Re-packaging a WAR file into a JAR file

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 Java 8 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 com.sun.net.httpserver.HttpServer;
import java.io.IOException;
import java.io.OutputStream;
import java.net.InetSocketAddress;

public class Main {

  public static void main(String[] args) throws IOException {
    // Create an instance of HttpServer bound to port defined by the 
    // PORT environment variable when present, otherwise on 8080.
    int port = Integer.parseInt(System.getenv().getOrDefault("PORT", "8080"));
    HttpServer server = HttpServer.create(new InetSocketAddress(port), 0);

    // Set root URI path.
    server.createContext("/", (var t) -> {
      byte[] response = "Hello World from Google App Engine Java 11.".getBytes();
      t.sendResponseHeaders(200, response.length);
      try (OutputStream os = t.getResponseBody()) {
        os.write(response);
      }
    });

    // Start the server.
    server.start();
  }
}

WAR migration example (Java 11)

The following instructions demonstrate how to repackage an App Engine Java 8 hello-world application as a JAR to run on the Java 11 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:

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 as a zip file and extract it.

Change to the directory that contains the sample code:

cd java-docs-samples/appengine-java11/appengine-simple-jetty-main/

Install the dependency locally:
```
mvn install
```

Add the following code to your project pom.xml file:

appengine-simple-jetty-main dependency:

<dependency>
  <groupId>com.example.appengine</groupId>
  <artifactId>simple-jetty-main</artifactId>
  <version>1</version>
  <scope>provided</scope>
</dependency>

maven-dependency plugin:

<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-dependency-plugin</artifactId>
  <version>3.6.1</version>
  <executions>
    <execution>
      <id>copy</id>
      <phase>prepare-package</phase>
      <goals>
        <goal>copy-dependencies</goal>
      </goals>
      <configuration>
        <outputDirectory>
          ${project.build.directory}/appengine-staging
        </outputDirectory>
      </configuration>
    </execution>
  </executions>
</plugin>

App Engine deploys files located in the ${build.directory}/appengine-staging directory. By adding the maven-dependency plugin to your build, App Engine installs your specified dependencies to the correct folder.

Create an entrypoint element in your app.yaml file to call the appengine-simple-jetty-main object and pass your WAR file as an argument. For example, see the helloworld-servlet sample app.yaml file:
```
runtime: java11
entrypoint: 'java -cp "*" com.example.appengine.jetty.Main helloworld.war'
```
To run your application locally:
1. Package your application:
```
mvn clean package
```
2. Start the server with your WAR file as an argument.
  
  For example, you can start the server in the helloworld-servlet sample by running the following command from your java-docs-samples/appengine-java11/appengine-simple-jetty-main/ folder:
```
mvn exec:java -Dexec.args="../helloworld-java8/target/helloworld.war"
```
3. In your web browser, enter the following address:
  
  http://localhost:8080
To deploy your application:
gcloud tooling
```
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 , you don't need to include the -Dapp.deploy.projectId property in the command you run.

Re-packaging a WAR file into a JAR file

WAR migration example (Java 11)

gcloud tooling

Maven plugin