In this chapter, we create a multi-module project that combines the examples from the two previous chapters. The simple-weather code developed in [customizing] will be combined with the simple-webapp project defined in [web] to create a web application that retrieves and displays weather forecast information on a web page. At the end of this chapter, you will be able to use Maven to develop complex, multi-module projects.
The multi-module project developed in this example consists of modified versions of the projects developed in [customizing] and [web], and we are not using the Maven Archetype plugin to generate this multi-module project. We strongly recommend downloading a copy of the example code to use as a supplemental reference while reading the content in this chapter. This chapter’s example project may be downloaded with the book’s example code at:
http://books.sonatype.com/mvnex-book/mvnex-examples.zip
Unzip this archive in any directory, and then go to the ch-multi/
directory. There you will see a directory named simple-parent,
which contains the multi-module Maven project developed in this
chapter. In this directory, you will see a pom.xml and the two
submodule directories, simple-weather and simple-webapp.
A multi-module project is defined by a parent POM referencing one or
more submodules. In the simple-parent/ directory, you will find the
parent POM (also called the top-level POM) in
simple-parent/pom.xml. See simple-parent Project POM.
<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/maven-v4_0_0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>org.sonatype.mavenbook.multi</groupId>
<artifactId>simple-parent</artifactId>
<packaging>pom</packaging>
<version>1.0</version>
<name>Multi Chapter Simple Parent Project</name>
<modules>
<module>simple-weather</module>
<module>simple-webapp</module>
</modules>
<build>
<pluginManagement>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<configuration>
<source>1.5</source>
<target>1.5</target>
</configuration>
</plugin>
</plugins>
</pluginManagement>
</build>
<dependencies>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>3.8.1</version>
<scope>test</scope>
</dependency>
</dependencies>
</project>
Notice that the parent defines a set of Maven coordinates: the
groupId is org.sonatype.mavenbook.multi, the artifactId is
simple-parent, and the version is 1.0. The parent project
doesn’t create a JAR or a WAR like our previous projects; instead, it
is simply a POM that refers to other Maven projects. The appropriate
packaging for a project like simple-parent that simply provides a
Project Object Model is pom. The next section in the pom.xml lists
the project’s submodules. These modules are defined in the modules
element, and each module element corresponds to a subdirectory of the
simple-parent directory. Maven knows to look in these directories
for pom.xml files, and it will add submodules to the list of Maven
projects included in a build.
Lastly, we define some settings which will be inherited by all
submodules. The simple-parent build configuration configures the
target for all Java compilation to be the Java 5 JVM. Since the
compiler plugin is bound to the lifecycle by default, we can use the
pluginManagement section do to this. We will discuss pluginManagement
in more detail in later chapters, but the separation between providing
configuration to default plugins and actually binding plugins is much
easier to see when they are separated this way. The dependencies
element adds JUnit 3.8.1 as a global dependency. Both the build
configuration and the dependencies are inherited by all
submodules. Using POM inheritance allows you to add common
dependencies for universal dependencies like JUnit or Log4J.
The first submodule we’re going to look at is the simple-weather submodule. This submodule contains all the code from the previous section [customizing].
<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/maven-v4_0_0.xsd">
<modelVersion>4.0.0</modelVersion>
<parent>
<groupId>org.sonatype.mavenbook.multi</groupId>
<artifactId>simple-parent</artifactId>
<version>1.0</version>
</parent>
<artifactId>simple-weather</artifactId>
<packaging>jar</packaging>
<name>Multi Chapter Simple Weather API</name>
<build>
<pluginManagement>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-surefire-plugin</artifactId>
<configuration>
<testFailureIgnore>true</testFailureIgnore>
</configuration>
</plugin>
</plugins>
</pluginManagement>
</build>
<dependencies>
<dependency>
<groupId>log4j</groupId>
<artifactId>log4j</artifactId>
<version>1.2.14</version>
</dependency>
<dependency>
<groupId>dom4j</groupId>
<artifactId>dom4j</artifactId>
<version>1.6.1</version>
</dependency>
<dependency>
<groupId>jaxen</groupId>
<artifactId>jaxen</artifactId>
<version>1.1.1</version>
</dependency>
<dependency>
<groupId>velocity</groupId>
<artifactId>velocity</artifactId>
<version>1.5</version>
</dependency>
<dependency>
<groupId>org.apache.commons</groupId>
<artifactId>commons-io</artifactId>
<version>1.3.2</version>
<scope>test</scope>
</dependency>
</dependencies>
</project>
In simple-weather's pom.xml file, we see this module referencing a
parent POM using a set of Maven coordinates. The parent POM for
simple-weather is identified by a groupId of
org.sonatype.mavenbook.multi, an artifactId of simple-parent,
and a version of 1.0.
