Spring MVC: how framework maps URI to bean

Notice

I wrote this article and was originally published on Qiita on 1 July 2021.

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What is org.springframework.web.servlet.HandlerMapping interface?

From the document from Spring Framework, it says "Interface to be implemented by objects that define a mapping between requests and handler objects.". In my notice board example application, following bean is created when started.

Order	Bean Name	Class	Where it created
2147483647	welcomePageHandlerMapping	org.springframework.boot.autoconfigure.web.servlet.WelcomePageHandlerMapping	org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration.EnableWebMvcConfiguration.welcomePageHandlerMapping(ApplicationContext, FormattingConversionService, ResourceUrlProvider)
2147483646	resourceHandlerMapping	org.springframework.web.servlet.handler.SimpleUrlHandlerMapping	org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport.resourceHandlerMapping()
2	beanNameHandlerMapping	org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping	org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport.beanNameHandlerMapping()
0	requestMappingHandlerMapping	org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping	org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration.EnableWebMvcConfiguration.requestMappingHandlerMapping()
-2147483647	faviconHandlerMapping	org.springframework.web.servlet.handler.SimpleUrlHandlerMapping	org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration.WebMvcAutoConfigurationAdapter.FaviconConfiguration.faviconHandlerMapping()

requestMappingHandlerMapping handles bean class which have org.springframework.web.bind.annotation.RequestMapping annotation.

How I know this? Please refer to here.

When it been loaded

First, beans are created by ApplicationContext instance.

Then when org.springframework.web.servlet.DispatcherServlet instance is created and during initialization, org.springframework.web.servlet.DispatcherServlet.initHandlerMappings(ApplicationContext) is called and all bean with org.springframework.web.servlet.HandlerMapping interface will been lookup. Its reference will be saved into DispatcherServlet's internal structure. Following is related code.

/**
 * Initialize the HandlerMappings used by this class.
 * <p>If no HandlerMapping beans are defined in the BeanFactory for this namespace,
 * we default to BeanNameUrlHandlerMapping.
 */
private void initHandlerMappings(ApplicationContext context) {
    this.handlerMappings = null;

    if (this.detectAllHandlerMappings) {
        // Find all HandlerMappings in the ApplicationContext, including ancestor contexts.
        Map<String, HandlerMapping> matchingBeans =
                BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);
        if (!matchingBeans.isEmpty()) {
            this.handlerMappings = new ArrayList<>(matchingBeans.values());
            // We keep HandlerMappings in sorted order.
            AnnotationAwareOrderComparator.sort(this.handlerMappings);
        }
    }
    else {
        ...
    }
    ...
}

Look up which bean handles particular URI

In method org.springframework.web.servlet.DispatcherServlet.doDispatch(HttpServletRequest, HttpServletResponse) you can see how Spring MVC actually handle a request. The code which lookup URI bean mapping is

mappedHandler = getHandler(processedRequest);
if (mappedHandler == null) {
    noHandlerFound(processedRequest, response);
    return;
}

And in method org.springframework.web.servlet.DispatcherServlet.getHandler(HttpServletRequest), request will be passed into list of instances with org.springframework.web.servlet.HandlerMapping interface which is built before. If mapping is found in one instance, lookup will be stopped and org.springframework.web.servlet.HandlerExecutionChain will be returned. Following is the code.

/**
 * Return the HandlerExecutionChain for this request.
 * <p>Tries all handler mappings in order.
 * @param request current HTTP request
 * @return the HandlerExecutionChain, or {@code null} if no handler could be found
 */
@Nullable
protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
    if (this.handlerMappings != null) {
        for (HandlerMapping mapping : this.handlerMappings) {
            HandlerExecutionChain handler = mapping.getHandler(request);
            if (handler != null) {
                return handler;
            }
        }
    }
    return null;
}