How do I update an App Engine project to Java 11 without web.xml?

Question

I have an App Engine project. Here is a sample repo, but it only contains a few files:

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>

  <groupId>io.happycoding</groupId>
  <artifactId>google-cloud-hello-world</artifactId>
  <version>1</version>
  <packaging>war</packaging>

  <properties>
    <!-- App Engine currently supports Java 8 -->
    <maven.compiler.source>1.8</maven.compiler.source>
    <maven.compiler.target>1.8</maven.compiler.target>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <failOnMissingWebXml>false</failOnMissingWebXml>
  </properties>

  <dependencies>
    <dependency>
      <groupId>javax.servlet</groupId>
      <artifactId>javax.servlet-api</artifactId>
      <version>4.0.1</version>
      <scope>provided</scope>
    </dependency>
  </dependencies>

  <build>
    <plugins>
      <plugin>
        <groupId>com.google.appengine</groupId>
        <artifactId>appengine-maven-plugin</artifactId>
        <version>1.9.71</version>
      </plugin>
    </plugins>
  </build>
</project>

appengine-web.xml

<?xml version="1.0" encoding="utf-8"?>
<appengine-web-app xmlns="http://appengine.google.com/ns/1.0">
  <application>MY_PROJECT_ID_HERE</application>
  <version>1</version>
  <threadsafe>false</threadsafe>
  <sessions-enabled>true</sessions-enabled>
  <runtime>java8</runtime>
</appengine-web-app>

HelloWorldServlet.java

package io.happycoding.servlets;

import java.io.IOException;

import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet("/hello")
public class HelloWorldServlet extends HttpServlet {

  @Override
  public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
    response.setContentType("text/html;");
    response.getOutputStream().println("<h1>Hello world!</h1>");
  }
}

I do not have a web.xml file because I'm using the @WebServlet annotation instead. This has worked perfectly for years.

The only problem was that I was restricted to using Java 8, so I was happy to see App Engine announce support for Java 11. I am now trying to upgrade my App Engine project to Java 11.

I started by changing the appengine-web.xml file to contain this line:

<runtime>java11</runtime>

I also changed the pom.xml file:

<maven.compiler.source>11</maven.compiler.source>
<maven.compiler.target>11</maven.compiler.target>

I run mvn appengine:devserver (which works fine before this change), and I get this error:

ClassLoader is jdk.internal.loader.ClassLoaders$AppClassLoader@78308db1, not a URLClassLoader.

I gather that this is because the App Engine Maven plugin itself requires Java 8. I also learn that the App Engine Maven plugin is deprecated , and that I should upgrade to the Cloud SDK Maven plugin. Okay fine.

I follow this guide and I change the plugin in my pom.xml file to this:

<plugin>
   <groupId>com.google.cloud.tools</groupId>
   <artifactId>appengine-maven-plugin</artifactId>
   <version>2.2.0</version>
</plugin>

I then run mvn package appengine:run (because of course the command to run a devserver changed too), but now I get this error:

 java.io.FileNotFoundException: /home/kevin/gcloud-tutorials/hello-world/target/hello-world-1/WEB-INF/web.xml (No such file or directory)

The error says it can't find a web.xml file, but I shouldn't need one because I'm using the @WebServlet annotation! My pom.xml file also contains a <failOnMissingWebXml>false</failOnMissingWebXml> property, but I don't know whether that does anything with the new plugin.

Am I missing a step or property? How do I upgrade my App Engine project to use Java 11, without also requiring a web.xml file?

Answer 1

There are some pretty huge differences between App Engine's Java 8 runtime and its Java 11 runtime.

Specifically, the Java 8 runtime included a Jetty web server that automatically ran your code, but the Java 11 runtime no longer includes this, so you have to include it yourself.

There is a migration guide here but I found that very confusing to follow, so I'll try to outline the steps here:

Step 1: Migrate from appengine-web.xml to app.yaml .

Delete your appengine-web.xml file, and create a new app.yaml file. My app.yaml file only contained a single line:

runtime: java11

Step 2: Add a main entry point class that runs a web server.

There are many ways to do this, but there's what I did:

package io.happycoding;

import java.net.URL;
import org.eclipse.jetty.annotations.AnnotationConfiguration;
import org.eclipse.jetty.server.Handler;
import org.eclipse.jetty.server.handler.DefaultHandler;
import org.eclipse.jetty.server.Server;
import org.eclipse.jetty.servlet.DefaultServlet;
import org.eclipse.jetty.webapp.Configuration;
import org.eclipse.jetty.webapp.WebAppContext;
import org.eclipse.jetty.webapp.WebInfConfiguration;

/**
 * Starts up the server, including a DefaultServlet that handles static files,
 * and any servlet classes annotated with the @WebServlet annotation.
 */
public class ServerMain {

  public static void main(String[] args) throws Exception {

    // Create a server that listens on port 8080.
    Server server = new Server(8080);
    WebAppContext webAppContext = new WebAppContext();
    server.setHandler(webAppContext);

