改变图片的大小，并且给图片设置白色背景

/**
	 * 改变图片的大小
	 * 
	 * @param source
	 *            源文件
	 * @param targetW
	 *            目标长
	 * @param targetH
	 *            目标宽
	 * @return
	 * @throws IOException 
	 */
	public static BufferedImage resize(BufferedImage source, int targetW, int targetH) throws IOException
	{
		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
		{
			//固定宽高，宽高一定要比原图片大
			//target = new BufferedImage(targetW, targetH, type);
			target = new BufferedImage(800, 600, type);
		}
		
		Graphics2D g = target.createGraphics();
		
		//写入背景
		g.drawImage(ImageIO.read(new File("ok/blank.png")), 0, 0, null);
		
		// 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 main(String[] args) throws IOException
	{
		ImgUtil iu = new ImgUtil();

		BufferedImage image = ImageIO.read(new File("ok/1111661748321.jpg"));
		image = iu.resize(image, 512, 512);

		ImageIO.write(image, "png", new FileOutputStream("ok/test.JPG"));
	}