下面是加水印的函式，直接在上傳圖片的地方呼叫就好了，兩個引數分別是File和String。

/**
	 * 加水印
	 * @param ifile
	 */
	public void addWaterMark(File ifile,String url){
		 try {
			String waterMarkContent="論壇報名專用"; 
			Image srcImg = ImageIO.read(ifile);
			int srcImgWidth = srcImg.getWidth(null);
			int srcImgHeight = srcImg.getHeight(null);
			 // 加水印
			BufferedImage bufImg = new BufferedImage(srcImgWidth, srcImgHeight, BufferedImage.TYPE_INT_RGB);
			Graphics2D g = bufImg.createGraphics();
			g.drawImage(srcImg, 0, 0, srcImgWidth, srcImgHeight, null);
			g.setColor(new Color(255,0,0)); //根據圖片的背景設定水印顏色
			g.setFont(new Font("宋體",Font.PLAIN,50));              //設定字型

			//設定水印的座標
			int x = 50;//srcImgWidth - 2*getWatermarkLength(waterMarkContent, g);  
			int y = 100;//srcImgHeight - 2*getWatermarkLength(waterMarkContent, g);  
			g.drawString(waterMarkContent, x, y);  //畫出水印
			g.dispose();  
			// 輸出圖片  
			FileOutputStream outImgStream = new FileOutputStream(url);  
			ImageIO.write(bufImg, "jpg", outImgStream);
			System.out.println("新增水印完成");  
			outImgStream.flush();  
			outImgStream.close();
		} catch (FileNotFoundException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}  
	}
	
		private int getWatermarkLength(String waterMarkContent, Graphics2D g) {
			 return g.getFontMetrics(g.getFont()).charsWidth(waterMarkContent.toCharArray(), 0, waterMarkContent.length());  
	}