import javax.imageio.ImageIO;
import java.awt.image.BufferedImage;
import java.awt.image.ColorModel;
import java.awt.image.WritableRaster;
import java.awt.*;
import java.awt.geom.AffineTransform;
import java.io.InputStream;
import java.io.File;
import java.io.FileOutputStream;

public class Test {
 public static BufferedImage resize(BufferedImage source, int targetW, int targetH) {
 // targetW，targetH分别表示目标长和宽
 int type = source.getType();
 BufferedImage target = null;
 double sx = (double) targetW / source.getWidth();
 double sy = (double) targetH / source.getHeight();
 //这里想实现在targetW，targetH范围内实现等比缩放。如果不需要等比缩放
 //则将下面的if else语句注释即可
 if(sx>sy)
 {
 sx = sy;
 targetW = (int)(sx * source.getWidth());
 }else{
 sy = sx;
 targetH = (int)(sy * source.getHeight());
 }
 if (type == BufferedImage.TYPE_CUSTOM) { //handmade
 ColorModel cm = source.getColorModel();
 WritableRaster raster = cm.createCompatibleWritableRaster(targetW, targetH);
 boolean alphaPremultiplied = cm.isAlphaPremultiplied();
 target = new BufferedImage(cm, raster, alphaPremultiplied, null);
 } else
 target = new BufferedImage(targetW, targetH, type);
 Graphics2D g = target.createGraphics();
 //smoother than exlax:
 g.setRenderingHint(RenderingHints.KEY_RENDERING, RenderingHints.VALUE_RENDER_QUALITY );
 g.drawRenderedImage(source, AffineTransform.getScaleInstance(sx, sy));
 g.dispose();
 return target;
 }
 public static void saveImageAsJpg (String fromFileStr,String saveToFileStr,int width,int hight)
 throws Exception {
 BufferedImage srcImage;
// String ex = fromFileStr.substring(fromFileStr.indexOf("."),fromFileStr.length());
 String imgType = "JPEG";
 if (fromFileStr.toLowerCase().endsWith(".png")) {
 imgType = "PNG";
 }
// System.out.println(ex);
 File saveFile=new File(saveToFileStr);
 File fromFile=new File(fromFileStr);
 srcImage = ImageIO.read(fromFile);
 if(width > 0 || hight > 0)
 {
 srcImage = resize(srcImage, width, hight);
 }
 ImageIO.write(srcImage, imgType, saveFile);

 }
 
 public static void main (String argv[]) {
 try{
 //参数1(from),参数2(to),参数3(宽),参数4(高)
 Test.saveImageAsJpg("E:/Document/My Pictures/3.gif","c:/6.gif",50,50);
 } catch(Exception e)
 {
 e.printStackTrace();
 }

 }
}

 import java.io.*;
 import java.util.*;
 import com.sun.image.codec.jpeg.*;
 import java.awt.image.*;
 import java.awt.*;
 import java.net.*;
 import java.applet.*;
 import java.sql.*;

//缩略图类，
//本java类能将jpg图片文件，进行等比或非等比的大小转换。
//具体使用方法
//s_pic(大图片路径,生成小图片路径,大图片文件名,生成小图片文名,生成小图片宽度,生成小图片高度,是否等比缩放(默认为true))
 public class Tes {
 String InputDir; //输入图路径
 String OutputDir; //输出图路径
 String InputFileName; //输入图文件名
 String OutputFileName; //输出图文件名
 int OutputWidth = 80; //默认输出图片宽
 int OutputHeight = 80; //默认输出图片高
 int rate = 0;
 boolean proportion = true; //是否等比缩放标记(默认为等比缩放)

 public Tes() {
//初始化变量
 InputDir = "";
 OutputDir = "";
 InputFileName = "";
 OutputFileName = "";
 OutputWidth = 80;
 OutputHeight = 80;
 rate = 0;
 }

 public void setInputDir(String InputDir) {
 this.InputDir = InputDir;
 }

 public void setOutputDir(String OutputDir) {
 this.OutputDir = OutputDir;
 }

 public void setInputFileName(String InputFileName) {
 this.InputFileName = InputFileName;
 }

 public void setOutputFileName(String OutputFileName) {
 this.OutputFileName = OutputFileName;
 }

 public void setOutputWidth(int OutputWidth) {
 this.OutputWidth = OutputWidth;
 }

 public void setOutputHeight(int OutputHeight) {
 this.OutputHeight = OutputHeight;
 }

 public void setW_H(int width, int height) {
 this.OutputWidth = width;
 this.OutputHeight = height;
 }

