后端心得

后端心得

传统servlet

@WebServlet("/exitServlet")
public class ExitServlet extends HttpServlet {
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        request.getSession().invalidate();

        response.sendRedirect(request.getContextPath()+"/login.html");
    }

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.doPost(request, response);
    }
}

1. 注解 @WebServlet("/activeUserServlet")里面对应url，就是访问该方法的网址。html页面请求的也是该方法

2. doGet方法没什么用，只用post

   对Servlet抽取，来实现解耦

   public class BaseServlet extends HttpServlet {
       //不需要被访问到，其他的Servlet来继承
    @Override
       protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
   //         当子类Servlet被访问，就运行这个方法(方法继承)
   //         获取请求路径，方法名称，方法对象，执行方法（反射）
   //         这里运行方法是通过网址来获取的
           String uri = req.getRequestURI();
           // /user/add
           String methodName = uri.substring(uri.lastIndexOf('/') + 1);
           //在这里this对象就是UserServlet 谁调用，就是谁
           try {
               //getDeclaredMethod,忽略修饰符
               Method method = this.getClass()
                       .getDeclaredMethod(methodName, HttpServletRequest.class, HttpServletResponse.class);
               method.setAccessible(true);
               // 暴力反射
               method.invoke(this, req, resp);
           } catch (NoSuchMethodException e) {
               e.printStackTrace();
           } catch (IllegalAccessException e) {
               e.printStackTrace();
           } catch (InvocationTargetException e) {
               e.printStackTrace();
           }
       }
       public void writeJsonBack(Object object, HttpServletResponse response) throws IOException {
           ObjectMapper mapper = new ObjectMapper();
           response.setContentType("application/json;charset=utf-8");
           mapper.writeValue(response.getOutputStream(), object);
       }
   
       public String writeValueAsString(Object obj) throws JsonProcessingException {
           ObjectMapper mapper = new ObjectMapper();
           return mapper.writeValueAsString(obj);
       }
   }
   
   

   这里多出的两个方法用于回写数据，数据写进流中，只有流能在网络中传输

   @WebServlet("/category/*")
   public class CategoryServlet extends BaseServlet {
       private CategoryService service = new CategoryServiceImpl();
   
       protected void findAll(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
           List<Category> cs = service.findAll();			 writeJsonBack(cs,response); }}
   

   另一种方法，将方法直接写在一个Servlet中，改写注解

   @WebServlet("/user/*")
   public class UserServlet extends BaseServlet {
       //复用代码的抽取
       private UserService userService = new UserServiceImpl();
   
       public void regist(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
           .......
       }
   
       public void findOneUser(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
           User user = (User) request.getSession().getAttribute("user");
           writeJsonBack(user, response);
       }
   
       public void exit(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
           request.getSession().invalidate();
           response.sendRedirect(request.getContextPath() + "/login.html");
       }}
   
   

   Service

   servlet只用于前台和后台Service之间的交互桥梁，而Service用于Dao的连接，将查询到的数据给Servlet，再通过servlet回写给前台

   public class UserServiceImpl implements UserService {
       private UserDao userDao = new UserDaoImpl();
    @Override
       public User login(User user) {
   
           return userDao.findByUsernameAndPassword(user.getUsername(), user.getPassword());
       }	}
   

   Dao

   private JdbcTemplate template = new JdbcTemplate(JDBCUtils.getDataSource());
   
       @Override
       public List<Category> findAll() {
           String sql = "select * from tab_category ";
           return template.query(sql, new BeanPropertyRowMapper<Category>(Category.class));
           //BeanPropertyRowMapper(Category.class) 按照Category.class，把数据写成类
       }
       
       public int findCountByRid(int rid) {
           String sql = "select count(*) from tab_favorite where rid = ?";
   
           return template.queryForObject(sql, Integer.class, rid);
       }
   	
   @Override
       public void add(int rid, int uid) {
           String sql = "insert into tab_favorite values(?,?,?)";
           template.update(sql, rid, new Date(), uid);
       }
   

   使用jdbc来连接数据库，按照返回类型的不同，使用不同的方法。