Re-package a WAR file into a JAR file

This document describes how to repackage a Java 8 application as a JAR file to run on the Java 11/17 runtimes.

Your application must have a Main class that starts a web server which responds to HTTP requests on the port specified by the PORT environment variable, typically 8081.

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

The following instructions demonstrate how to repackage an App Engine Java 8 hello-world application as a JAR to run on the Java 11/17 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 the 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. The WAR version must be same as the version listed in your pom.xml file:

runtime: java
env: flex
runtime_config:
  operating_system: ubuntu18
  runtime_version: 11
entrypoint: 'java -cp "*" com.example.appengine.jetty.Main helloworld-1.war'
handlers:
- url: /.*
  script: this field is required, but ignored

manual_scaling:
  instances: 1

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-war sample by running the following command from your java-docs-samples/appengine-java11/appengine-simple-jetty-main/ folder:
```
mvn exec:java -Dexec.args="../flexible/java-11/helloworld-war/target/helloworld-1.war"
```
3. In your web browser, enter the following address:
  
  http://localhost:8080
To deploy your application using 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.