Using Captcha in portlet:

In view.jsp:

<%@ taglib uri="http://java.sun.com/portlet_2_0" prefix="portlet" %>

<%@ taglib prefix="liferay-ui" uri="http://liferay.com/tld/ui" %>

<portlet:defineObjects />

<portlet:resourceURL var="captchaURL"/>

<form action="<portlet:actionURL/>" method="post">

      <liferay-ui:captcha url="<%= captchaURL %>"/>

      <input type="submit" value="verify"></input>

</form>

In controller class:

public void serveResource(ResourceRequest resourceRequest, ResourceResponse resourceResponse) throws IOException, PortletException {

    try {

        com.liferay.portal.kernel.captcha.CaptchaUtil.serveImage(resourceRequest, resourceResponse);

    } catch (Exception e) {    e.printStackTrace(); }

    }

public synchronized void processAction(ActionRequest request, ActionResponse response) throws PortletException, IOException {

        String eText = ParamUtil.getString(request, "captchaText");

        String cText = getCaptchaText( request );

        if ( eText.equals( cText ) ) {

               /* Captcha OK */

               } else {

               /* Captcha KO */

        }

    }

private String getCaptchaText( ActionRequest request ) {

    PortletSession session = request.getPortletSession();

    Enumeration<String> atNames = session.getAttributeNames();

    while (atNames.hasMoreElements()) {

        String name = atNames.nextElement();

        if (name.contains("CAPTCHA_TEXT")) {

                       String cText=(String) session.getAttribute(name);

                       return cText;

               }

        }

    return "";

    }