PrintWriter writer = null; response.setCharacterEncoding("UTF-8"); response.setContentType("application/json; charset=utf-8"); try { writer = response.getWriter(); writer.print(JSONObject.toJSONString(R.error(401, "请先登录"))); } finally { if(writer != null){ writer.close(); } } return false; } }

这段代码是一个 Java Servlet 中的代码，主要作用是在用户未登录时返回一个错误消息。具体来说，代码首先设置响应的编码和内容类型为 JSON，然后尝试获取响应的输出流并使用 JSON 序列化框架将一个错误的 JSON 对象写入到输出流中，最后关闭输出流并返回 false。

需要注意的是，代码中使用了 try-finally 语句块来确保无论发生什么情况，输出流都能够被正确地关闭，避免资源泄漏。

相关问题

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // TODO Auto-generated method stub //response.getWriter().append("Served at: ").append(request.getContextPath()); request.setCharacterEncoding("utf-8"); response.setContentType("text/html;charset=utf-8"); response.setCharacterEncoding("utf-8");//解码 //向网页输出内容 PrintWriter out=response.getWriter(); //获取用户的姓名和密码 String b_username = request.getParameter("b_username"); String b_password = request.getParameter("b_password"); System.out.println(b_password+" "+b_username+"==="); //调用Login方法进行判断 BuyerobjDao dao = new BuyerobjDao(); boolean success = dao.login(b_username, b_password); System.out.println(success); if(success==true){ System.out.println("登录成功"); response.sendRedirect("shouyu.html"); }else{ System.out.println("登录失败"); response.sendRedirect("jingdong.jsp"); } } 改错

代码中主要存在的问题是：

1. doGet方法中的response.getWriter()方法只能调用一次，不能重复调用，否则会报错。

2. 在设置response的字符编码时，应该在获取PrintWriter对象之前进行设置，否则会无效。

修改后的代码如下：

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    request.setCharacterEncoding("utf-8");
    response.setContentType("text/html;charset=utf-8");
    response.setCharacterEncoding("utf-8");
    //向网页输出内容
    PrintWriter out = response.getWriter();

    //获取用户的姓名和密码
    String b_username = request.getParameter("b_username");
    String b_password = request.getParameter("b_password");
    System.out.println(b_password + " " + b_username + "===");

    //调用Login方法进行判断
    BuyerobjDao dao = new BuyerobjDao();
    boolean success = dao.login(b_username, b_password);
    System.out.println(success);

    if (success) {
        System.out.println("登录成功");
        response.sendRedirect("shouyu.html");
    } else {
        System.out.println("登录失败");
        response.sendRedirect("jingdong.jsp");
    }
    out.close();
}

修改的主要内容包括：

1. 删除了无用的注释和代码。

2. 将获取PrintWriter对象的代码移到设置response字符编码的前面。

3. 修改了if语句中的判断条件，使其更加简洁。

代码改错：@WebServlet("/update_teacher") public class update_teacher extends HttpServlet { protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html;charset=utf-8"); response.setCharacterEncoding("utf-8"); request.setCharacterEncoding("utf-8"); PrintWriter out = response.getWriter(); HttpSession session = request.getSession(); TeacherDao teacherD = new TeacherDao(); String uid = request.getParameter("uid"); String name = request.getParameter("name"); String sex = request.getParameter("sex"); String email = request.getParameter("email"); String password = request.getParameter("password"); try { Teacher teacher = teacherD.updateTeacher(uid, name, sex, email, password); session.setAttribute("info", teacher); out.print("<script>alert(\"保存成功！\");location.href = \"teacher/personal.jsp\";</script>"); } catch (Exception e) { out.print(e); } } protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { } }

这段代码看起来没有明显的错误，但是建议做一些改进：

1. 类名应该以大写字母开头，所以建议将类名 `update_teacher` 改为 `UpdateTeacher`。

2. 在 `catch` 块中，如果出现异常应该将信息打印到日志中而不是直接输出到页面，因为这样可以避免暴露敏感信息。

3. 如果更新教师信息失败，应该给出友好的提示信息。

下面是改进后的代码：

@WebServlet("/update_teacher")
public class UpdateTeacher extends HttpServlet {

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html;charset=utf-8");
        response.setCharacterEncoding("utf-8");
        request.setCharacterEncoding("utf-8");

        PrintWriter out = response.getWriter();
        HttpSession session = request.getSession();

        TeacherDao teacherDao = new TeacherDao();

        String uid = request.getParameter("uid");
        String name = request.getParameter("name");
        String sex = request.getParameter("sex");
        String email = request.getParameter("email");
        String password = request.getParameter("password");

        try {
            Teacher teacher = teacherDao.updateTeacher(uid, name, sex, email, password);
            session.setAttribute("info", teacher);
            out.print("<script>alert(\"保存成功！\");location.href = \"teacher/personal.jsp\";</script>");
        } catch (Exception e) {
            Logger.getLogger(UpdateTeacher.class.getName()).log(Level.SEVERE, null, e);
            out.print("<script>alert(\"保存失败，请稍后重试！\");location.href = \"teacher/personal.jsp\";</script>");
        }
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    }
}