RestEasy 3.0.x implementation with your existing servlet based web application

Web.xml entry:

<context-param>
		<param-name>resteasy.servlet.mapping.prefix</param-name>
		<param-value>/rest</param-value>
	</context-param>
 <context-param>
		<param-name>resteasy.resources</param-name>
		<param-value>com.abc.xyz.UserREST</param-value>
	</context-param>
    <servlet>
        <servlet-name>resteasy-servlet</servlet-name>
        <servlet-class>
            org.jboss.resteasy.plugins.server.servlet.HttpServletDispatcher
        </servlet-class>
    </servlet>  
    <servlet-mapping>
		<servlet-name>resteasy-servlet</servlet-name>
		<url-pattern>/rest/*</url-pattern>
	</servlet-mapping>

Your REST class:

package com.abc.xyz;

import javax.ws.rs.*;

@Path("/Users")
public class UserREST {

	@GET
	@Path("/getUserById")
	public String doGet(@QueryParam("userId") String userId, @HeaderParam("authorization") String sessionToValidate)
			throws Exception {
			return "SomeString";
	}
}

Minimum jar files needed from jboss resteasy 3.0.19 bundle:

resteasy-jaxrs-3.0.19.Final.jar
jboss-jaxrs-api_2.0_spec-1.0.0.Final.jar
jboss-logging-3.1.4.GA.jar

And call : http(s)://yourhost:port/yourapplication/rest/Users/getUserById?userId=xx
You may choose to pass your own authorization in header, use Postman, but you can ignore it also.