The restClient object throws error in Freemarker templates on JBOSS EAP

Issue

• The "restClient" object throws an error in a Freemarker template within a Web Content Template, Application Display Template, a Fragment or any other kind.
• The error message displayed on the HTML page is something similar to:

  Java method "com.liferay.portal.vulcan.internal.template.RESTClientTemplateContextContributor$RESTClient.get(String)" threw an exception when invoked on com.liferay.portal.vulcan.internal.template.RESTClientTemplateContextContributor$RESTClient object "…"; see cause exception in the Java stack trace.

• No errors are logged in the server logs or browser console.

Environment

• Liferay DXP 7.4 and Quarterly Releases
• JBoss EAP

Resolution

1. Add the allow-non-standard-wrappers="true" attribute to the <servlet-container> tag in your standalone.xml file:

   <servlet-container name="default" allow-non-standard-wrappers="true">
       ...
   </servlet-container>

   • This is needed because Liferay's RESTClient uses a dynamic proxy approach (ProxyUtil.newDelegateProxyInstance()) that doesn't comply with JBOSS's strict servlet specification enforcement, and will be eventually documented in the future: Installing on JBoss EAP.
2. You also need to verify if your Liferay patch level includes the fix for LPD-42480 (Fixed in 2024.Q3.11+ and 2025.Q1.0+), and request a hotfix if needed.

Additional Information

• Creating a Widget Template - Calling Headless APIs in the Template Editor 
• LPD-42480 - RESTClientTemplateContextContributor.java: Incorrect DispatcherType and Path on JBoss 
• Installing on JBoss EAP 
• (External link) WildFly 8 Undertow subsystem configuration - Servlet Container Configuration