The WeatherService class shown in The WeatherService Class is
defined in src/main/java/org/sonatype/mavenbook/weather, and it
simply calls out to the three objects defined in [customizing]. In
this chapter’s example, we’re creating a separate project that
contains service objects that are referenced in the web application
project. This is a common model in enterprise Java development; often
a complex application consists of more than just a single, simple web
application. You might have an enterprise application that consists of
multiple web applications and some command-line applications. Often,
you’ll want to refactor common logic to a service class that can be
reused across a number of projects. This is the justification for
creating a WeatherService class; by doing so, you can see how the
simple-webapp project references a service object defined in
simple-weather.
package org.sonatype.mavenbook.weather;
import java.io.InputStream;
public class WeatherService {
public WeatherService() {}
public String retrieveForecast( String zip ) throws Exception {
// Retrieve Data
InputStream dataIn = new YahooRetriever().retrieve( zip );
// Parse Data
Weather weather = new YahooParser().parse( dataIn );
// Format (Print) Data
return new WeatherFormatter().format( weather );
}
}
The retrieveForecast() method takes a String containing a zip code. This zip code parameter is then passed to the YahooRetriever's retrieve() method, which gets the XML from Yahoo Weather. The XML returned from YahooRetriever is then passed to the parse() method on YahooParser which returns a Weather object. This Weather object is then formatted into a presentable String by the WeatherFormatter.
The simple-webapp module is the second submodule referenced in the simple-parent project. This web application project depends upon the simple-weather module, and it contains some simple servlets that present the results of the Yahoo weather service query.
<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/maven-v4_0_0.xsd">
<modelVersion>4.0.0</modelVersion>
<parent>
<groupId>org.sonatype.mavenbook.multi</groupId>
<artifactId>simple-parent</artifactId>
<version>1.0</version>
</parent>
<artifactId>simple-webapp</artifactId>
<packaging>war</packaging>
<name>simple-webapp Maven Webapp</name>
<dependencies>
<dependency>
<groupId>javax.servlet</groupId>
<artifactId>servlet-api</artifactId>
<version>2.4</version>
<scope>provided</scope>
</dependency>
<dependency>
<groupId>org.sonatype.mavenbook.multi</groupId>
<artifactId>simple-weather</artifactId>
<version>1.0</version>
</dependency>
</dependencies>
<build>
<finalName>simple-webapp</finalName>
<plugins>
<plugin>
<groupId>org.mortbay.jetty</groupId>
<artifactId>maven-jetty-plugin</artifactId>
</plugin>
</plugins>
</build>
</project>
This simple-webapp module defines a very simple servlet that reads a zip code from an HTTP request, calls the WeatherService shown in The WeatherService Class, and prints the results to the response’s Writer.
package org.sonatype.mavenbook.web;
import org.sonatype.mavenbook.weather.WeatherService;
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class WeatherServlet extends HttpServlet {
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
String zip = request.getParameter("zip" );
WeatherService weatherService = new WeatherService();
PrintWriter out = response.getWriter();
try {
out.println( weatherService.retrieveForecast( zip ) );
} catch( Exception e ) {
out.println( "Error Retrieving Forecast: " + e.getMessage() );
}
out.flush();
out.close();
}
}
In WeatherServlet, we instantiate an instance of the WeatherService class defined in simple-weather. The zip code supplied in the request parameter is passed to the retrieveForecast() method and the resulting test is printed to the response’s Writer.
Finally, to tie all of this together is the web.xml for
simple-webapp in src/main/webapp/WEB-INF. The servlet and
servlet-mapping elements in the web.xml shown in
simple-webapp web.xml map the request path /weather to the
WeatherServlet.
