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Using Apache Maven

Apache Maven is a software project management and comprehension tool. It is capable of building WAR files for deployment into App Engine. The App Engine team provides both a plugin and Maven Archetypes for the purpose of speeding up development.

Requirements

You need the following:

  1. You must use Java 7. If you don't have Java 7, Download and install it.

  2. Set your JAVA_HOME environment variable. If you are a bash user

    1. For a typical Linux installation, add a line similar to the following to your .bashrc file: 

      export JAVA_HOME=/usr/local/tools/java/jdk1.7.0_45.jdk

    2. If you use Mac OSX and the default Terminal app, your shell session doesn't load .bashrc by default. So you may need to add a line similar to the following to your .bash_profile : 

      [ -r ~/.bashrc ] && source ~/.bashrc

    3. If you use Mac OSX but don't use the default terminal app, for example, you use a terminal management app such as tmux, you may need to add a line similar to the following line to your .bashrc file: 

      export JAVA_HOME=/Library/Java/JavaVirtualMachines/jdk1.7.0_45.jdk/Contents/Home

  3. If you don't have Maven installed, download and install Maven.

A note about Maven projects and Eclipse

A Maven project has a different layout than an Eclipse project. So, if you wish to use a Maven project with Eclipse, you'll need to use Eclipse for Java Enterprise Edition (EE), which has Maven support, and you'll need to use one of the following approaches:

Maven terms you need to know

During project creation, Maven prompts you to supply groupId , artifactId , version , and the package for the project. What are these in Maven?

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.

Installing Maven

If you do not already have Maven version 3.1 or greater installed on your system, visit the Apache Maven web site to download and install it.

When Maven is installed and the mvn command is on your command path, you can run the following command to verify that it works, and see which version is installed: 

mvn -v

Make sure the version is 3.1 or greater.

Maven App Engine archetypes

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. The current App Engine artifacts are listed in the table below:

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 App Engine applications or backend APIs using the archetypes

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.

App Engine App

To create an App Engine App:

  1. Change directory to a directory where you want to build the project.

  2. Invoke the following Maven command: 

        mvn archetype:generate

  3. When prompted to Choose a number or apply filter , supply the value com.google.appengine.archetypes: . The various App Engine archetypes will be located and displayed in a short list preceded with a number, for example: 1: remote -> com.google.appengine.archetypes:appengine-skeleton-archetype (-)

  4. If you want to create the complete, ready-to-run demo Guestbook app, supply the number corresponding to guestbook-archetype .

    If you want to create an empty project that contains the required directory structure and files, ready for your own classes, supply the number corresponding to appengine-skeleton-archetype .

  5. If prompted, select the most recent version from the displayed list of available archetype versions.

  6. When prompted to Define value for property 'groupId' , supply the namespace for your app; for example, com.mycompany.myapp . (For the Guestbook demo app, supply the value com.google.guestbook .)

  7. When prompted to Define value for property 'artifactId' , supply the project name; for example, myapp . (For the Guestbook demo app, supply the value guestbook .)

  8. When prompted to Define value for property 'version' , accept the default value.

  9. When prompted to Define value for property 'package' , accept the default value.

  10. When prompted to confirm your choices, accept the default value ( Y ).

  11. Wait for the project to finish generating. then change directories to the new project directory, for example guestbook/ .

  12. Build the project by invoking 

    mvn clean install

  13. 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 2013
 Final Memory: 24M/213M

  14. If you created the sample Guestbook demo app:

    1. Test the application locally in the development server as follows: 

      mvn appengine:devserver

      Wait for the dev server to start up. When it finishes starting up, you will see a message similar to this: 

       Jul 04, 2013 2:56:42 PM com.google.appengine.tools.development.DevAppServerImpl start
 INFO: The server is running at http://localhost:8080/
 Jul 04, 2013 2:56:42 PM com.google.appengine.tools.development.DevAppServerImpl start
 INFO: The admin console is running at http://localhost:8080/_ah/admin

    2. Visit the application at the default URL and port http://localhost:8080/ used by the development server. You will see the guestbook demo app.

    3. To shut down the app and the development server, press Control+C in the Windows/Linux terminal window you started it in, or CMD+C on the Mac.

