title: How to inject single json parameter value in @PostMapping
body: <p>I have a simple <code>POST</code> servlet and want to inject only one single property of a <code>JSON</code> request:</p>
  
  <pre><code>@RestController
  public class TestServlet {
      @PostMapping(value = "/test", consumes = APPLICATION_JSON_VALUE)
      public String test(@Valid @NotBlank @RequestBody String test) {
          return test;
      }
  }
  </code></pre>
  
  <p>Request:</p>
  
  <pre><code>{
      "test": "random value",
      "some": "more"
  }
  </code></pre>
  
  <p>Result: the <code>test</code> parameter contains the whole json, not the parameter value only. Why? How can I achieve this <em>without</em> having to introduce an extra Bean?</p>