/**
     * 縮放圖片(壓縮圖片質量，改變圖片尺寸)
     * 若原圖寬度小于新寬度，則寬度不變！
     * @param newWidth 新的寬度
     * @param quality 圖片質量參數(shù) 0.7f 相當于70%質量
         * 2015年12月11日
     */
    public static void resize(File originalFile, File resizedFile,  
            int newWidth, float quality) throws IOException {  
   
        if (quality > 1) {  
            throw new IllegalArgumentException(  
                    "Quality has to be between 0 and 1");  
        }  
   
        ImageIcon ii = new ImageIcon(originalFile.getCanonicalPath());  
        Image i = ii.getImage();  
        Image resizedImage = null;  
   
        int iWidth = i.getWidth(null);  
        int iHeight = i.getHeight(null);  
   
        if(iWidth < newWidth){
            newWidth = iWidth;
        }
        if (iWidth > iHeight) {  
            resizedImage = i.getScaledInstance(newWidth, (newWidth * iHeight)  
                    / iWidth, Image.SCALE_SMOOTH);  
        } else {  
            resizedImage = i.getScaledInstance((newWidth * iWidth) / iHeight,  
                    newWidth, Image.SCALE_SMOOTH);  
        }  
   
        // This code ensures that all the pixels in the image are loaded.  
        Image temp = new ImageIcon(resizedImage).getImage();  
   
        // Create the buffered image.  
        BufferedImage bufferedImage = new BufferedImage(temp.getWidth(null),  
                temp.getHeight(null), BufferedImage.TYPE_INT_RGB);  
   
        // Copy image to buffered image.  
        Graphics g = bufferedImage.createGraphics();  
   
        // Clear background and paint the image.  
        g.setColor(Color.white);  
        g.fillRect(0, 0, temp.getWidth(null), temp.getHeight(null));  
        g.drawImage(temp, 0, 0, null);  
        g.dispose();  
   
        // Soften.  
        float softenFactor = 0.05f;  
        float[] softenArray = { 0, softenFactor, 0, softenFactor,  
                1 - (softenFactor * 4), softenFactor, 0, softenFactor, 0 };  
        Kernel kernel = new Kernel(3, 3, softenArray);  
        ConvolveOp cOp = new ConvolveOp(kernel, ConvolveOp.EDGE_NO_OP, null);  
        bufferedImage = cOp.filter(bufferedImage, null);  
   
        // Write the jpeg to a file.  
        FileOutputStream out = new FileOutputStream(resizedFile);  
   
        // Encodes image as a JPEG data stream  
        JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(out);  
   
        JPEGEncodeParam param = encoder  
                .getDefaultJPEGEncodeParam(bufferedImage);  
   
        param.setQuality(quality, true);  
   
        encoder.setJPEGEncodeParam(param);  
        encoder.encode(bufferedImage);  
    } // Example usage  
   
    public static void main(String[] args) throws IOException {  
//       File originalImage = new File("C:\\11.jpg");  
//       resize(originalImage, new File("c:\\11-0.jpg"),150, 0.7f);  
//       resize(originalImage, new File("c:\\11-1.jpg"),150, 1f);  
         File originalImage = new File("d:\\testImg\\1.jpg");  
         System.out.println("源文件大小" + originalImage.length());
//         File resizedImg = new File("d:\\testImg\\11.jpg");
//         resize(originalImage, resizedImg, 850, 1f);  
//         System.out.println("0.5轉換后文件大小" + resizedImg.length());
//         File resizedImg1 = new File("d:\\testImg\\111.jpg");
         File resizedImg1 = new File("/alidata/zkyj/dashixiong/tempImgFile/11.jpg");
         resize(originalImage, resizedImg1, 1550, 0.7f);  
         System.out.println("0.7轉換后文件大小" + resizedImg1.length());
    }  

import com.sun.image.codec.jpeg.JPEGCodec;
import com.sun.image.codec.jpeg.JPEGEncodeParam;
import com.sun.image.codec.jpeg.JPEGImageEncoder;
import javax.imageio.ImageIO;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
public class DPIHandleHelper {
private static int DPI = 300;
public static void main(String[] args) {
    String path = "C:\\test.jpg";
    File file = new File(path);
    handleDpi(file, 300, 300);
}
/**
 * 改變圖片DPI
 *
 * @param file
 * @param xDensity
 * @param yDensity
 */
public static void handleDpi(File file, int xDensity, int yDensity) {
    try {
        BufferedImage image = ImageIO.read(file);
        JPEGImageEncoder jpegEncoder = JPEGCodec.createJPEGEncoder(new FileOutputStream(file));
        JPEGEncodeParam jpegEncodeParam = jpegEncoder.getDefaultJPEGEncodeParam(image);
        jpegEncodeParam.setDensityUnit(JPEGEncodeParam.DENSITY_UNIT_DOTS_INCH);
        jpegEncoder.setJPEGEncodeParam(jpegEncodeParam);
        jpegEncodeParam.setQuality(0.75f, false);
        jpegEncodeParam.setXDensity(xDensity);
        jpegEncodeParam.setYDensity(yDensity);
        jpegEncoder.encode(image, jpegEncodeParam);
        image.flush();
    } catch (IOException e) {
        e.printStackTrace();
    }
}
}