadoop/text.gz的扩展
名判断使用哪个解压工具进行解压的。解压后的路径就是去掉扩展名。

　　进行文件压缩后，在执行命令./hadoop fs -ls /user/hadoop/查看文件信息，如下：

1 [hadoop@master bin]$ ./hadoop fs -ls /user/hadoop/
2 Found 7 items
3 -rw-r–r–   3 hadoop supergroup   76805248 2013-06-17 23:55 /user/hadoop/aa.mp4
4 -rw-r–r–   3 hadoop supergroup        520 2013-06-17 22:29 /user/hadoop/aa.txt
5 drwxr-xr-x   - hadoop supergroup          0 2013-06-16 17:19 /user/hadoop/input
6 drwxr-xr-x   - hadoop supergroup          0 2013-06-16 19:32 /user/hadoop/output
7 drwxr-xr-x   - hadoop supergroup          0 2013-06-18 17:08 /user/hadoop/test
8 drwxr-xr-x   - hadoop supergroup          0 2013-06-18 19:45 /user/hadoop/test1
9 -rw-r–r–   3 hadoop supergroup         46 2013-06-19 20:09 /user/hadoop/text.gz

第4行为压缩之前的文件，大小为520个字节。第9行为压缩后的文件，大小为46个字节。由此可以看出上面讲的压缩的两大好处了。