1. Overview
In this tutorial, we’ll explore Spring Web annotations from the org.springframework.web.bind.annotation package.
2. @RequestMapping
Simply put, @RequestMapping marks request handler methods inside @Controller classes; it can be configured using:
- path, or its aliases, name, and value: which URL the method is mapped to
- method: compatible HTTP methods
- params: filters requests based on presence, absence, or value of HTTP parameters
- headers: filters requests based on presence, absence, or value of HTTP headers
- consumes: which media types the method can consume in the HTTP request body
- produces: which media types the method can produce in the HTTP response body
Here’s a quick example of what that looks like:
@Controller
class VehicleController {
@RequestMapping(value = "/vehicles/home", method = RequestMethod.GET)
String home() {
return "home";
}
}
We can provide default settings for all handler methods in a @Controller class if we apply this annotation on the class level. The only exception is the URL which Spring won’t override with method level settings but appends the two path parts.
For example, the following configuration has the same effect as the one above:
@Controller
@RequestMapping(value = "/vehicles", method = RequestMethod.GET)
class VehicleController {
@RequestMapping("/home")
String home() {
return "home";
}
}
Moreover, @GetMapping, @PostMapping, @PutMapping, @DeleteMapping, and @PatchMapping are different variants of @RequestMapping with the HTTP method already set to GET, POST, PUT, DELETE, and PATCH respectively.
These are available since Spring 4.3 release.
3. @RequestBody
Let’s move on to @RequestBody – which maps the body of the HTTP request to an object:
@PostMapping("/save")
void saveVehicle(@RequestBody Vehicle vehicle) {
// ...
}
The deserialization is automatic and depends on the content type of the request.
4. @PathVariable
Next, let’s talk about @PathVariable.
This annotation indicates that a method argument is bound to a URI template variable. We can specify the URI template with the @RequestMapping annotation and bind a method argument to one of the template parts with @PathVariable.
We can achieve this with the name or its alias, the value argument:
@RequestMapping("/{id}")
Vehicle getVehicle(@PathVariable("id") long id) {
// ...
}
If the name of the part in the template matches the name of the method argument, we don’t have to specify it in the annotation:
@RequestMapping("/{id}")
Vehicle getVehicle(@PathVariable long id) {
// ...
}
Moreover, we can mark a path variable optional by setting the argument required to false:
@RequestMapping("/{id}")
Vehicle getVehicle(@PathVariable(required = false) long id) {
// ...
}
5. @RequestParam
We use @RequestParam for accessing HTTP request parameters:
@RequestMapping
Vehicle getVehicleByParam(@RequestParam("id") long id) {
// ...
}
It has the same configuration options as the @PathVariable annotation.
In addition to those settings, with @RequestParam we can specify an injected value when Spring finds no or empty value in the request. To achieve this, we have to set the defaultValue argument.
Providing a default value implicitly sets required to false:
@RequestMapping("/buy")
Car buyCar(@RequestParam(defaultValue = "5") int seatCount) {
// ...
}
Besides parameters, there are other HTTP request parts we can access: cookies and headers. We can access them with the annotations @CookieValue and @RequestHeader respectively.
We can configure them the same way as @RequestParam.
6. Response Handling Annotations
In the next sections, we will see the most common annotations to manipulate HTTP responses in Spring MVC.
6.1. @ResponseBody
If we mark a request handler method with @ResponseBody, Spring treats the result of the method as the response itself:
@ResponseBody
@RequestMapping("/hello")
String hello() {
return "Hello World!";
}
If we annotate a @Controller class with this annotation, all request handler methods will use it.
6.2. @ExceptionHandler
With this annotation, we can declare a custom error handler method. Spring calls this method when a request handler method throws any of the specified exceptions.
The caught exception can be passed to the method as an argument:
@ExceptionHandler(IllegalArgumentException.class)
void onIllegalArgumentException(IllegalArgumentException exception) {
// ...
}
6.3. @ResponseStatus
We can specify the desired HTTP status of the response if we annotate a request handler method with this annotation. We can declare the status code with the code argument, or its alias, the value argument.
Also, we can provide a reason using the reason argument.
We also can use it along with @ExceptionHandler:
@ExceptionHandler(IllegalArgumentException.class)
@ResponseStatus(HttpStatus.BAD_REQUEST)
void onIllegalArgumentException(IllegalArgumentException exception) {
// ...
}
For more information about HTTP response status, please visit this article.
7. Other Web Annotations
Some annotations don’t manage HTTP requests or responses directly. In the next sections, we’ll introduce the most common ones.
7.1. @RestController
The @RestController combines @Controller and @ResponseBody.
Therefore, the following declarations are equivalent:
@Controller
@ResponseBody
class VehicleRestController {
// ...
}
@RestController
class VehicleRestController {
// ...
}
7.2. @ModelAttribute
With this annotation we can access elements that are already in the model of an MVC @Controller, by providing the model key:
@PostMapping("/assemble")
void assembleVehicle(@ModelAttribute("vehicle") Vehicle vehicleInModel) {
// ...
}
Like with @PathVariable and @RequestParam, we don’t have to specify the model key if the argument has the same name:
@PostMapping("/assemble")
void assembleVehicle(@ModelAttribute Vehicle vehicle) {
// ...
}
Besides, @ModelAttribute has another use: if we annotate a method with it, Spring will automatically add the method’s return value to the model:
@ModelAttribute("vehicle")
Vehicle getVehicle() {
// ...
}
Like before, we don’t have to specify the model key, Spring uses the method’s name by default:
@ModelAttribute
Vehicle vehicle() {
// ...
}
Before Spring calls a request handler method, it invokes all @ModelAttribute annotated methods in the class.
More information about @ModelAttribute can be found in this article.
7.3. @CrossOrigin
@CrossOrigin enables cross-domain communication for the annotated request handler methods:
@CrossOrigin
@RequestMapping("/hello")
String hello() {
return "Hello World!";
}
If we mark a class with it, it applies to all request handler methods in it.
We can fine-tune CORS behavior with this annotation’s arguments.
For more details, please visit this article.
8. Conclusion
In this article, we saw how we can handle HTTP requests and responses with Spring MVC.
As usual, the examples are available over on GitHub.