    // Load static content from inside the jar file.
    URL webAppDir =
        ServerMain.class.getClassLoader().getResource("META-INF/resources");
    webAppContext.setResourceBase(webAppDir.toURI().toString());

    // Enable annotations so the server sees classes annotated with @WebServlet.
    webAppContext.setConfigurations(new Configuration[]{ 
      new AnnotationConfiguration(),
      new WebInfConfiguration(), 
    });

    // Look for annotations in the classes directory (dev server) and in the
    // jar file (live server)
    webAppContext.setAttribute(
        "org.eclipse.jetty.server.webapp.ContainerIncludeJarPattern", 
        ".*/target/classes/|.*\\.jar");

    // Handle static resources, e.g. html files.
    webAppContext.addServlet(DefaultServlet.class, "/");

    // Start the server! 🚀
    server.start();
    System.out.println("Server started!");

    // Keep the main thread alive while the server is running.
    server.join();
  }
}

This class uses Jetty to run a web server, and then adds the rest of your code to that web server. This code assumes that everything will be packaged in the same .jar file.

Step 3: Modify pom.xml

Your pom.xml needs a few things:

1. Dependencies for the web server you're running. I used Jetty.
2. Plugins for packaging your code. I chose to package mine as a single uber jar, so I used maven-resources-plugin and maven-shade-plugin .
3. Plugins for running your code locally. The old appengine-maven-plugin does not work for deploying locally, because it still requires appengine-web.xml for some reason. Because I chose the uber jar approach, I used exec-maven-plugin .
4. The appengine-maven-plugin does still work for deploying to the live server, so you still need it for that.

If that sounds confusing, you're right. But putting it all together, here's what I came up with:

<project xmlns="http://maven.apache.org/POM/4.0.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>

  <groupId>io.happycoding</groupId>
  <artifactId>app-engine-hello-world</artifactId>
  <version>1</version>

  <properties>
    <!-- App Engine currently supports Java 11 -->
    <maven.compiler.source>11</maven.compiler.source>
    <maven.compiler.target>11</maven.compiler.target>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <jetty.version>9.4.31.v20200723</jetty.version>

    <!-- Project-specific properties -->
    <mainClass>io.happycoding.ServerMain</mainClass>
    <googleCloudProjectId>YOUR_PROJECT_ID_HERE</googleCloudProjectId>
  </properties>

  <dependencies>
    <!-- Java Servlets API -->
    <dependency>
      <groupId>javax.servlet</groupId>
      <artifactId>javax.servlet-api</artifactId>
      <version>4.0.1</version>
    </dependency>

    <!-- Jetty -->
    <dependency>
      <groupId>org.eclipse.jetty</groupId>
      <artifactId>jetty-server</artifactId>
      <version>${jetty.version}</version>
    </dependency>
    <dependency>
      <groupId>org.eclipse.jetty</groupId>
      <artifactId>jetty-annotations</artifactId>
      <version>${jetty.version}</version>
    </dependency>
  </dependencies>

  <build>
    <plugins>
      <!-- Copy static resources like html files into the output jar file. -->
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-resources-plugin</artifactId>
        <version>2.7</version>
        <executions>
          <execution>
            <id>copy-web-resources</id>
            <phase>compile</phase>
            <goals><goal>copy-resources</goal></goals>
            <configuration>
              <outputDirectory>
                ${project.build.directory}/classes/META-INF/resources
              </outputDirectory>
              <resources>
                <resource><directory>./src/main/webapp</directory></resource>
              </resources>
            </configuration>
          </execution>
        </executions>
      </plugin>

      <!-- Package everything into a single executable jar file. -->
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-shade-plugin</artifactId>
        <version>3.2.4</version>
        <executions>
          <execution>
            <phase>package</phase>
            <goals><goal>shade</goal></goals>
            <configuration>
              <createDependencyReducedPom>false</createDependencyReducedPom>
              <transformers>
                <transformer implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
                  <mainClass>${mainClass}</mainClass>
                </transformer>
              </transformers>
            </configuration>
          </execution>
        </executions>
      </plugin>

      <!-- Exec plugin for deploying the local server. -->
      <plugin>
        <groupId>org.codehaus.mojo</groupId>
        <artifactId>exec-maven-plugin</artifactId>
        <version>3.0.0</version>
        <executions>
          <execution>
            <goals>
              <goal>java</goal>
            </goals>
          </execution>
        </executions>
        <configuration>
          <mainClass>${mainClass}</mainClass>
        </configuration>
      </plugin>

      <!-- App Engine plugin for deploying to the live site. -->
      <plugin>
        <groupId>com.google.cloud.tools</groupId>
        <artifactId>appengine-maven-plugin</artifactId>
        <version>2.2.0</version>
        <configuration>
          <projectId>${googleCloudProjectId}</projectId>
          <version>1</version>
        </configuration>
      </plugin>
    </plugins>
  </build>
</project>

I put all of this into an example project available here .