问答平台（3），统一处理异常

统一处理异常

@ControllerAdvice
- 用于修饰类，表示该类是 Controller 的全局配置类。
- 在此类中，可以对 Controller 进行如下三种全局配置：异常处理方案、绑定数据方案、绑定参数方案。
@ExceptionHandle（已使用）
- 用于修饰方法，该方法会在 Controller（出现异常后）被调用，用于处理捕获到的异常。
@ModelAttribute（未使用）
- 用于修饰方法，该方法会在 Controller（方法执行前）被调用，用于为 Model 对象绑定参数。
@DataBinder（未使用）
- 用于修饰方法，该方法会在 Controller（方法执行前）被调用，用于绑定参数的转换器。

表现层

// HomeController.java
@RequestMapping(path = "/error", method = RequestMethod.GET)
public String getErrorPage() {
    return "/error/500";
}
// 拒绝访问时的提示页面
@RequestMapping(path = "/denied", method = RequestMethod.GET)
public String getDeniedPage() {
    return "/error/404";
}

制造异常

• 普通请求

  // MessageController.java
  @RequestMapping(path = "/letter/list", method = RequestMethod.GET)
  public String getLetterList(Model model, Page page) {
      // 制造异常
      Integer.valueOf("abc");
  
      User user = hostHolder.getUser();
      // 分页信息
      page.setLimit(5);
      page.setPath("/letter/list");
      page.setRows(messageService.findConversationCount(user.getId()));
  
      // 会话列表
      List<Message> conversationList = messageService.findConversations(
              user.getId(), page.getOffset(), page.getLimit());
      List<Map<String, Object>> conversations = new ArrayList<>();
      if (conversationList != null) {
          for (Message message : conversationList) {
              Map<String, Object> map = new HashMap<>();
              map.put("conversation", message);
              map.put("letterCount", messageService.findLetterCount(message.getConversationId()));
              map.put("unreadCount", messageService.findLetterUnreadCount(user.getId(), message.getConversationId()));
              int targetId = user.getId() == message.getFromId() ? message.getToId() : message.getFromId();
              map.put("target", userService.findUserById(targetId));
              conversations.add(map);
          }
      }
      model.addAttribute("conversations", conversations);
  
      // 查询未读消息数量
      int letterUnreadCount = messageService.findLetterUnreadCount(user.getId(), null);
      model.addAttribute("letterUnreadCount", letterUnreadCount);
      int noticeUnreadCount = messageService.findNoticeUnreadCount(user.getId(), null);
      model.addAttribute("noticeUnreadCount", noticeUnreadCount);
  
      return "/site/letter";
  }

• 异步请求

  // MessageController.java
  @RequestMapping(path = "/letter/send", method = RequestMethod.POST)
  @ResponseBody
  public String sendLetter(String toName, String content) {
      // 制造异常
      Integer.valueOf("abc");
  
      User target = userService.findUserByName(toName);
      if (target == null) {
          return CommunityUtil.getJSONString(1, "目标用户不存在!");
      }
  
      Message message = new Message();
      message.setFromId(hostHolder.getUser().getId());
      message.setToId(target.getId());
      if (message.getFromId() < message.getToId()) {
          message.setConversationId(message.getFromId() + "_" + message.getToId());
      } else {
          message.setConversationId(message.getToId() + "_" + message.getFromId());
      }
      message.setContent(content);
      message.setCreateTime(new Date());
      messageService.addMessage(message);
  
      return CommunityUtil.getJSONString(0);
  }

异常处理

• 包: controller -> advice

  // ExceptionAdvice.java
  @ControllerAdvice(annotations = Controller.class)
  public class ExceptionAdvice {
  
      private static final Logger logger = LoggerFactory.getLogger(ExceptionAdvice.class);
  
      @ExceptionHandler({Exception.class})
      public void handleException(Exception e, HttpServletRequest request, HttpServletResponse response) throws IOException {
          logger.error("服务器发生异常: " + e.getMessage());
          for (StackTraceElement element : e.getStackTrace()) {
              logger.error(element.toString());
          }
  
          String xRequestedWith = request.getHeader("x-requested-with");
          if ("XMLHttpRequest".equals(xRequestedWith)) {
              response.setContentType("application/plain;charset=utf-8");
              PrintWriter writer = response.getWriter();
              writer.write(CommunityUtil.getJSONString(1, "服务器异常!"));
          } else {
              response.sendRedirect(request.getContextPath() + "/error");
          }
      }
  }

页面

• 404.html

  <!doctype html>
  <html lang="en" xmlns:th="http://www.thymeleaf.org">
  <head>
      <meta charset="utf-8">
      <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
      <link rel="icon" href="/img/logo.png"/>
      <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css" crossorigin="anonymous">
      <link rel="stylesheet" th:href="@{/css/global.css}"/>
      <title>问答社区-404</title>
  </head>
  <body>
      <div class="nk-container">
          <!-- 头部 -->
  
          <!-- 内容 -->
          <div class="main">
              <div class="container pl-5 pr-5 pt-3 pb-3 mt-3 mb-3">
                  <img th:src="@{/img/404.png}">
              </div>
          </div>
  
          <!-- 尾部 -->
  </body>
  </html>

• 500.html

  <!doctype html>
  <html lang="en" xmlns:th="http://www.thymeleaf.org">
  <head>
      <meta charset="utf-8">
      <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
      <link rel="icon" href="/img/logo.png"/>
      <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css" crossorigin="anonymous">
      <link rel="stylesheet" th:href="@{/css/global.css}"/>
      <title>问答社区-500</title>
  </head>
  <body>
      <div class="nk-container">
          <!-- 头部 -->
  
          <!-- 内容 -->
          <div class="main">
              <div class="container pl-5 pr-5 pt-3 pb-3 mt-3 mb-3">
                  <img th:src="@{/img/error.png}">
              </div>
          </div>
  
          <!-- 尾部 -->
  </body>
  </html>