如何通过jsp、html上传图片到数据库，及如何从数据库中取出blob类型并显示到页面中

1、通过jsp、html上传图片到数据库

先在addProduct.jsp前面加上文件类型控件，
页面上显示
然后选择文件，提交给后端接口的doPost或service方法，获得输入流：

     InputStream inputStream=null;
     Part filePart = req.getPart("picture");
        if(filePart!=null) {
       	  System.out.println("file name :"+filePart.getName()+" size"+filePart.getSize()+" file type"+filePart.getContentType());
       	  inputStream=filePart.getInputStream();
         }

接着将对象、该输入流传给dao,存入数据库：

public int save(Connection con, Product product, InputStream inputStream) {
		log.debug("saving product instance");
		int flag = 0;
		try {
			String sql = "insert into Product(productname,productdescription,picture,price,"
					+ "categoryid)values(?,?,?,?,?)";
			PreparedStatement statement = con.prepareStatement(sql);
			statement.setString(1, product.getProductName());
			statement.setString(2, product.getProductDescription());
			if(inputStream!=null)
				statement.setBlob(3, inputStream);
			statement.setDouble(4, product.getPrice());
			statement.setInt(5, product.getCategoryID());
			flag = statement.executeUpdate();
			log.debug("save successful");
		} catch (Exception re) {
			log.error("save failed", re);
			try {
				throw re;
			} catch (Exception e) {
				e.printStackTrace();
			}
		}
		return flag;
	}

当flag>0,存入成功。

2、从数据库中取出blob类型并显示到页面中
这里面有点巧妙，直接在想要显示的img的src里面设置新的访问请求（/getImg），自动访问获取图片的servlet，并通过该serlet显示出来。

basePath在前面heade.jsp有，

String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";

这时会发送新的请求/getImg?id,在GetImgServlet里面

protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		resp.setContentType("text/html");
		ProductDao dao = new ProductDao();
		int id=0;
		if(req.getParameter("id")!=null) {
			id=Integer.parseInt(req.getParameter("id"));
			System.out.println("id"+id);
		}
		byte[] imgById = dao.getImgById(conn, id);
		if(imgById!=null) {
			resp.setContentType("image/gif");
			ServletOutputStream os = resp.getOutputStream();
			os.write(imgById);
			os.flush();
		}
	}

调用dao的通过对象id找到相应的图片的byte数组方法：

public byte[] getImgById(Connection con, int id) {
		log.debug("finding picture by productId");
		byte[] imgByte=null;
		try {
			String queryString = "select picture from  Product where productId=?";
			PreparedStatement	statement = con.prepareStatement(queryString);
			statement.setInt(1, id);
			ResultSet resultSet = statement.executeQuery();
			while(resultSet.next()){
				Blob blob =resultSet.getBlob("picture");
				imgByte=blob.getBytes(1, (int)blob.length());
			}
		
		} catch (Exception re) {
			log.error("find picture failed", re);
			try {
				throw re;
			} catch (Exception e) {
				e.printStackTrace();
			}
		}
		return imgByte;	}//end getImgById

在doGet()里

resp.setContentType("image/gif");
			ServletOutputStream os = resp.getOutputStream();
			os.write(imgById);

将该byte数组通过ServletOutputStream 显示给jsp/html页面。