@RestController
public class TestServlet {
    @RequestMapping(value = "/test1")
    public String test1() {
        return "test1";
    }

    @RequestMapping(value = "/test2")
    public String test2(@RequestBody TestClass req) {
        return "test2";
    }

    public static class TestClass {
        private String value;

        public String getValue() {
            return value;
        }

        public void setValue(String value) {
            this.value = value;
        }
    }
}

public String test2(@Valid TestClass req) {
        return "test2";
    }

Content-Type: application/json
Accept: text/html (As your method returns only a String)

{
"value":"the provided value"
}