M05 Q05 How is an incoming request mapped to a controller and mapped to a method?

Incoming request is mapped to a controller and a method by DispatcherServlet, which uses HandlerMapping and HandlerAdapter components for this purpose.

HandlerMapping components are used during Spring initialization to scan classpath for @Controller or @RestController classes with one of request mapping annotations that are part of annotation-based programming model:

• @RequestMapping
• @GetMapping
• @PostMapping
• @PutMapping
• @PatchMapping
• @DeleteMapping

HandlerAdapter components are responsible for execution of method identified as handler candidate for the request.

Now let's look at the lifecycle of the request when it is being created against the server. When request is performed against the server following steps are executed:

1. Application Server (Standalone or Embedded) searches for Servlet that can handle request, DispatcherServlet is selected based on Servlet Registration and url-pattern.
2. DispatcherServlet uses HandlerMapping classes to get request mapping information and HandlerAdapter.
3. DispatcherServlet uses HandlerAdapter to execute controller method that will handle request.
4. DispatcherServlet interprets results of method execution and renders View with help of ViewResolver classes.

@RequestMapping allows you to specify conditions that request has to match for a method to be used as request handler. @RequestMapping can be used at class or method level, when used at the class level, all method level mappings inherit this primary mapping, narrowing it to a specific handler method.

For example, below controllers are supposed to map GET /say/hello requests, even though request mapping is defined differently, all are equal.

@RequestMapping annotation allows you to specify following criteria for request:

• path - uri path/paths for request, for example /api/books
• method – supported HTTP method/methods: GET, POST, HEAD, OPTIONS, PUT, PATCH, DELETE, TRACE
• params – required parameters of request, for example key1=value1, key2!=value2, key1, !key1
• headers – header needs to match specified condition, for example, header1=value1, header2!=value2, header1, !header1, content-type=text/*
• consumes – media types that can be consumed by request, for example, application/json
• produces – media types that are produced by method handling the request, for example, application/pdf

Spring MVC also supports composed annotations for request mapping:

• @GetMapping
• @PostMapping
• @PutMapping
• @PatchMapping
• @DeleteMapping

Each of those annotations allows you to specify same conditions as @RequestMapping except for HTTP method field, following fields in @*Mapping are aliases to @RequestMapping: path, params, headers, consumes, produces.

In most of the cases it is possible to translate mappings between those annotations, one example when this is not possible is when creating HTTP HEAD request mapping.

More examples

This index() method has several uri to reach the index page

The second example is a header request.

The next example allows you to specify HTTP POST request and it allows you to search for the article based on some specified criteria.

And one more, the method to delete the article by id.