Monday, October 5, 2015

Add Spring to a Google App Engine application

Spring is a Java based framework that takes a lot of work out from developing web applications that you can "just run". I tried to install Spring on Google's App Engine platform as a service (PAAS) but there are a few "gotchas".

Google App Engine runs the older Java 1.7 SDK and hence you must adjust the development environment accordingly. Java 1.8 cannot be used otherwise you will encounter no class found when trying to deploy your Spring application. The Servlet API should be version 2.5 only. Once you keep to the above versions, you only need to follow the steps below to add Spring to a skeleton Google App Engine template. For more information on creating the skeleton with Maven, see Google's documentation at https://cloud.google.com/appengine/docs/java/tools/maven.

Creating the skeleton

  1. Open up a Command Prompt. Change to a parent folder where you want to build the project, e.g. C:\MyProjects\.

    C:\> cd \MyProjects
  2. Type in the Maven command to create a skeleton.

    C:\> mvn archetype:generate -Dappengine-version=1.9.27 -Dapplication-id=your-app-id -Dfilter=com.google.appengine.archetypes:appengine-skeleton-archetype

  3. When prompted for the groupId, type in a name space, e.g. com.mycompany.myapp.
  4. When prompted for the artifactId, type in e.g. myapp.
  5. When prompted for the version, type in a value or accept the default.
  6. When prompted for the package, type in a value or accept the default.
  7. When prompted to confirm, type in Y.

    The skeleton is created.
Add the application's Java code and logic
  1. Using your favorite text editor or IDE, create the Java source code in the skeleton project's src\main folder , e.g. Application.java and MySvc.java.
    Application.java with Spring configuration annotations

    MySvc.java with Spring controller annotations. This just prints out a message "Greetings from Spring Boot" when invoked. 


Add in Spring configurations in the pom.xml file
  1. Open up the skeleton's pom.xml file in a text editor.
  2. In the properties section of the pom.xml file, add in a property for defining the Spring version for convenience.

    <spring.version>4.2.1.RELEASE</spring.version>
  3. In the dependencies section of the pom.xml file, add in the all the Spring dependencies you want to use in the application, making use of the Spring version property defined earlier.

    An example listing for the edited pom.xml file.
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
 
<modelVersion>4.0.0</modelVersion>
<packaging>war</packaging>
<version>1.0-SNAPSHOT</version>
 
<groupId>com.mycompany.myapp</groupId>
<artifactId>myapp</artifactId>
 
<properties>
<app.id>com.mycompany.myapp</app.id>
<app.version>1</app.version>
<appengine.version>1.9.27</appengine.version>
<gcloud.plugin.version>2.0.9.74.v20150814</gcloud.plugin.version>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<maven.compiler.showDeprecation>true</maven.compiler.showDeprecation>

<!-- Add a property for Spring -->
<spring.version>4.2.1.RELEASE</spring.version>
</properties>
 
<prerequisites>
<maven>3.1.0</maven>
</prerequisites>
 
<dependencies>
<!-- Compile/runtime dependencies -->
<dependency>
<groupId>com.google.appengine</groupId>
<artifactId>appengine-api-1.0-sdk</artifactId>
<version>${appengine.version}</version>
</dependency>
<dependency>
<groupId>javax.servlet</groupId>
<artifactId>servlet-api</artifactId>
<version>2.5</version>
<scope>provided</scope>
</dependency>
<dependency>
<groupId>jstl</groupId>
<artifactId>jstl</artifactId>
<version>1.2</version>
</dependency>
 
<!-- Test Dependencies -->
<dependency>
<groupId>com.google.appengine</groupId>
<artifactId>appengine-testing</artifactId>
<version>${appengine.version}</version>
<scope>test</scope>
</dependency>
<dependency>
<groupId>com.google.appengine</groupId>
<artifactId>appengine-api-stubs</artifactId>
<version>${appengine.version}</version>
<scope>test</scope>
</dependency>

<!-- Spring Dependencies -->
 
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-aop</artifactId>
<version>${spring.version}</version>
</dependency>
 
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-context</artifactId>
<version>${spring.version}</version>
</dependency>
 
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-beans</artifactId>
<version>${spring.version}</version>
</dependency>
 
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-context-support</artifactId>
<version>${spring.version}</version>
</dependency>
 
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-core</artifactId>
<version>${spring.version}</version>
</dependency>
 
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-expression</artifactId>
<version>${spring.version}</version>
</dependency>
 
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-web</artifactId>
<version>${spring.version}</version>
</dependency>
 
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-webmvc</artifactId>
<version>${spring.version}</version>
</dependency>


