Using Apache Maven and the App Engine Plugin

This page explains how to manage an App Engine project using Apache Maven, a software project management and comprehension tool. It is capable of building WAR files for deployment into App Engine. Google provides a plugin and Maven Archetypes which are included with Maven 3.1 and greater.

When you use Maven, you don't need to download the Java libraries from the Google App Engine SDK. Maven does that for you. You can also use Maven to test your app locally and upload (deploy) it to production App Engine.

Setting up Maven

Configuring Java

1. If you don't have Java, download, install and configure it.

2. You can use Java 7 or Java 8, but you must use Java 7 bytecode level. If you are using Java 8, configure your app to target Java 7 in your project's pom.xml:

   <properties>  <!-- App Engine Standard currently requires Java 7 -->
     <maven.compiler.target>1.7</maven.compiler.target>
     <maven.compiler.source>1.7</maven.compiler.source>
   </properties>

Installing Maven 3.3.9+

To determine whether Maven is installed and which version you have, invoke the following command:

 mvn -v

If you don't have the proper version of Maven installed:

1. Download Maven 3.3.9 or greater from the Maven website.
2. Install Maven 3.3.9 or greater on your local machine.

Setting up and validating your Cloud Platform project

You need to set up your Cloud Platform project and install the App Engine SDK:

1. Use the Google Cloud Platform Console to create and set up your Cloud Platform project:

   Go to App Engine

   1. Select or create a new Cloud Platform project.
   2. If you need to create an App Engine application for your project, you are prompted to select the region where you want your App Engine application located.
   3. The Dashboard opens after your App Engine application has been created in your project.

2. Install the App Engine SDK for Java and add the directory to your PATH.

Using Maven

Adding the App Engine plugin to an existing project (optional)

To add the Google App Engine Maven plugin to an existing Maven project, add the following into the plugins section in the project pom.xml file:

<plugin>
   <groupId>com.google.appengine</groupId>
   <artifactId>appengine-maven-plugin</artifactId>
   <version>1.9.48</version>
</plugin>

Choosing an archetype

Maven Archetypes allow users to create Maven projects using templates that cover common scenarios. App Engine takes advantage of this Maven feature to provide some useful App Engine archetypes at Maven Central. Select an App Engine archetype appropriate for your app:

Application Type	Artifact	Description
App Engine app*	guestbook-archetype	Generates the guestbook demo sample, complete and ready to run and test.
App Engine app*	appengine-skeleton-archetype	Generates a new, empty App Engine project ready for your own classes and resources, but with required files and directories.
Cloud Endpoints API backend	hello-endpoints-archetype	Generates a simple starter Cloud Endpoints backend API project, ready to build and run.
Cloud Endpoints API backend	endpoints-skeleton-archetype	Generates a new, empty Cloud Endpoints backend API project ready for your own classes and resources, with required files and directories.

* App Engine app in this context means a regular App Engine app, not an app serving as a Cloud Endpoints backend API.

Creating a new project

During project creation, Maven prompts you to supply groupId, artifactId, version, and the package for the project.

Term	Meaning
groupId	A namespace within Maven to keep track of your artifacts. When people consume your project in their own Maven Project, it will serve as an attribute of the dependency they will end up specifying.
artifactId	The name of your project within Maven. It is also specified by consumers of your project when they depend on you in their own Maven projects.
version	The initial Maven version you want to have your project generated with. It's a good idea to have version suffixed by -SNAPSHOT because this will provide support in the Maven release plugin for versions that are under development. For more information, see the Maven guide to using the release plugin.
package	The Java package created during the generation.

The following table shows how to use App Engine Maven archetypes to create an App Engine app or a Cloud Endpoints backend API. Click on the desired app type to see the instructions.

Managing a Maven project

Compiling and building your project

To build an app created with the Maven App Engine archetypes:

1. Change directory to the main directory for your project, for example, guestbook/ http://maven.apache.org/archetype/maven-archetype-plugin

2. Invoke Maven:

   mvn clean install
   

3. Wait for the project to build. When the project successfully finishes you will see a message similar to this one:

   BUILD SUCCESS
    Total time: 10.724s
    Finished at: Thur Jul 04 14:50:06 PST 2014
    Final Memory: 24M/213M
   

4. Optionally, test the application using the following procedure.

Testing your app with the development server

During the development phase, you can run and test your app at any time in the development server by invoking the App Engine Maven plugin. The procedure varies slightly depending on the artifact used to create the project, so click on the appropriate tab below:

Specifying a port for local testing

When you run your app in the local development server, the default port is 8080. You can change this default by modifying the plugin entry for appengine-maven-plugin (or adding it if it doesn't exist). For example, we specify port and address in the following <plugin> entry within <plugins> inside the main app directory pom.xml file (myapp/pom.xml):

    <plugin>
         <groupId>com.google.appengine</groupId>
         <artifactId>appengine-maven-plugin</artifactId>
         <version>1.9.48</version>
         <configuration>
             <enableJarClasses>false</enableJarClasses>
             <port>8181</port>
             <address>0.0.0.0</address>
         </configuration>
    </plugin>

Notice that the <port> sets the port here to 8181 as shown, and the address 0.0.0.0 is specified, which means the development server will listen to requests coming in from the local network.

Deploying your app

See the Deploying your app guide.

