<dependencies>
    <dependency>
        <groupId>org.geoserver</groupId>
        <artifactId>community</artifactId>
        <version>2.21-RC</version>
    </dependency>
    <dependency>
        <groupId>org.geoserver</groupId>
        <artifactId>gs-main</artifactId>
        <version>2.21-RC</version>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>com.google.errorprone</groupId>
        <artifactId>javac-shaded</artifactId>
        <version>9+181-r4173-1</version>
    </dependency>
</dependencies>

<!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN//EN" "http://www.springframework.org/dtd/spring-beans.dtd">

<beans>
    <!-- Spring will reference the instance of the HelloWorld class
            by the id name "helloService" -->
    <bean id="helloService" class="HelloWorld"/>

    <!-- This creates a Service descriptor, which allows the org.geoserver.ows.Dispatcher
        to locate it. -->
    <bean id="helloService-1.0.0" class="org.geoserver.platform.Service">

        <!-- used to reference the service in the URL -->
        <constructor-arg index="0" value="hello"/>

        <!-- our actual service POJO defined previously -->
        <constructor-arg index="1" ref="helloService"/>

        <!-- a version number for this service -->
        <constructor-arg index="2" value="1.0.0"/>

        <!-- a list of functions for this service -->
        <constructor-arg index="3">
            <list>
                <value>sayHello</value>
            </list>
        </constructor-arg>

    </bean>
</beans>

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class HelloWorld {
    public HelloWorld() {
        // Do nothing
    }

    public void sayHello(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.getOutputStream().write("Hello world".getBytes());
    }
}