  15. If you created a new, empty app, you need to do some quick configuration.

    1. Before starting to code your own classes for the app, familiarize yourself with the basic project layout and the required project files is complete: inside the directory where you created the project, you'll have a subdirectory named myapp , which contains a pom.xml file and two subdirectories: myapp-ear and myapp-war , similar to the layout shown here:

      Maven Project Layout

      We'll describe what to do inside these two subdirectories later. (Notice that the location where you'll add your own Java source code is inside myapp-war .)

    2. Change directory to the main app directory, for example, myapp , which contains a pom.xml file and the myapp-ear and myapp-war subdirectories.

    3. Edit pom.xml in myapp so that points to the most recent App Engine SDK version, which currently is 1.9.9: 

      <properties>
  <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
  <appengine.target.version>1.9.9</appengine.target.version>
</properties>

    4. Now add your classes as desired.

Cloud Endpoints Backend API

To create a Cloud Endpoints backend API project:

  1. Change directory to a directory where you want to build the project.

  2. Invoke the following Maven command: 

         mvn archetype:generate -Dappengine-version=1.9.9 -Dfilter=com.google.appengine.archetypes:

    where -Dappengine-version is set to the most recent App Engine Java SDK version.

  3. If you want to create the complete, ready-to-run Hello Endpoints backend API, supply the number corresponding to hello-endpoints-archetype .

    If you want to create an empty project that contains the required directory structure and files, ready for your own classes, supply the number corresponding to endpoints-skeleton-archetype .

  4. Select the most recent version from the displayed list of available archetype versions by accepting the default.

  5. When prompted to Define value for property 'groupId' , supply the namespace for your app; for example, com.mycompany.myapp . (For the Hello Endpoints sample backend API, supply the value com.google.appengine.samples.helloendpoints .)

  6. When prompted to Define value for property 'artifactId' , supply the project name; for example, myapp . (For the Hello Endpoints sample, supply the value helloendpoints .)

  7. When prompted to Define value for property 'version' , accept the default value.

  8. When prompted to Define value for property 'package' , accept the default value.

  9. When prompted to confirm your choices, accept the default value ( Y ).

  10. Wait for the project to finish generating. then change directories to the new project directory, for example helloendpoints/ .

  11. Build the project by invoking 

    mvn clean install

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

    [INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 14.846s
[INFO] Finished at: Tue Jun 03 09:43:09 PDT 2014
[INFO] Final Memory: 24M/331M

  13. If you created the sample Hello Endpoints sample backend API:

    1. Test the application locally in the development server as follows: 

      mvn appengine:devserver

      Wait for the dev server to start up. When it finishes starting up, you will see a message similar to this one: 

       [INFO] INFO: Module instance default is running at http://localhost:8080/
 [INFO] Jun 03, 2014 9:44:47 AM com.google.appengine.tools.development.AbstractModule startup
 [INFO] INFO: The admin console is running at http://localhost:8080/_ah/admin
 [INFO] Jun 03, 2014 9:44:47 AM com.google.appengine.tools.development.DevAppServerImpl doStart
 [INFO] INFO: Dev App Server is now running

    2. Visit the URL http://localhost:8080 to send requests to the backend API

  14. If you created a new, empty app:

    1. Familiarize yourself with the basic project layout shown here:

      Maven Project Layout

      YourFirstAPI.java is a starter file for your own API. (You aren't required to use this name.)

    2. Add your classes as desired.

Compile and build your project using Maven

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

  1. Change directory to the main directory for your project, for example, guestbook/

  2. Invoke Maven as follows: 

    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:

App Engine App

To test your app:

  1. If you haven't already done so, build your app ( mvn clean install ).

  2. Change directory to the top level of your project's EAR hierarchy (for example, to myapp-ear ) and invoke Maven as follows: 

    mvn appengine:devserver

    Wait for the the server to start. When the server is completely started with your app running, you will see a message similar to this one: 

     Jul 04, 2014 2:56:42 PM com.google.appengine.tools.development.DevAppServerImpl start
 INFO: The server is running at http://localhost:8080/
 Jul 04, 2013 2:56:42 PM com.google.appengine.tools.development.DevAppServerImpl start
 INFO: The admin console is running at http://localhost:8080/_ah/admin

  3. Use your browser to visit http://localhost:8080/ to access your app.

  4. Shut down the app and the development server by pressing Control+C in the Windows/Linux terminal window where you started it, or CMD+C on the Mac.