</dependencies>
 
<build>
<!-- for hot reload of the web application-->
<outputDirectory>${project.build.directory}/${project.build.finalName}/WEB-INF/classes</outputDirectory>
<plugins>
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>versions-maven-plugin</artifactId>
<version>2.1</version>
<executions>
<execution>
<phase>compile</phase>
<goals>
<goal>display-dependency-updates</goal>
<goal>display-plugin-updates</goal>
</goals>
</execution>
</executions>
</plugin>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<version>3.1</version>
<artifactId>maven-compiler-plugin</artifactId>
<configuration>
<source>1.7</source>
<target>1.7</target>
</configuration>
</plugin>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-war-plugin</artifactId>
<version>2.4</version>
<configuration>
<archiveClasses>true</archiveClasses>
<webResources>
<!-- in order to interpolate version from pom into appengine-web.xml -->
<resource>
<directory>${basedir}/src/main/webapp/WEB-INF</directory>
<filtering>true</filtering>
<targetPath>WEB-INF</targetPath>
</resource>
</webResources>
</configuration>
</plugin>
 
<plugin>
<groupId>com.google.appengine</groupId>
<artifactId>appengine-maven-plugin</artifactId>
<version>${appengine.version}</version>
<configuration>
<enableJarClasses>false</enableJarClasses>
<version>${app.version}</version>
<!-- Comment in the below snippet to bind to all IPs instead of just localhost -->
<!-- address>0.0.0.0</address>
                    <port>8080</port -->
<!-- Comment in the below snippet to enable local debugging with a remote debugger
                         like those included with Eclipse or IntelliJ -->
<!-- jvmFlags>
                      <jvmFlag>-agentlib:jdwp=transport=dt_socket,address=8000,server=y,suspend=n</jvmFlag>
                    </jvmFlags -->
</configuration>
</plugin>
<plugin>
<groupId>com.google.appengine</groupId>
<artifactId>gcloud-maven-plugin</artifactId>
<version>${gcloud.plugin.version}</version>
<configuration>
<set_default>true</set_default>
</configuration>
</plugin>
</plugins>
<pluginManagement>
<plugins>
<!--This plugin's configuration is used to store Eclipse m2e settings only. It has no influence on the Maven build itself.-->
<plugin>
<groupId>org.eclipse.m2e</groupId>
<artifactId>lifecycle-mapping</artifactId>
<version>1.0.0</version>
<configuration>
<lifecycleMappingMetadata>
<pluginExecutions>
<pluginExecution>
<pluginExecutionFilter>
<groupId>
org.codehaus.mojo
</groupId>
<artifactId>
versions-maven-plugin
</artifactId>
<versionRange>
[2.1,)
</versionRange>
<goals>
<goal>
display-plugin-updates
</goal>
<goal>
display-dependency-updates
</goal>
</goals>
</pluginExecutionFilter>
<action>
<ignore></ignore>
</action>
</pluginExecution>
</pluginExecutions>
</lifecycleMappingMetadata>
</configuration>
</plugin>
</plugins>
</pluginManagement>
</build>
</project>

Modify the Servlet's web.xml  file to add in Spring configurations

  1. Open up the skeleton project's web.xml file in [project]\src\main\webapp\WEB-INF\ folder in a text editor.
  2. Add in sections for defining the Spring configuration context, listener, dispatcher sections.
  3. In the context-param section's contextConfigLocation param-name, define your controller's class name e.g. com.mycompany.myapp.Application.
  4. In the servlet section's contextConfigLocation init-param, define your controller's class name e.g. com.mycompany.myapp.Application.

    An example web.xml file is shown below
    .
<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
 
<context-param>
<param-name>contextClass</param-name>
<param-value>
org.springframework.web.context.support.AnnotationConfigWebApplicationContext
</param-value>
</context-param>
 
<context-param>
<param-name>log4jConfigLocation</param-name>
<param-value>/WEB-INF/classes/log4j.properties</param-value>
</context-param>
 
<!-- Configuration locations must consist of one or more comma- or space-delimited 
        fully-qualified @Configuration classes. Fully-qualified packages may also 
        be specified for component-scanning -->
<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>com.mycompany.myapp.Application</param-value>
</context-param>
 
<!-- Bootstrap the root application context as usual using ContextLoaderListener -->
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>
 
<servlet>
<servlet-name>dispatcher</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
<init-param>
<param-name>contextClass</param-name>
<param-value>org.springframework.web.context.support.AnnotationConfigWebApplicationContext</param-value>
</init-param>
<init-param>
<param-name>contextConfigLocation</param-name>
<param-value>com.mycompany.myapp.Application</param-value>
</init-param>        
</servlet>
 
<servlet-mapping>
<servlet-name>dispatcher</servlet-name>
<url-pattern>/*</url-pattern>
</servlet-mapping>
 
 
</web-app>


Building and deploying to the development server

  1. Open a Command Prompt. Change directory to the root of the skeleton project e.g. C:\MyProjects\myapp\.

    C:\> cd \MyProjects\myapp
  2. Type in the Maven build command.

    C;\gt; mvn clean install
  3. To run the Spring application in the Google App Engine development server, type in the command:

    C:\> mvn appengine:devserver



    The Spring example can be accessed from a browser as shown below.

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