<!DOCTYPE web-app PUBLIC
"-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
"http://java.sun.com/dtd/web-app_2_3.dtd" >
<web-app>
<display-name>Archetype Created Web Application</display-name>
<servlet>
<servlet-name>simple</servlet-name>
<servlet-class>
org.sonatype.mavenbook.web.SimpleServlet
</servlet-class>
</servlet>
<servlet>
<servlet-name>weather</servlet-name>
<servlet-class>
org.sonatype.mavenbook.web.WeatherServlet
</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>simple</servlet-name>
<url-pattern>/simple</url-pattern>
</servlet-mapping>
<servlet-mapping>
<servlet-name>weather</servlet-name>
<url-pattern>/weather</url-pattern>
</servlet-mapping>
</web-app>
With the simple-weather project containing all WAR file. To do this, you will want to compile and install both projects in the appropriate order; since simple-webapp depends on simple-weather, the simple-weather JAR needs to be created before the simple-webapp project can compile. To do this, you will run mvn clean install command from the simple-parent project:
~/examples/ch-multi/simple-parent$ mvn clean install [INFO] Scanning for projects... [INFO] Reactor build order: [INFO] Simple Parent Project [INFO] simple-weather [INFO] simple-webapp Maven Webapp [INFO] ----------------------------------------- [INFO] Building simple-weather [INFO]task-segment: [clean, install] [INFO] ----------------------------------------- [...] [INFO] [install:install] [INFO] Installing simple-weather-1.0.jar to simple-weather-1.0.jar [INFO] ----------------------------------------- [INFO] Building simple-webapp Maven Webapp [INFO]task-segment: [clean, install] [INFO] ----------------------------------------- [...] [INFO] [install:install] [INFO] Installing simple-webapp.war to simple-webapp-1.0.war [INFO] [INFO] ----------------------------------------- [INFO] Reactor Summary: [INFO] ----------------------------------------- [INFO] Simple Parent Project .................. SUCCESS [3.041s] [INFO] simple-weather ......................... SUCCESS [4.802s] [INFO] simple-webapp Maven Webapp ............. SUCCESS [3.065s] [INFO] -----------------------------------------
When Maven is executed against a project with submodules, Maven first loads the parent POM and locates all of the submodule POMs. Maven then puts all of these project POMs into something called the Maven Reactor which analyzes the dependencies between modules. The Reactor takes care of ordering components to ensure that interdependent modules are compiled and installed in the proper order.
|
Note
|
The Reactor preserves the order of modules as defined in the POM unless changes need to be made. A helpful mental model for this is to picture that modules with dependencies on sibling projects are "pushed down" the list until the dependency ordering is satisfied. On rare occasions, it may be handy to rearrange the module order of your build — for example if you want a frequently unstable module towards the beginning of the build. |
Once the Reactor figures out the order in which projects must be built, Maven then executes the specified goals for every module in the multi-module build. In this example, you can see that Maven builds simple-weather before simple-webapp, effectively executing mvn clean install for each submodule.
|
Note
|
When you run Maven from the command line you’ll frequently want to specify the clean lifecycle phase before any other lifecycle stages. When you specify clean, you make sure that Maven is going to remove old output before it compiles and packages an application. Running clean isn’t necessary, but it is a useful precaution to make sure that you are performing a "clean build". |
Once the multi-module project has been installed with mvn install, you
can run the web application with mvn jetty:run.
~/examples/ch-multi/simple-parent/simple-webapp $ mvn jetty:run [INFO] ----------------------------------------- [INFO] Building simple-webapp Maven Webapp [INFO]task-segment: [jetty:run] [INFO] ----------------------------------------- [...] [INFO] [jetty:run] [INFO] Configuring Jetty for project: simple-webapp Maven Webapp [...] [INFO] Webapp directory = ~/examples/ch-multi/simple-parent/\ simple-webapp/src/main/webapp [INFO] Starting jetty 6.1.6rc1 ... 2007-11-18 1:58:26.980::INFO: jetty-6.1.6rc1 2007-11-18 1:58:26.125::INFO: No Transaction manager found 2007-11-18 1:58:27.633::INFO: Started SelectChannelConnector@0.0.0.0:8080 [INFO] Started Jetty Server
Once Jetty has started, load http://localhost:8080/simple-webapp/weather?zip=01201 in a browser and you should see the formatted weather output.