Snappy Java API简介

Snappy是Google开源的压缩/解压缩库。和其他压缩库相比，snappy的压缩率并不是最高的，兼容性也并非最好的。相反，它的诞生旨在以极高的压缩/解压缩速率提供合理的压缩率。Snappy官指出：在64位单核core-i7处理器下，snappy的压缩率能够达到250MB/S，而解压缩速度则能达到500MB/S。目前很多软件使用（或支持）snappy作为压缩库，如MongoDB,Cassandra,Hadoop,Lucene…本文简单介绍snappy Java Api的使用。

Snappy提供了很多高级API以方便对byte[],Sting,double[]等进行压缩和解压缩，示例代码如下：

import org.xerial.snappy.Snappy;
String input = "Hello snappy-java! Snappy-java is a JNI-based wrapper of Snappy, a fast compresser/decompresser.";
{
    byte[] compressed = Snappy.compress(input.getBytes("UTF-8"));
    byte[] uncompressed = Snappy.uncompress(compressed);
    String result = new String(uncompressed, "UTF-8");
    System.out.println(result);
}

{
    byte[] compressed = Snappy.compress(input);
    System.out.println(Snappy.uncompressString(compressed));
}

{
    double [] arr = new double[]{123.456,234.567,345.678};
    byte[] compressed = Snappy.compress(arr);
    double [] unarr = Snappy.uncompressDoubleArray(compressed);
    System.out.println(Arrays.toString(unarr));
}

同时Snappy仍然保留着低级API，用于最小化内存的拷贝，例如

/*
*inputAddr:待压缩数据的内存地址
*inputSize:待压缩数据的byte size
*destAddr:压缩结果的保存地址
*return：返回压缩后数据的大小
*/
public static long rawCompress(long inputAddr,long inputSize, long destAddr)throws java.io.IOException;
/*
*inputAddr:待解压缩数据的内存地址
*inputSize:待解压缩数据的byte size
*destAddr:解压缩结果的保存地址
*return：返回解压缩后数据的大小
*/
public static long rawUncompress(long inputAddr,long inputSize,long destAddr)throws java.io.IOException;

基于输入/输出流的Snappy解压缩
SnappyOutputStream和SnappyInputStream分别用于流数据的压缩/解压缩。此外，从snappy v1.1.0开始提供了Framing-format(帧格式)输入输出流的压缩/解压缩的方法：SnappyFramedOutputStream和SnappyFramedInputStream。需要注意的是以SnappyOutputStream压缩的数据不能以SnappyFramedInputStream方法解开，反之亦然。下面以SnappyOutputStream和SnappyInputStream为例介绍。
SnappyOutputStream继承自java.io OutputStream,重写了常用的的write,close,flush:

File file = new File("..."); //待压缩文件
File out = new File("./", file.getName() + ".snappy"); //压缩结果文件

byte[] buffer = new byte[1024 * 1024 * 8];
FileInputStream  fi = null;
FileOutputStream fo = null;
SnappyOutputStream sout = null;
try
{
    fi = new FileInputStream(file);
    fo = new FileOutputStream(out);
    sout = new SnappyOutputStream(fo);
    while(true)
    {
        int count = fi.read(buffer, 0, buffer.length);
        if(count == -1) { break; }
        sout.write(buffer, 0, count);
    }
    sout.flush();
}
catch(Throwable ex)
{
    ex.printStackTrace();
}
finally
{
    if(sout != null) {try { sout.close();} catch (Exception e) {}}
    if(fi != null) { try { fi.close(); } catch(Exception x) {} }
    if(fo != null) { try { fo.close(); } catch(Exception x) {} }
}

SnappyInputStream 继承自java.io InputStream,重写了常用的read,close方法等

File file = new File("xxx"); //待解压文件
File out = new File("xxx");  //解压后文件

byte[] buffer = new byte[1024 * 1024 * 8];
FileInputStream  fi = null;
FileOutputStream fo = null;
SnappyInputStream sin = null;
try
{
    fo = new FileOutputStream(out);
    fi = new FileInputStream(file.getPath());
    sin = new SnappyInputStream(fi);

    while(true)
    {
        int count = sin.read(buffer, 0, buffer.length);
        if(count == -1) { break; }
        fo.write(buffer, 0, count);
    }
    fo.flush();
}
catch(Throwable ex)
{
    ex.printStackTrace();
}
finally
{
    if(sin != null) { try { sin.close(); } catch(Exception x) {} }
    if(fi != null) { try { fi.close(); } catch(Exception x) {} }
    if(fo != null) { try { fo.close(); } catch(Exception x) {} }
}

xtof

Snappy Java API简介