import java.awt.Image;
import java.awt.image.BufferedImage;


Image img = ImageIO.read(file.getInputStream());//读入图片

BufferedImage tag = new BufferedImage(newWidth, newHeight, BufferedImage.TYPE_INT_RGB);

//缩略算法                    tag.getGraphics().drawImage(img.getScaledInstance(newWidth, newHeight, Image.SCALE_SMOOTH), 0, 0, null);

ByteArrayOutputStream output = new ByteArrayOutputStream();
ImageIO.write(tag, "jpg", output);

或

File destFile = new File("C:\\temp\\456.jpg");  
FileOutputStream out = new FileOutputStream(destFile); // 输出到文件流  
 // 可以正常实现bmp、png、gif转jpg  
 JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(out);  
 encoder.encode(image); // JPEG编码