What does configureDefaultServletHandling means?

As JB Nizet already tried to explain both are used to serve static resources.

So your question is that your Java based Spring configuration has

@Override public void addResourceHandlers(ResourceHandlerRegistry registry) {         registry.addResourceHandler("/assets/**").addResourceLocations("/resources/bootstrap/"); }

then why do you need

@Overridepublic void configureDefaultServletHandling(DefaultServletHandlerConfigurer configurer) {    configurer.enable();}

or why <mvc:default-servlet-handler/> if you have

<mvc:resources mapping="/assets/**" location="/resources/bootstrap/" />

in terms of xml configuration.

To answer your question based on the requirements you have put you don't need to override configureDefaultServletHandling() as you have already overridden and provided your static resource mappings.

By overriding addResourceHandlers() method you are essentially asking ResourceHttpRequestHandler to serve the resources mentioned resource location.

However if you override configureDefaultServletHandling() and enabling it you are essentially asking default servlet (mapped to "/") to serve the resources. There are couple of things you need to take care here if you are using this. Quoting from docs -

This allows for mapping the DispatcherServlet to "/" (thus overriding the mapping of the container’s default Servlet), while still allowing static resource requests to be handled by the container’s default Servlet. It configures a DefaultServletHttpRequestHandler with a URL mapping of "/**" and the lowest priority relative to other URL mappings.

This handler will forward all requests to the default Servlet. Therefore it is important that it remains last in the order of all other URL HandlerMappings. That will be the case if you use or alternatively if you are setting up your own customized HandlerMapping instance be sure to set its order property to a value lower than that of the DefaultServletHttpRequestHandler, which is Integer.MAX_VALUE.