The DispatcherServlet

Every request in Spring MVC passes through DispatcherServlet.doDispatch(). Understanding this method eliminates all mystery behind web requests.

1. 011. Check Multipart: If multipart/form-data, wrap request in MultipartHttpServletRequest.
2. 022. Determine Handler: Query HandlerMapping list to find HandlerExecutionChain (controller method + interceptors).
3. 033. Determine Adapter: Locate HandlerAdapter capable of executing the handler (RequestMappingHandlerAdapter).
4. 044. Apply Pre-Interceptors: Execute HandlerInterceptor.preHandle() in order.
5. 055. Execute Handler: HandlerAdapter invokes the controller method with resolved arguments.
6. 066. Apply Post-Interceptors: Execute HandlerInterceptor.postHandle().
7. 077. Process Dispatch Result: Handle exceptions or render view, followed by triggerAfterCompletion() cleanup.

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@Component
public class CorrelationIdInterceptor implements HandlerInterceptor {
    @Override
    public boolean preHandle(HttpServletRequest req, HttpServletResponse res, Object handler) {
        String correlationId = Optional.ofNullable(req.getHeader("X-Correlation-ID"))
            .orElseGet(() -> UUID.randomUUID().toString());
        MDC.put("correlationId", correlationId);
        res.setHeader("X-Correlation-ID", correlationId);
        return true; // Continue dispatch pipeline!
    }

    @Override
    public void afterCompletion(HttpServletRequest req, HttpServletResponse res, Object handler, Exception ex) {
        MDC.remove("correlationId");
    }
}

If a HandlerInterceptor.preHandle() method returns false (to reject a request) but forgets to set a status code or write to the response, the client receives an empty 200 OK with no body!