title: How to validate a form from a config.jelly on Jenkins?
body: <p>I can do it from a global.jelly file, but it doesn't work from a config.jelly.
  Here is the procedure for a global.jelly file:</p>
  <h3>Jelly:</h3>
  <pre><code>&lt;f:entry title=&quot;Value&quot; field=&quot;value&quot;&gt;
      &lt;f:textbox /&gt;
  &lt;/f:entry&gt;
  </code></pre>
  <hr />
  <h3>JAVA:</h3>
  <pre><code>public static final class Descriptor extends BuildStepDescriptor&lt;Builder&gt;{
  
      //descriptor's code
  
      /**
       * Performs on-the-fly validation of the form field 'value'.
       * 
       * @param value
       *            This parameter receives the value that the user has typed.
       * @return Indicates the outcome of the validation. This is sent to the
       *         browser.
       */
      public FormValidation doCheckValue(@QueryParameter String value) throws IOException, ServletException {
          if(value.isEmpty()) {
              return FormValidation.warning(&quot;You must fill this box!&quot;);
          }
          return FormValidation.ok();
      }
  }
  </code></pre>
  <p>This doesn't apply anymore when the jelly code is placed in a configuration file (config.jelly), no matter whether the <code>doCheckValue</code> method is placed in the plugin class or in its descriptor.</p>