 public String s_pic() {
 BufferedImage image;
 String NewFileName;
//建立输出文件对象
 File file = new File(OutputDir + OutputFileName);
 FileOutputStream tempout = null;
 try {
 tempout = new FileOutputStream(file);
 } catch (Exception ex) {
 System.out.println(ex.toString());
 }
 Image img = null;
 Toolkit tk = Toolkit.getDefaultToolkit();
 Applet app = new Applet();
 MediaTracker mt = new MediaTracker(app);
 try {
 img = tk.getImage(InputDir + InputFileName);
 mt.addImage(img, 0);
 mt.waitForID(0);
 } catch (Exception e) {
 e.printStackTrace();
 }

 if (img.getWidth(null) == -1) {
 System.out.println(" can't read,retry!" + "<BR>");
 return "no";
 } else {
 int new_w;
 int new_h;
 if (this.proportion == true) { //判断是否是等比缩放.
//为等比缩放计算输出的图片宽度及高度
 double rate1 = ((double) img.getWidth(null)) /
 (double) OutputWidth + 0.1;
 double rate2 = ((double) img.getHeight(null)) /
 (double) OutputHeight + 0.1;
 double rate = rate1 > rate2 ? rate1 : rate2;
 new_w = (int) (((double) img.getWidth(null)) / rate);
 new_h = (int) (((double) img.getHeight(null)) / rate);
 } else {
 new_w = OutputWidth; //输出的图片宽度
 new_h = OutputHeight; //输出的图片高度
 }
 BufferedImage buffImg = new BufferedImage(new_w, new_h,
 BufferedImage.TYPE_INT_RGB);

 Graphics g = buffImg.createGraphics();

 g.setColor(Color.white);
 g.fillRect(0, 0, new_w, new_h);

 g.drawImage(img, 0, 0, new_w, new_h, null);
 g.dispose();

 JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(tempout);
 try {
 encoder.encode(buffImg);
 tempout.close();
 } catch (IOException ex) {
 System.out.println(ex.toString());
 }
 }
 return "ok";
 }

 public String s_pic(String InputDir, String OutputDir, String InputFileName,
 String OutputFileName) {
//输入图路径
 this.InputDir = InputDir;
//输出图路径
 this.OutputDir = OutputDir;
//输入图文件名
 this.InputFileName = InputFileName;
//输出图文件名
 this.OutputFileName = OutputFileName;
 return s_pic();
 }

 public String s_pic(String InputDir, String OutputDir, String InputFileName,
 String OutputFileName, int width, int height,
 boolean gp) {
//输入图路径
 this.InputDir = InputDir;
//输出图路径
 this.OutputDir = OutputDir;
//输入图文件名
 this.InputFileName = InputFileName;
//输出图文件名
 this.OutputFileName = OutputFileName;
//设置图片长宽
 setW_H(width, height);
//是否是等比缩放 标记
 this.proportion = gp;
 return s_pic();
 }

 public static void main(String[] a) {
//s_pic(大图片路径,生成小图片路径,大图片文件名,生成小图片文名,生成小图片宽度,生成小图片高度)
 Tes mypic = new Tes();
 System.out.println(
 mypic.s_pic("E:\\Document\\My Pictures\\",
 "E:\\Document\\My Pictures\\",
 "topbg-3.gif", "3.gif", 400, 400, true)
 );

 }
 }

java.io.*,java.awt.Image,java.awt.image.*,com.sun.image.codec.jpeg.*,

  try
    {
 java.io.File file = new java.io.File("E:\\Document\\My Pictures\\3.gif");
 String newurl="E:\\Document\\My Pictures\\32.gif"; //新的缩略图保存地址
 Image src = javax.imageio.ImageIO.read(file); //构造Image对象
 float tagsize=200;
 int old_w=src.getWidth(null); //得到源图宽
 int old_h=src.getHeight(null);
 int new_w=0;
 int new_h=0; //得到源图长
 int tempsize;
 float tempdouble;
 if(old_w>old_h){
 tempdouble=old_w/tagsize;
 }else{
 tempdouble=old_h/tagsize;
 }
 new_w=Math.round(old_w/tempdouble);
 new_h=Math.round(old_h/tempdouble);//计算新图长宽
 BufferedImage tag = new BufferedImage(new_w,new_h,BufferedImage.TYPE_INT_RGB);
 tag.getGraphics().drawImage(src,0,0,new_w,new_h,null); //绘制缩小后的图
 FileOutputStream newimage=new FileOutputStream(newurl); //输出到文件流
 JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(newimage);
 encoder.encode(tag); //近JPEG编码
 newimage.close();

}catch (Exception e){

e.toString();

}