Java压缩技术（一） ZLib

有关ZLib可参见官方主页  http://www.zlib.net/  
ZLib可以简单的理解为压缩/解压缩算法，它与ZIP、RAR等归档算法有所不同，与bzip2比较接近。 

压缩工具代码如下： 

 /**
  * 2009-9-9
  */
 package org.zlex.commons.io;

 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.OutputStream;
 import java.util.zip.Deflater;
 import java.util.zip.DeflaterOutputStream;
 import java.util.zip.Inflater;
 import java.util.zip.InflaterInputStream;

 /**
  * ZLib压缩工具
  *
  * @author <a href="mailto:[email protected]">梁栋</a>
  * @version 1.0
  * @since 1.0
  */
 public abstract class ZLibUtils {

     /**
      * 压缩
      *
      * @param data
      *            待压缩数据
      * @return byte[] 压缩后的数据
      */
     public static byte[] compress(byte[] data) {
         byte[] output = new byte[0];

         Deflater compresser = new Deflater();

         compresser.reset();
         compresser.setInput(data);
         compresser.finish();
         ByteArrayOutputStream bos = new ByteArrayOutputStream(data.length);
         try {
             byte[] buf = new byte[1024];
             while (!compresser.finished()) {
                 int i = compresser.deflate(buf);
                 bos.write(buf, 0, i);
             }
             output = bos.toByteArray();
         } catch (Exception e) {
             output = data;
             e.printStackTrace();
         } finally {
             try {
                 bos.close();
             } catch (IOException e) {
                 e.printStackTrace();
             }
         }
         compresser.end();
         return output;
     }

     /**
      * 压缩
      *
      * @param data
      *            待压缩数据
      *
      * @param os
      *            输出流
      */
     public static void compress(byte[] data, OutputStream os) {
         DeflaterOutputStream dos = new DeflaterOutputStream(os);

         try {
             dos.write(data, 0, data.length);

             dos.finish();

             dos.flush();
         } catch (IOException e) {
             e.printStackTrace();
         }
     }

     /**
      * 解压缩
      *
      * @param data
      *            待压缩的数据
      * @return byte[] 解压缩后的数据
      */
     public static byte[] decompress(byte[] data) {
         byte[] output = new byte[0];

         Inflater decompresser = new Inflater();
         decompresser.reset();
         decompresser.setInput(data);

         ByteArrayOutputStream o = new ByteArrayOutputStream(data.length);
         try {
             byte[] buf = new byte[1024];
             while (!decompresser.finished()) {
                 int i = decompresser.inflate(buf);
                 o.write(buf, 0, i);
             }
             output = o.toByteArray();
         } catch (Exception e) {
             output = data;
             e.printStackTrace();
         } finally {
             try {
                 o.close();
             } catch (IOException e) {
                 e.printStackTrace();
             }
         }

         decompresser.end();
         return output;
     }

     /**
      * 解压缩
      *
      * @param is
      *            输入流
      * @return byte[] 解压缩后的数据
      */
     public static byte[] decompress(InputStream is) {
         InflaterInputStream iis = new InflaterInputStream(is);
         ByteArrayOutputStream o = new ByteArrayOutputStream(1024);
         try {
             int i = 1024;
             byte[] buf = new byte[i];

             while ((i = iis.read(buf, 0, i)) > 0) {
                 o.write(buf, 0, i);
             }

         } catch (IOException e) {
             e.printStackTrace();
         }
         return o.toByteArray();
     }
 }

测试用例代码如下： 

 /**
  * 2009-9-9
  */
 package org.zlex.commons.io;

 import static org.junit.Assert.*;

 import java.io.File;
 import java.io.FileInputStream;
 import java.io.FileOutputStream;

 import org.junit.Test;

 /**
  * ZLib压缩测试用例
  *
  * @author <a href="mailto:[email protected]">梁栋</a>
  * @version 1.0
  * @since 1.0
  */
 public class ZLibUtilsTest {

     @Test
     public final void testBytes() {
         System.err.println("字节压缩／解压缩测试");
         String inputStr = "[email protected];[email protected];[email protected]";
         System.err.println("输入字符串:\t" + inputStr);
         byte[] input = inputStr.getBytes();
         System.err.println("输入字节长度:\t" + input.length);

         byte[] data = ZLibUtils.compress(input);
         System.err.println("压缩后字节长度:\t" + data.length);

         byte[] output = ZLibUtils.decompress(data);
         System.err.println("解压缩后字节长度:\t" + output.length);
         String outputStr = new String(output);
         System.err.println("输出字符串:\t" + outputStr);

         assertEquals(inputStr, outputStr);
     }

     @Test
     public final void testFile() {
         String filename = "zlib";
         File file = new File(filename);
         System.err.println("文件压缩／解压缩测试");
         String inputStr = "[email protected];[email protected];[email protected]";
         System.err.println("输入字符串:\t" + inputStr);
         byte[] input = inputStr.getBytes();
         System.err.println("输入字节长度:\t" + input.length);

         try {

             FileOutputStream fos = new FileOutputStream(file);
             ZLibUtils.compress(input, fos);
             fos.close();
             System.err.println("压缩后字节长度:\t" + file.length());
         } catch (Exception e) {
             fail(e.getMessage());
         }

         byte[] output = null;

         try {
             FileInputStream fis = new FileInputStream(file);
             output = ZLibUtils.decompress(fis);
             fis.close();

         } catch (Exception e) {
             fail(e.getMessage());
         }
         System.err.println("解压缩后字节长度:\t" + output.length);
         String outputStr = new String(output);
         System.err.println("输出字符串:\t" + outputStr);

         assertEquals(inputStr, outputStr);
     }
 }

输入结果 

 字节压缩／解压缩测试
 输入字符串:  [email protected];[email protected];[email protected]
 输入字节长度: 59
 压缩后字节长度:    39
 解压缩后字节长度:   59
 输出字符串:  [email protected];[email protected];[email protected]
 文件压缩／解压缩测试
 输入字符串:  [email protected];[email protected];[email protected]
 输入字节长度: 59
 压缩后字节长度:    39
 解压缩后字节长度:   59
 输出字符串:  [email protected];[email protected];[email protected]

应该怎么计算呢？原数据长度59字节，压缩后39字节，大约是33％的压缩率！ 

ZLib压缩对大字节数据压缩，才能反映出压缩效果。