如果給出縮略圖的寬度和高度，那么就會根據給出的寬度和高度生產縮略圖，如果只給出寬度或者高度值，那么就會根據比例生成縮略圖。
package com.founder.common.utils;

import java.awt.image.BufferedImage;
import java.io.File;
import javax.imageio.ImageIO;

//生成等比例高質量縮略圖
public class ThumbnailUtil {
 private static int width;
 private static int height;
 private static int scaleWidth;
 static double support = (double) 3.0;
 static double PI = (double) 3.14159265358978;
 static double[] contrib;
 static double[] normContrib;
 static double[] tmpContrib;
 static int startContrib, stopContrib;
 static int nDots;
 static int nHalfDots;

 public static void saveImage(String fromFileStr, String saveToFileStr, int formatWidth, int formatHeight) throws Exception {
  File saveFile = new File(saveToFileStr);
  File fromFile = new File(fromFileStr);
  saveImage(fromFile, saveFile, formatWidth, formatHeight);
 }
 
 public static void saveImage(File fromFile  , File saveFile, int formatWidth, int formatHeight) throws Exception {
  BufferedImage srcImage;
  srcImage = javax.imageio.ImageIO.read(fromFile); // construct image
  int imageWidth = srcImage.getWidth(null);
  int imageHeight = srcImage.getHeight(null);
  int changeToWidth = 0;
  int changeToHeight = 0;
  if (formatWidth > 0 && formatHeight > 0) {
   changeToWidth = formatWidth;
   changeToHeight = formatHeight;
  } else {
   if (imageWidth > 0 && imageHeight > 0) {
    if (imageWidth / imageHeight >= formatWidth / formatHeight) {
     if (imageWidth > formatWidth) {
      changeToWidth = formatWidth;
      changeToHeight = (imageHeight * formatWidth) / imageWidth;
     } else {
      changeToWidth = imageWidth;
      changeToHeight = imageHeight;
     }
    } else {
     if (imageHeight > formatHeight) {
      changeToHeight = formatHeight;
      changeToWidth = (imageWidth * formatHeight) / imageHeight;
     } else {
      changeToWidth = imageWidth;
      changeToHeight = imageHeight;
     }
    }
   }
  }
  
  srcImage = imageZoomOut(srcImage, changeToWidth, changeToHeight);
  ImageIO.write(srcImage, "JPEG", saveFile);
 }

 public static BufferedImage imageZoomOut(BufferedImage srcBufferImage, int w, int h) {
  width = srcBufferImage.getWidth();
  height = srcBufferImage.getHeight();
  scaleWidth = w;

  if (DetermineResultSize(w, h) == 1) {
   return srcBufferImage;
  }
  CalContrib();
  BufferedImage pbOut = HorizontalFiltering(srcBufferImage, w);
  BufferedImage pbFinalOut = VerticalFiltering(pbOut, h);
  return pbFinalOut;
 }

 /**
  * 決定圖像尺寸
  */
 private static int DetermineResultSize(int w, int h) {
  double scaleH, scaleV;
  scaleH = (double) w / (double) width;
  scaleV = (double) h / (double) height;
  // 需要判斷一下scaleH，scaleV，不做放大操作
  if (scaleH >= 1.0 && scaleV >= 1.0) {
   return 1;
  }
  return 0;

 }

 private static double Lanczos(int i, int inWidth, int outWidth, double Support) {
  double x;

  x = (double) i * (double) outWidth / (double) inWidth;

  return Math.sin(x * PI) / (x * PI) * Math.sin(x * PI / Support)
    / (x * PI / Support);

 }

 private static void CalContrib() {
  nHalfDots = (int) ((double) width * support / (double) scaleWidth);
  nDots = nHalfDots * 2 + 1;
  try {
   contrib = new double[nDots];
   normContrib = new double[nDots];
   tmpContrib = new double[nDots];
  } catch (Exception e) {
   System.out.println("init contrib,normContrib,tmpContrib" + e);
  }

  int center = nHalfDots;
  contrib[center] = 1.0;

  double weight = 0.0;
  int i = 0;
  for (i = 1; i <= center; i++) {
   contrib[center + i] = Lanczos(i, width, scaleWidth, support);
   weight += contrib[center + i];
  }

  for (i = center - 1; i >= 0; i--) {
   contrib[i] = contrib[center * 2 - i];
  }

  weight = weight * 2 + 1.0;

  for (i = 0; i <= center; i++) {
   normContrib[i] = contrib[i] / weight;
  }

  for (i = center + 1; i < nDots; i++) {
   normContrib[i] = normContrib[center * 2 - i];
  }
 }

