当我们请求/hello时, 传参数u.username如何注入到user中的username?

样例

@Controller
public class HelloController {
   

   @RequestMapping("/hello")
   public String hello(@ModelAttribute("user")User user) {
   
      System.out.println(user);
      System.out.println(employee);
      return "hello";
   }

   @InitBinder("user")
   public void initUser(WebDataBinder binder) {
   
      binder.setFieldDefaultPrefix("u.");
   }
}

原理

目标调用方法为hello方法, User参数如何被注入进去的? 先从原理出发

1. 通过反射的方式, 拿到hello方法的参数信息, 比如参数个数为1个, 参数类型为User等等
2. 反射创建出User对象, 用内省的方式(反射)给对象填充属性
3. 通过method.invoke(obj, args)调用到目标hello方法

流程

org.springframework.web.servlet.DispatcherServlet#doDispatch
关注ha.handle()方法, 处理器适配器的handle方法, 会对参数解析并注入, 后续再调用具体的方法

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
   
   HttpServletRequest processedRequest = request;
   HandlerExecutionChain mappedHandler = null;
   boolean multipartRequestParsed = false;

   WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

   try {
   
      ModelAndView mv = null;
      Exception dispatchException = null;

      try {
   
         processedRequest = checkMultipart(request);
         multipartRequestParsed = (processedRequest != request);

         // Determine handler for the current request.

         // HandlerMapping核心为getHandler方法, 获取该请求的调用执行链, 拦截器 + 具体处理的HandlerMethod
         mappedHandler = getHandler(processedRequest);
         if (mappedHandler == null) {
   
            noHandlerFound(processedRequest, response);
            return;
         }

         // Determine handler adapter for the current request.

         // mappedHandler.getHandler()获取的是HandlerMethod对象
         HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

         // Process last-modified header, if supported by the handler.
         String method = request.getMethod();
         boolean isGet = "GET".equals(method);
         if (isGet || "HEAD".equals(method)) {
   
            long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
            if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
   
               return;
            }
         }

         // 处理拦截器preHandle方法
         if (!mappedHandler.applyPreHandle(processedRequest, response)) {
   
            return;
         }

         // Actually invoke the handler.

         // 调用目标方法
         mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

         if (asyncManager.isConcurrentHandlingStarted()) {
   
            return;
         }

         applyDefaultViewName(processedRequest, mv);

         // 处理拦截器postHandle方法
         mappedHandler.applyPostHandle(processedRequest, response, mv);
      }
      catch (Exception ex) {
   
         dispatchException = ex;
      }
      catch (Throwable err) {
   
         // As of 4.3, we're processing Errors thrown from handler methods as well,
         // making them available for @ExceptionHandler methods and other scenarios.
         dispatchException = new NestedServletException("Handler dispatch failed", err);
      }
      processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
   }
   catch (Exception ex) {
   
      triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
   }
   catch (Throwable err) {
   
      triggerAfterCompletion(processedRequest, response, mappedHandler,
            new NestedServletException("Handler processing failed"