If you are migrating to the latest supported Java version and your app does not use legacy bundled services, you must re-package your App Engine web application into an executable JAR file. Your application must have a `Main` class that starts a web server that responds to HTTP requests on port 8080, which might be specified by the `PORT` environment variable. For example: import org.eclipse.jetty.server.Server; import org.eclipse.jetty.webapp.Configuration.ClassList; import org.eclipse.jetty.webapp.WebAppContext; /** Simple Jetty Main that can execute a WAR file when passed as an argument. */ public class Main { public static void main(String[] args) throws Exception { if (args.length != 1) { System.err.println("Usage: need a relative path to the war file to execute"); System.exit(1); } System.setProperty("org.eclipse.jetty.util.log.class", "org.eclipse.jetty.util.log.StrErrLog"); System.setProperty("org.eclipse.jetty.LEVEL", "INFO"); // Create a basic Jetty server object that will listen on port defined by // the PORT environment variable when present, otherwise on 8080. int port = Integer.parseInt(System.getenv().getOrDefault("PORT", "8080")); Server server = new Server(port); // The WebAppContext is the interface to provide configuration for a web // application. In this example, the context path is being set to "/" so // it is suitable for serving root context requests. WebAppContext webapp = new WebAppContext(); webapp.setContextPath("/"); webapp.setWar(args[0]); ClassList classlist = ClassList.setServerDefault(server); // Enable Annotation Scanning. classlist.addBefore( "org.eclipse.jetty.webapp.JettyWebXmlConfiguration", "org.eclipse.jetty.annotations.AnnotationConfiguration"); // Set the the WebAppContext as the ContextHandler for the server. server.setHandler(webapp); // Start the server! By using the server.join() the server thread will // join with the current thread. See // "http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/Thread.html#join()" // for more details. server.start(); server.join(); } } ## WAR migration example (Java 21) The following instructions demonstrate how to repackage an App Engine `hello-world` WAR application as a JAR to run on the Java 21 runtime. The migration uses the `appengine-simple-jetty-main` artifact. This provides a `Main` class with a simple Jetty web server that loads a WAR file and packages your app into an executable JAR file: 1. Clone the Embedded Jetty Server artifact to your local machine: git clone https://github.com/GoogleCloudPlatform/java-docs-samples Alternatively, you can [download the sample](https://github.com/GoogleCloudPlatform/java-docs-samples/archive/master.zip) as a zip file and extract it. 2. Change to the directory that contains the sample code: cd java-docs-samples/flexible/repacking-legacy-applications/appengine-simple-jetty-main/ 3. Build the Jetty runner JAR: mvn install This generates `jetty-jar-with-dependencies.jar` in your `target/` directory. 4. Configure the `pom.xml` build file for the Jetty server: - Compiler properties (Java 21): UTF-8 21 21 9.4.57.v20241219 - Embedded Jetty dependencies: org.eclipse.jetty jetty-server ${jetty.version} org.eclipse.jetty jetty-webapp ${jetty.version} jar org.eclipse.jetty jetty-util ${jetty.version} org.eclipse.jetty jetty-annotations ${jetty.version} org.eclipse.jetty apache-jsp ${jetty.version} - `maven-assembly-plugin` configuration: org.apache.maven.plugins maven-assembly-plugin 3.0.0 jetty jar-with-dependencies com.example.appengine.jetty.Main make-assembly package single 5. Place the generated `jetty-jar-with-dependencies.jar` and your application WAR file. For example, `helloworld.war` in the same directory. 6. Create an `app.yaml` file in your application directory to specify the Java 21 runtime and an `entrypoint` element to call the `jetty-jar-with-dependencies.jar` file and pass your WAR file as an argument: runtime: java21 entrypoint: 'java -jar jetty-jar-with-dependencies.jar helloworld.war' 7. To run your application locally: 1. Start the server with your WAR file as an argument: java -jar jetty-jar-with-dependencies.jar helloworld.war 2. In your web browser, enter the following address: 8. To deploy your application: ### gcloud tooling ```bash gcloud app deploy ``` ### Maven plugin ``` mvn package appengine:deploy -Dapp.deploy.projectId=PROJECT_ID ``` Replace PROJECT_ID with the ID of your Google Cloud project. If your `pom.xml` file already [specifies your project ID](https://docs.cloud.google.com/appengine/docs/standard/java-gen2/maven-reference#appenginedeploy) , you don't need to include the `-Dapp.deploy.projectId` property in the command you run.