 // 處理邊緣
 private static void CalTempContrib(int start, int stop) {
  double weight = 0;

  int i = 0;
  for (i = start; i <= stop; i++) {
   weight += contrib[i];
  }

  for (i = start; i <= stop; i++) {
   tmpContrib[i] = contrib[i] / weight;
  }

 }

 private static int GetRedValue(int rgbValue) {
  int temp = rgbValue & 0x00ff0000;
  return temp >> 16;
 }

 private static int GetGreenValue(int rgbValue) {
  int temp = rgbValue & 0x0000ff00;
  return temp >> 8;
 }

 private static int GetBlueValue(int rgbValue) {
  return rgbValue & 0x000000ff;
 }

 private static int ComRGB(int redValue, int greenValue, int blueValue) {

  return (redValue << 16) + (greenValue << 8) + blueValue;
 }

 // 行水平濾波
 private static int HorizontalFilter(BufferedImage bufImg, int startX, int stopX,
   int start, int stop, int y, double[] pContrib) {
  double valueRed = 0.0;
  double valueGreen = 0.0;
  double valueBlue = 0.0;
  int valueRGB = 0;
  int i, j;

  for (i = startX, j = start; i <= stopX; i++, j++) {
   valueRGB = bufImg.getRGB(i, y);

   valueRed += GetRedValue(valueRGB) * pContrib[j];
   valueGreen += GetGreenValue(valueRGB) * pContrib[j];
   valueBlue += GetBlueValue(valueRGB) * pContrib[j];
  }

  valueRGB = ComRGB(Clip((int) valueRed), Clip((int) valueGreen),
    Clip((int) valueBlue));
  return valueRGB;

 }

 // 圖片水平濾波
 private static BufferedImage HorizontalFiltering(BufferedImage bufImage, int iOutW) {
  int dwInW = bufImage.getWidth();
  int dwInH = bufImage.getHeight();
  int value = 0;
  BufferedImage pbOut = new BufferedImage(iOutW, dwInH,
    BufferedImage.TYPE_INT_RGB);

  for (int x = 0; x < iOutW; x++) {

   int startX;
   int start;
   int X = (int) (((double) x) * ((double) dwInW) / ((double) iOutW) + 0.5);
   int y = 0;

   startX = X - nHalfDots;
   if (startX < 0) {
    startX = 0;
    start = nHalfDots - X;
   } else {
    start = 0;
   }

   int stop;
   int stopX = X + nHalfDots;
   if (stopX > (dwInW - 1)) {
    stopX = dwInW - 1;
    stop = nHalfDots + (dwInW - 1 - X);
   } else {
    stop = nHalfDots * 2;
   }

   if (start > 0 || stop < nDots - 1) {
    CalTempContrib(start, stop);
    for (y = 0; y < dwInH; y++) {
     value = HorizontalFilter(bufImage, startX, stopX, start,
       stop, y, tmpContrib);
     pbOut.setRGB(x, y, value);
    }
   } else {
    for (y = 0; y < dwInH; y++) {
     value = HorizontalFilter(bufImage, startX, stopX, start,
       stop, y, normContrib);
     pbOut.setRGB(x, y, value);
    }
   }
  }

  return pbOut;

 }

 private static int VerticalFilter(BufferedImage pbInImage, int startY, int stopY,
   int start, int stop, int x, double[] pContrib) {
  double valueRed = 0.0;
  double valueGreen = 0.0;
  double valueBlue = 0.0;
  int valueRGB = 0;
  int i, j;

  for (i = startY, j = start; i <= stopY; i++, j++) {
   valueRGB = pbInImage.getRGB(x, i);

   valueRed += GetRedValue(valueRGB) * pContrib[j];
   valueGreen += GetGreenValue(valueRGB) * pContrib[j];
   valueBlue += GetBlueValue(valueRGB) * pContrib[j];
  }

  valueRGB = ComRGB(Clip((int) valueRed), Clip((int) valueGreen), Clip((int) valueBlue));
  
  return valueRGB;

 }

 private static BufferedImage VerticalFiltering(BufferedImage pbImage, int iOutH) {
  int iW = pbImage.getWidth();
  int iH = pbImage.getHeight();
  int value = 0;
  BufferedImage pbOut = new BufferedImage(iW, iOutH,
    BufferedImage.TYPE_INT_RGB);

  for (int y = 0; y < iOutH; y++) {

   int startY;
   int start;
   int Y = (int) (((double) y) * ((double) iH) / ((double) iOutH) + 0.5);

   startY = Y - nHalfDots;
   if (startY < 0) {
    startY = 0;
    start = nHalfDots - Y;
   } else {
    start = 0;
   }

   int stop;
   int stopY = Y + nHalfDots;
   if (stopY > (int) (iH - 1)) {
    stopY = iH - 1;
    stop = nHalfDots + (iH - 1 - Y);
   } else {
    stop = nHalfDots * 2;
   }

   if (start > 0 || stop < nDots - 1) {
    CalTempContrib(start, stop);
    for (int x = 0; x < iW; x++) {
     value = VerticalFilter(pbImage, startY, stopY, start, stop,
       x, tmpContrib);
     pbOut.setRGB(x, y, value);
    }
   } else {
    for (int x = 0; x < iW; x++) {
     value = VerticalFilter(pbImage, startY, stopY, start, stop,
       x, normContrib);
     pbOut.setRGB(x, y, value);
    }
   }

  }

  return pbOut;

 }

 static int Clip(int x) {
  if (x < 0)
   return 0;
  if (x > 255)
   return 255;
  return x;
 }
}