Understanding Content-Type and Its Use in Spring MVC

In HTTP requests the Content-Type header specifies the media type of the request body, allowing servers to correctly interpret the data format.

Media types follow the pattern type/subtype(;parameter)?, where type is a primary category (e.g., text or image), subtype refines the format (e.g., html, png), and an optional parameter such as charset can provide additional details.

Common media types include:

text/html (HTML), text/plain (plain text), text/xml (XML), image/gif (GIF), image/jpeg (JPEG), image/png (PNG), and several application/* types such as application/json, application/xml, application/pdf, application/octet-stream, application/x-www-form-urlencoded, and multipart/form-data for file uploads.

Spring MVC leverages these types through the @RequestMapping annotation. The annotation can declare consumes to restrict which request Content‑Types are accepted and produces to specify the response Content‑Type, while other attributes like value, method, params, and headers further refine request handling.

Example of the @RequestMapping definition:

@Target({ElementType.METHOD, ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Mapping
public @interface RequestMapping {
    String name() default "";
    @AliasFor("path") String[] value() default {};
    @AliasFor("value") String[] path() default {};
    RequestMethod[] method() default {};
    String[] params() default {};
    String[] headers() default {};
    String[] consumes() default {};
    String[] produces() default {};
}

Key attributes: value: the URL pattern (e.g., /user/details/info); method: HTTP method such as GET, POST; consumes: required request Content‑Type (e.g., application/json); produces: response Content‑Type when the request’s Accept header matches; params and headers: additional constraints.

Typical usage in a controller:

@Controller
@RequestMapping(value = "/addUser", method = RequestMethod.POST,
    consumes = "application/json", produces = "application/json")
@ResponseBody
public String addUser(@RequestBody User user) {
    // TODO: implement logic
    Map<String, String> result = new HashMap<>();
    result.put("errCode", "200");
    result.put("msg", "success");
    return result;
}

This method only processes requests whose Content-Type is application/json and returns a JSON response when the client’s Accept header includes application/json. The consumes and produces attributes thus act as filters based on Content‑Type, while headers can also be used for similar checks.

Developers can inspect actual request headers using browser developer tools to see which Content‑Type values are sent by various endpoints.