build.gradle

// Apply the java plugin to add support for Java
apply plugin: 'java'
apply plugin: 'war'
apply from: 'https://raw.github.com/akhikhl/gretty/master/pluginScripts/gretty.plugin'

// In this section you declare where to find the dependencies of your project
repositories {
    // Use 'jcenter' for resolving your dependencies.
    // You can declare any Maven/Ivy/file repository here.
    jcenter()
}

// In this section you declare the dependencies for your production and test code
dependencies {
    // The production code uses the SLF4J logging API at compile time
    compile 'org.slf4j:slf4j-api:1.7.21'
    // https://mvnrepository.com/artifact/javax.servlet/javax.servlet-api
    compile group: 'javax.servlet', name: 'javax.servlet-api', version: '3.1.0'
    compile group: 'com.twilio.sdk', name: 'twilio', version: '7.2.0'

    // Declare the dependency for your favourite test framework you want to use in your tests.
    // TestNG is also supported by the Gradle Test task. Just change the
    // testCompile dependency to testCompile 'org.testng:testng:6.8.1' and add
    // 'test.useTestNG()' to your build script.
    testCompile 'junit:junit:4.12'
}

gretty {
    port = 8080
    contextPath = '/java-guide'
}

HelloWorldServlet.java

package guide;

import com.twilio.twiml.Say;
import com.twilio.twiml.TwiMLException;
import com.twilio.twiml.VoiceResponse;

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

public class HelloWorldServlet extends HttpServlet {
    @Override
    public void doGet(HttpServletRequest req, HttpServletResponse response) throws IOException {
        // Create a TwiML response and add our friendly message.
        VoiceResponse voiceResponse = new VoiceResponse.Builder()
                .say(new Say.Builder("Hello World!").build())
                .build();

        response.setContentType("application/xml");
        try {
            response.getWriter().print(voiceResponse.toXml());
        } catch (TwiMLException e) {
            e.printStackTrace();
        }
    }
}

web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="3.0"
         metadata-complete="true"
         xmlns="http://java.sun.com/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">

    <servlet>
        <servlet-name>helloWorld</servlet-name>
        <servlet-class>guide.HelloWorldServlet</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>helloWorld</servlet-name>
        <url-pattern>/</url-pattern>
    </servlet-mapping>
</web-app>