众所周知缩放图片时用imagemagick的效果好。不过用起来不是很方便。在linux下倒还方便。在win下用就没那么方便了。
对效果要求不高可以用awt来缩放就可以了。下面就简要介绍一下其方法：

public final class ThumbUtils { private ThumbUtils() {        }     
    /*** @param in    * @param out   * @param formatName      
   * @param size         * @throws IOException         */    
    public static void encodeThumb(InputStream in, OutputStream out, PictureSize limitSize, String formatName)
 throws IOException
{   BufferedImage image = ImageIO.read(in);        
        encodeThumb(image, out, limitSize, formatName);       
 }     
    public static void encodeThumb(BufferedImage image, OutputStream out, PictureSize limitSize, String formatName)
throws IOException {            
    PictureSize originalSize = new PictureSize(image.getWidth(null), image.getHeight(null));      
          PictureSize thumbSize = getThumbSize(limitSize, originalSize);     
           int width = thumbSize.getWidth(), height = thumbSize.getHeight();      
           ColorModel dstCM = image.getColorModel();        
        BufferedImage dst = new BufferedImage(dstCM, dstCM.createCompatibleWritableRaster(width, height),
dstCM.isAlphaPremultiplied(), null);            
     Image scaleImage = image.getScaledInstance(width, height, Image.SCALE_SMOOTH);           
     Graphics2D g = dst.createGraphics();             
   g.drawImage(scaleImage, 0, 0, width, height, null);      
          g.dispose();            
     ImageIO.write(dst, formatName, out);        }  
       /**         * @param limitSize         * @param originalSize         * @return         */  
      public static PictureSize getThumbSize(PictureSize limitSize, PictureSize originalSize)
 {             
    int maxWidth = limitSize.getWidth();     
           int maxHeight = limitSize.getHeight();  
              if (maxWidth == 0 && maxHeight == 0) {    
                    maxWidth = maxHeight = DEFAULT_THUMB_SIZE;         
       }             
   int originalWidth = originalSize.getWidth();      
          int originalHeight = originalSize.getHeight();      
          int width = DEFAULT_THUMB_SIZE, height = DEFAULT_THUMB_SIZE;       
         float rate = 0;            
    if (maxWidth > 0 && maxHeight > 0)
{              
          rate = Math.min(maxWidth * 1f / originalWidth, maxHeight * 1f / originalHeight);      
          } else if (maxWidth > 0)
 {         
               rate = maxWidth * 1f / originalWidth;       
         } else if (maxHeight > 0)
{           
             rate = maxHeight * 1f / originalHeight;         
       }
else {                 
       // no happen.        
        }          
      width = (int) (originalWidth * rate);     
           height = (int) (originalHeight * rate);    
            return new PictureSize(width, height);   
      }      
   public static final int DEFAULT_THUMB_SIZE = 96; }