Reference: Available goals

Once the App Engine Maven plugin is added to the project's pom.xml file, several App Engine-specific Maven goals are available. To see all of the available goals, invoke the command:

    mvn help:describe -Dplugin=appengine

The App Engine Maven plugin goals can be categorized as devserver goals, app and project management goals, and Endpoints goals.

Development server goals

These are the development server goals:

appengine:devserver

Runs the App Engine development server. When the server is running, it continuously checks to determine whether appengine-web.xml has changed. If it has, the server does a hot reload of the application. This means that you do not need to stop and restart your application because of changes to appengine-web.xml. The following parameters are available:

• <fullScanSeconds>
• <address>
• <disableUpdateCheck>
• <jvmFlags>
• <port>
• <server>

For example, to enable running the server in debug mode on port 8000 without suspending at start time, you can use the following flags:

<jvmFlags>
  <jvmFlag>-Xdebug</jvmFlag>
  <jvmFlag>-agentlib:jdwp=transport=dt_socket,address=8000,server=y,suspend=n</jvmFlag>
</jvmFlags>

By default, the <fullScanSeconds> flag is set to 5 seconds, which means server is checking every 5 seconds for changes in the web application files, and reloads the application automatically. This is useful with IDEs that support the compile on save feature like NetBeans. In order to use this feature, you must configure the <build> section:

<build>
   <outputDirectory>target/${project.artifactId}-${project.version}/WEB-INF/classes</outputDirectory>
   <plugins>
      ....
   </plugins>
</build>

appengine:devserver_start

Performs an asynchronous start for the devserver and then returns to the command line. When this goal runs, the behavior is the same as the devserver goal except that Maven continues processing goals and exits after the server is up and running.

appengine:devserver_stop

Stops the development server. Available only if you started the development server with appengine:devserver_start.

Application management goals

For application and project management, the goals are listed in the following table:

Goal	Description	Corresponding gcloud Command
appengine:backends_stop	This stops any running development server listening on the port as configured in your pom.xml file. This goal can be used in conjunction with the devserver_start command to do integration tests with the Maven plugin.
appengine:backends_configure	Configure the specified backend.
appengine:backends_delete	Delete the specified backend.
appengine:backends_rollback	Roll back a previously in-progress update.
appengine:backends_start	Start the specified backend.
appengine:backends_update	Update the specified backend or (if no backend is specified) all backends.
appengine:enhance	Runs the App Engine Datanucleus JDO enhancer.
appengine:rollback	Rollback an in-progress update.	gcloud app versions start, gcloud app versions stop
appengine:set_default_version	Set the default application version.	gcloud app services set-traffic
appengine:update	Create or update an app version.	gcloud app deploy
appengine:update_cron	Update application cron jobs.	gcloud app deploy
appengine:update_dispatch	Update the application dispatch configuration.	gcloud app deploy
appengine:update_dos	Update application DoS protection configuration.	gcloud app deploy
appengine:update_indexes	Update application indexes.	gcloud preview datastore create-indexes [INDEX_YAML]
appengine:update_queues	Update application task queue definitions.	gcloud app deploy
appengine:vacuum_indexes	Delete unused indexes from application.	gcloud preview datastore cleanup-indexes [INDEX_YAML]
appengine:start_module_version	Start the specified module version.	gcloud app versions start
appengine:stop_module_version	Stop the specified module version.	gcloud app versions stop

Troubleshooting upload errors

If you use the update goal, your update attempt may fail with a message similar to this one: 404 Not Found This application does not exist (app_id=u'your-app-ID'). This error will occur if you have multiple Google accounts and are using the wrong account to perform the update.

To solve this issue, change directories to ~, locate a file named .appcfg_oauth2_tokens_java, and rename it. Then try updating again. http://maven.apache.org/archetype/maven-archetype-plugin

Cloud Endpoints goals

These are the Endpoints goals:

appengine:endpoints_get_client_lib

Generate a zip file in the directory ${project.build.directory}/generated-sources/appengine-endpoints/WEB-INF for the Java client library for your endpoints.

appengine:endpoints_get_discovery_doc

A combination of both gen-api-config and gen-discovery-doc commands. The Endpoints API and discovery documents for both REST and RPC are generated in the ${project.build.directory}/generated-sources/appengine-endpoints/WEB-INF directory. The source web.xml is automatically changed with the addition of the Endpoints servlet and the correct Endpoints classes. In order to use the generated artifacts, you need to configure the Maven WAR plugin:

<plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-war-plugin</artifactId>
        <version>2.3</version>
        <configuration>
          <webXml>${project.build.directory}/generated-sources/appengine-endpoints/WEB-INF/web.xml</webXml>
          <webResources>
            <resource>
              <directory>${project.build.directory}/generated-sources/appengine-endpoints</directory>
              <includes>
                <include>WEB-INF/*.discovery</include>
                <include>WEB-INF/*.api</include>
              </includes>
            </resource>
          </webResources>
        </configuration>
      </plugin>

You can automatically call this goal as part of a Maven build by configuring the App Engine Maven plugin:

    <plugin>
            <groupId>com.google.appengine</groupId>
            <artifactId>appengine-maven-plugin</artifactId>
            <version>1.9.48</version>
            <configuration>
              <enableJarClasses>false</enableJarClasses>
            </configuration>
            <executions>
              <execution>
                <goals>
                  <goal>endpoints_get_discovery_doc</goal>
                </goals>
              </execution>
            </executions>
          </plugin>

For a reference configuration, see the Endpoint sample application appengine-tictactoe-java-maven.