Spring Let the DispatcherServlet get the HandlerMapping in advance

Currently, DispatcherServlet#initHandlerMappings is initialized in org.springframework.web.servlet.HttpServletBean#init, But it is already executed in the servlet thread http-nio-8081-exec-1，I hope it can be executed in advance to play the role of preloading，In this way, I can also get HandlerMapping earlier through DispatcherServlet#getHandlerMappings

for example：DispatcherServlet can impl SmartInitializingSingleton and invoke DispatcherServlet#initHandlerMappings in method SmartInitializingSingleton#afterSingletonsInstantiated

Comment From: poutsma

initHandlerMappings is called by Servlet::init, part of the Servlet environment (i.e. Tomcat). Servlet::init is called by the servlet container to indicate to a servlet that the servlet is being placed into service. Before init is called, the ServletConfig is not yet available, and that's required for the web application context to be instantiated. Therefore, init is the earliest possible moment where Spring Framework can safely instantiate beans such as handler mappings.

Comment From: brucelwl

@poutsma but we can get the ServletContext before Servlet::init For example, implement the following interfaces: javax.servlet.ServletContainerInitializer#onStartup(Set<Class<?>> c, ServletContext ctx)和javax.servlet.ServletContextListener#contextInitialized(ServletContextEvent sce)

Comment From: poutsma

What problem are you trying to solve by having the HandlerMappings available earlier than Servlet::init?

Comment From: brucelwl

What problem are you trying to solve by having the HandlerMappings available earlier than Servlet::init?

Because I want to intercept the handler corresponding to the httpServletRequest through the javax.servlet.Filter, so I can add my own current limit statistics to the handler. Therefore, I must know all HandlerMapping, Coincidentally, I found that all HandlerMapping will be loaded in DispatcherServlet, and method DispatcherServlet#getHandlerMappings is also provided, but the initialization time is a little over

Comment From: brucelwl

What problem are you trying to solve by having the HandlerMappings available earlier than Servlet::init?

In addition, loading HandlerMapping in advance can warm up, instead of initializing the HandlerMapping after the user request arrives, because it is locked in the servlet thread http-nio-8081-exec-xxx, so the first request will be slow.

Therefore, it is suggested that the whole method DispatcherServlet#initStrategies(applicationContext) should be initialized in advance, not in the servlet thread

Comment From: brucelwl

@poutsma Can you tell me how to deal with this issue in the future?

Comment From: poutsma

Looking at the code you shared, I think you're running into problems because your component is dependant on the Servlet lifecycle as well as the Spring lifecycle.

So, instead of implementing a Servlet Filter, you should consider implementing org.springframework.web.servlet.HandlerInterceptor, as handler interceptor are Spring components that live in the Spring lifecycle. Then, you can simply detect the handler mappings in your ApplicationContextAware::setApplicationContext implementation, which will be called before your HandlerInterceptor method implementations. I would also not implement SmartLifecycle, as all the initialization you need can be done in setApplicationContext.

As for the suggestion made in your original comment: DispatcherServlet is a Servlet that is instantiated by the web container (i.e. Tomcat). DispatcherServlet is not a Spring Bean, and therefore implementing SmartInitializingSingleton will not work.

As for implementing ServletContainerInitializer: I don't think that's going to help either, because Filters are executed (and initialized) before Servlets are, so you will most likely face the same problem of the servlet not being initialized yet. By implementing a handler interceptor as I suggested above, you are part of the same lifecycle of the DispatcherServlet that you're trying to query, so that seems the best way forward.