Cloud Endpoints Backend API

To test your app:

  1. If you haven't already done so, build your app ( mvn clean install ).

  2. Change directory to your project's main directory /myapp ) and invoke Maven as follows: 

    mvn appengine:devserver

    Wait for the the server to start. When the server is completely started with your app running, you will see a message similar to this one: 

     Jul 04, 2014 2:56:42 PM com.google.appengine.tools.development.DevAppServerImpl start
 INFO: The server is running at http://localhost:8080/
 Jul 04, 2013 2:56:42 PM com.google.appengine.tools.development.DevAppServerImpl start
 INFO: The admin console is running at http://localhost:8080/_ah/admin

  3. Use your browser to visit http://localhost:8080/ to access your app, or, alternatively, to test the API using the built-in Google API Explorer, visit http://localhost:8080/_ah/api/explorer .

  4. Shut down the app and the development server by pressing Control+C in the Windows/Linux terminal window where you started it, or CMD+C on the Mac.

Uploading your app to production App Engine

The upload procedure varies slightly depending on the artifact used to create the project, so click on the appropriate tab below:

App Engine App

To upload an app created with the the appengine-skeleton-archetype :

  1. Change directory to the top level of your project's EAR hierarchy (for example, myapp/myapp-ear ) and invoke Maven as follows: 

    mvn appengine:update

  2. You will be prompted for an authorization code in the terminal window and your web browser will launch with a consent screen which you must accept in order to be authorized. Follow the prompts to copy any codes from the browser to the command line.

Cloud Endpoints Backend API

To upload an app created with the the endpoints-skeleton-archetype :

  1. Change directory to your project's main directory (for example, /myapp ) and invoke Maven as follows: 

    mvn appengine:update

  2. You will be prompted for an authorization code in the terminal window and your web browser will launch with a consent screen which you must accept in order to be authorized. Follow the prompts to copy any codes from the browser to the command line.

Configuring pom.xml settings

If you use the App Engine Maven skeleton-archetype to create your project, all of the required pom.xml settings are set for you in your project's main pom.xml file ( myapp/pom.xml ), although you should specify the latest App Engine SDK version for the <appengine.target.version> .

If you want to use JUnit dependencies (optional), you can do so by setting them in myapp/myapp-war/pom.xml , as follows: 

    <dependency>
        <groupId>junit</groupId>
        <artifactId>junit</artifactId>
        <version>4.10</version>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>org.mockito</groupId>
        <artifactId>mockito-all</artifactId>
        <version>1.9.0</version>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>com.google.appengine</groupId>
        <artifactId>appengine-testing</artifactId>
        <version>1.9.9</version>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>com.google.appengine</groupId>
        <artifactId>appengine-api-stubs</artifactId>
        <version>1.9.9</version>
        <scope>test</scope>
    </dependency>

Notice that the scope is set to test ):

Adding the App Engine Maven plugin to an existing Maven project

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>1.9.9</version>
</plugin>

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>1.9.9</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.

Managing and running a project with the App Engine Maven plugin

The App Engine Maven plugin enables support for App Engine within Maven. It provides the ability to use the development server and the majority of the functionality of the appcfg tool.

The plugin also provides Google Cloud Endpoints goals for discovery doc generation and client library generation.

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

App Engine Maven plugin goals

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 as follows: 

<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
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.
appengine:set_default_version Set the default application version.
appengine:update Create or update an app version.
appengine:update_cron Update application cron jobs.
appengine:update_dispatch Update the application dispatch configuration.
appengine:update_dos Update application DoS protection configuration.
appengine:update_indexes Update application indexes.
appengine:update_queues Update application task queue definitions.
appengine:vacuum_indexes Delete unused indexes from application.
appengine:start_module_version Start the specified module version.
appengine:stop_module_version Stop the specified module version.
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.

Managing Versions with versions-maven-plugin command options

The versions-maven-plugin provides several useful command options for managing versions in your Maven project. The following table lists useful activities supported by versions-maven-plugin :

Command Description
mvn versions:display-dependency-updates Shows dependency updates (including app engine libs)
mvn versions:display-plugin-updates Shows plugin updates (including appengine-maven-plugin)
mvn versions:use-latest-versions Automatically updates to the latest versions and makes a backup of the original pom.xml .
mvn versions:commit Removes the backup and makes the change permanent.
mvn versions:revert Restores pom.xml with the contents of the backup pom.xml .

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 as follows: 

<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 as follows: 

    <plugin>
            <groupId>com.google.appengine</groupId>
            <artifactId>appengine-maven-plugin</artifactId>
            <version>1.9.9</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 .

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